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observations have revealed the presence and topology of magnetic fields on the surfaces of some main sequence massive stars. these stars possess a convective core that supports strong dynamo action. this core is linked to the dynamics of the rest of the star through overshooting convection and magnetic fields and may influence the surface magnetism. such effects are captured through 3-d mhd simulations of a 10 m⊙ b-type star, using the anelastic spherical harmonic (ash) code. these simulations capture the inner 65% of the star by radius, encompassing the convective core and an extensive portion of the radiative exterior. vigorous dynamo action is achieved in the convective core with self-consistent super-equipartition (se) states sustained over a range of rotation rates. indeed, the ratio of magnetic to convective kinetic energy shows a distinct scaling with elsasser and coriolis number. the impact of this dynamo action upon the differential rotation of the core is assessed by contrasting hydrodynamic and magnetohydrodynamic simulations. the processes that permit the maintenance of such se states are examined. we further study how the magnetic field generated during main-sequence dynamo action may carry over into later evolutionary stages. | super-equipartition convective dynamo action in the cores of b-type stars |
we examine how 3d mhd simulations can deliver clues on the mechanisms at the origin of angular momentum loss saturation of rapidly rotating solar-like stars. based on a study of six targets, whose magnetic field has been observed by zeeman doppler imaging (zdi), we find that the saturation could be explained by a extremely strong coverage of the stellar surface of a large scale dipolar mode, in disagreement with recent works. | spin evolution and saturation: new insights through 3d mhd simulations of young solar analogs |
the saturation and nonlinear evolution of the spatial spectrum of the weibel instability, which is caused by an anisotropy of the particle velocity distribution and generates quasi-magnetostatic turbulence during various transient processes, are important for a wide class of phenomena in the nonequilibrium cosmic and laboratory plasmas [1]. complex anisotropic particle distributions, such as kappa and other non-maxwellian distributions, are observed and/or expected for a laser plasma, solar wind, active regions in stellar coronas, and collisionless shock waves. the present talk is based on an original (quasi-linear) code for one- and two-dimensional modeling of the nonlinear development of the spatial harmonics of the weibel tm-instability. we reveal the qualitative dynamical features of quasi-magnetostatic turbulence originated from different (bi-maxwellian and bi-kappa) initial electron distributions in a collisionless homogeneous plasma. the phenomenon is described by the maxwell-vlasov equations which are solved by the leapfrog method for many hundreds of harmonics with wave vectors lying in the simulation plane containing the anisotropy axis defined by the maximum effective particle temperature. in a wide range of both small and large values of the degree of initial electron anisotropy, the saturating value of the magnetic field is found, its further slow nonlinear decay is traced, the evolution of the main part of the energy-carrying harmonics of turbulence is established, the law of variation in the width of their spectrum along and across the anisotropy axis is determined. also, the typical patterns of deformation (flattening) of the velocity distribution function are picked out and characteristic evolution of both the change in effective temperatures and the degree of electron anisotropy at the nonlinear stage of instability development are investigated. a comparative analysis of the results obtained for (i) one- and two-dimensional problems, (ii) bi-maxwellian and bi-kappa distributions of electrons, (iii) small and large (compared to unity) initial values of their degree of anisotropy, (iv) a number of effective temperatures and kappa indices is carried out. particular attention is paid to the nonlinear and/or quasi-linear effects of the interaction of various spatial harmonics of the self-generated magnetic field with each other and with certain fractions of electrons. the novel results are compared with the known results of the analysis of the nonlinear evolution of the weibel instability in those parameter regions where the available analytical (for example, quasi-linear) and numerical (for example, particles-in-cells) methods are comparative with the more efficient method we use. finally, we present the original analytical estimates of the saturation level and the dynamical features of weibel turbulence that are based on the previously developed one-dimensional quasi-linear theory and turned out to be in agreement with the advanced results of numerical simulation in the region of applicability. the investigation of the nonlinear process of the weibel instability in various anisotropic plasmas was supported by the theoretical physics and mathematics advancement foundation "basis", project no. 20-1-1-37-1. simulations were carried out partially in the supercomputer center of the keldysh institute of applied mathematics of the russian academy of sciences. 1. kocharovsky vl. v., kocharovsky v. v., martyanov v. y., tarasov s. v. analytical theory of self-consistent current structures in a collisionless plasma // phys. usp. 2016. v. 59, n. 12. p. 1165-1210. doi: 10.3367/ufne.2016.08.037893. | saturated magnetic field of the weibel tm-instability and dynamics of its spatial spectrum in a plasma with the anisotropic kappa or maxwellian distribution of electrons |
our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. such fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution more generally. a major difficulty is the effect of stable stratification, which generally suppresses dynamo action. we explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a nonhelical large-scale dynamo mechanism known as the magnetic shear-current effect. we find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). both are plausibly sourced in the presence of differential rotation. our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. additionally, a scan over magnetic reynolds number shows no change in the growth or saturation of the large-scale dynamo, providing good numerical evidence of a dynamo mechanism immune to catastrophic quenching, which has been an issue for helical dynamos. these properties—the lack of catastrophic quenching and robustness to stable stratification—make the mechanism a plausible candidate for generating in-situ large-scale magnetic fields in stellar radiative zones, and more specifically, the solar tachocline. | large-scale dynamo with stable stratification: the magnetic shear-current effect |
the role played by magnetic fields in high-mass star formation is not yet fully clear. theoretical simulations have shown that magnetic fields appear to suppress fragmentation in the star forming cloud, to enhance accretion via disc and to provide feedback in the form of outflows and jets. however, models require specific magnetic configurations and need more observational constraints to properly test the impact of magnetic fields. the identification of massive protostars is complicated due to their quick evolution, and their location inside distant, dense, and dark clusters. in the past few years, masers have been successfully used to probe the magnetic field strength and morphology at the small scales of about 10 astronomical units (au), around massive protostars. thanks to the narrow and strong spectral lines of masers, we can measure linear polarization angles and zeeman splitting and obtain information about the magnetic field intensity and geometry. radio-interferometers, such as the multi-element radio linked interferometer network (merlin), can provide the sensitivity and the spatial and spectral resolution needed to detect the signatures of protostellar processes at the required scale of few au. in this work we make use of merlin data to investigate the magnetic field structure of the massive protostar iras18089-1722, analyzing 6.7 ghz methanol maser observations. iras18089-1732 is a well studied high mass protostar, showing a hot core chemistry, an accretion disc and a bipolar outflow. an ordered magnetic field oriented around its disc has been detected from previous observations of polarized dust. this gives us the chance to investigate how the magnetic field at the small scale probed by masers relates to the large scale field probed by the dust. our analysis of the 6.7 ghz polarized methanol maser observations, indicates that the magnetic field in the maser region is consistent with the magnetic field constrained by the previous dust polarized observations. we find that the magnetic field in the maser region presents the same orientation as in the disc. thus the large scale field component, even at the few au scale of the masers, dominates over any small scale field fluctuations. we present a tentative detection of circularly polarized line emission, from which we obtain a field strength along the line of sight of 5.5 mg, consistent with previous estimates. | methanol masers reveal the magnetic field of a high mass protostar |
star formation in giant molecular clouds (gmcs) is of fundamental importance to our understanding of star formation in our universe since it is in these massive, transient objects where almost all stars are formed. to understand the process of star formation in gmcs one must understand how star formation feedback from the massive stars that form within them acts to halt the star formation process. massive stars (m∗ ≳ 8m⊙) can inject significant amounts of energy in to the surrounding cloud, blowing apart the cloud and halting star formation. one way this can happen is when the radiative energy from these stars is injected directly in to their atmospheres, resulting in massive outflows moving at velocities of > 1000kms‑1. these winds shock when they impact the ambient medium, creating highly over-pressurized, extremely hot t ∼ 108 k gas that expands rapidly in to the surrounding cloud. in this thesis, we seek to understand this process in the context of realistic star forming environments. in this dissertation, we show how turbulent mixing between the wind-driven bubble and the surroundings can lead to efficient cooling at the t ∼ 104 k gas created through this mixing. we demonstrate that this cooling can be sufficient to cause the bubble's evolution to become momentum-driven, rather than energy driven, drastically decreasing the winds power to disperse its surroundings. in chapter 2 we lay out the details of this theory, examine how the internal structure of the wind-driven bubble changes in this scenario, and show how such a theory would explain several outstanding observational mysteries. in chapter 3, we validate this picture with a large suite of three-dimensional, hydrodynamic simulations. in chapter 4 we test this theory in the context of self-gravitating, self-consistently star forming clouds and show that turbulent cooling is indeed the dominant mechanism for the removal of stellar wind energy. finally, in chapter 5 we present the final work of this thesis, where we investigate the effects of background magnetic fields and photo-ionized gas on this picture for stellar wind-driven bubble evolution before reviewing the prospects for future work in chapter 6. | the dynamics of stellar wind-driven bubbles and their effect on star formation in giant molecular clouds |
simulations of the terrestrial paleo-magnetosphere for early stages of the solar system are of particular importance for studying the evolution and mass loss of the earth's atmosphere. within this presentation, we will present simulations of the paleo-magnetosphere of the earth for the late hadean, i.e. for ~4.1 billion years ago. these were performed with an adapted version of the paraboloid magnetospheric model (pmm) of the skobeltsyn institute for nuclear physics of the moscow state university, which serves as an iso standard for the magnetosphere . as an input parameter, the new measurements of the paleomagnetic field strength by tarduno et al., 2015, are taken. these data from zircons between 3.3 billion and 4.2 billion years old vary between 1.0 and 0.12 of today's equatorial field strength. available data at ~4.1 billion years ago are among the lowest field strength values. another input into the adapted pmm is the solar wind pressure, which was derived from a newly developed solar/stellar wind evolution model, which is strongly dependent on the rotation rate of the early sun.our simulations of the terrestrial paleo-magnetosphere with the adapted pmm show that for the most extreme case of a fast rotating sun and a paleomagnetic field strength with 0.12 of today's value, the stand-off distance of the magnetopause rs shrinks significantly down from today's 10-11 re to 3.43 re (i.e. 2.43 re above the earth's surface, where re is the earth's surface). even for the least extreme case - i.e. the same magnetic field strength as that of today and a slow rotating sun - rs shrinks down to 8.23 re. another outcome of the modelling is that the polar cap was significantly broader ~4.1 billion years ago than today.these results have implications for the early terrestrial atmosphere. since the euv flux during the late hadean eon was significantly higher, the exobase of a nitrogen dominated atmosphere would most probably have reached the magnetopause, leading to enhanced atmospheric erosion. however, a significantly higher amount of co2 during the late hadean than at present-day may have prevented atmospheric loss in such a scenario. first results of these erosion processes will be presented within this presentation. | on the earth's paleo-magnetosphere for the late hadean eon |
magnetars are neutron stars with ultra strong magnetic fields, $b\sim 10^{14\text{-}15} \text{g}$, and their high surface luminosity, $l_{s}\sim 10^{35} \text{erg} \text{s}^{-1}$, is a direct evidence of their magnetic activity. for a young magnetar, with internal temperatures $t\gtrsim10^{9} \text{k}$, particles in the core are strongly coupled by collisional forces but can convert into each other by $\beta$-decays (urca reactions), in the so called "strong coupling regime". at this stage the magnetic field induces small fluid displacements, changing the local chemical composition and generating pressure gradient forces, which tend to be erased by urca reactions. depending on the strength of the chemical departure, these reactions can lead to a non-trivial thermal evolution as a consequence of the magnetic feedback, which can even produce a net heating of the core. this mechanism converts magnetic to thermal energy and could explain the high surface luminosity in newly born magnetars. in this work, we present the first long-term magneto-thermal simulation of a neutron star core in this regime and explore the possibility of this heating mechanism. | magneto-thermal evolution of neutron star cores in the "strong-coupling regime" |
understanding the magnetic field strength and morphology of astrophysical regions is of great importance in understanding their dynamics. there exist a number of methods astronomers can employ to trace magnetic field structures, and each have their own limitations. a promising technique to trace the magnetic field morphology around evolved stars, or on the smallest scales of (high-mass) star forming regions, is (sub-)millimeter spectral line polarization observations. line (linear) polarization can either arise in association with maser radiative transfer, or alternatively, molecular lines polarize through the goldreich-kylafis effect. in both cases, the polarization angle traces the magnetic field with a 90-degree ambiguity. in order to remove this ambiguity, and to estimate the observational viability of particular line polarization measurements, polarized line radiative transfer needs to be employed. in this talk, i present (i) polarized radiative transfer tools that quantify the polarization of maser radiation, (ii) a three-dimensional polarized line radiative transfer tool: portal. portal simulates the emergence of thermal molecular line polarization in astrophysical objects of arbitrary geometry and magnetic field morphology, (iii) a novel polarization mechanism: collisional polarization. which provides the possibility of directly detecting ambipolar diffusion in disks through the polarization of molecular ions, and i will discuss observations of molecular line polarization around evolved stars and on the smallest scales of (high-mass) star forming regions | tracing cosmic magnetic fields using molecules |
observations of agn feedback have been revolutionized in recent years by the direct detection of galaxy-scale outflows driven by supermassive black holes. these agndriven galactic winds are now observed in several observational windows, including emission and absorption in the uv, optical, x-ray, radio, and far ir. the outflows are routinely observed in several molecules, indicating that even the cold and dense gas that can form stars is directly affected by agn feedback. however, the basic physics of these outflows, how to interpret their observational signatures, and their effects on galaxy evolution remain poorly understood. building on pilot studies, we will use two complementary types of simulations to address these questions and make predictions testable by several nasa observatories. first, we will use controlled numerical experiments to develop a robust understanding of the physical processes involved. we have recently simulated, for the first time, the formation of new molecules in cooling agn wind shocks, providing an explanation for observed molecular outflows. in that pilot study, an ambient medium with uniform density and constant dust abundance were assumed. next, we will augment our timedependent chemistry network to self-consistently follow the destruction and growth of dust grains, a major uncertainty since dust is essential for efficient molecule formation. we will study how wind propagation and molecular chemistry are affected by inhomogeneities in the ambient medium. we will also study for the first time the effects of thermal conduction and magnetic fields on agn wind dynamics. in stellar wind bubbles, thermal conduction strongly affects the dynamics but conduction has so far been neglected in agn-driven wind models. after gaining a solid understanding of the basic physics, we will extend our models to fully dynamic simulations of realistic galaxies with a multiphase ism. such simulations are needed to understand how agn outflows affect galaxies and to make detailed predictions that can be directly compared to observations. in these realistic galaxy simulations, agn outflows interact self-consistently with stellar feedback. we will include non-equilibrium ism chemistry in a subset of these realistic galaxy simulations, which will enable us to quantify how the mass and energy of agn outflows is partitioned between different phases, including molecular gas. our simulation suite will include quasars fueled by galaxy mergers, lower-luminosity seyferts, and typical star-forming galaxies. we will investigate both the negative and positive effects of agn winds on star formation. we will calibrate the energetics of our agn wind models by comparing our simulations with observations of galaxy-scale outflows. we will then evolve fully cosmological simulations including observationally-calibrated agn winds to study the effects of agn feedback not only on galaxies but also on their gaseous halos ("preventive" feedback). we will process our simulations with radiative transfer to produce spatially-resolved spectra in several important observational tracers, including warm h2 ir lines (observable by jwst), oh (observed by herschel), co, [cii], and dust continuum (observable by alma and other ground-based observatories). we recently showed that a large fraction of the molecular gas formed in agn outflows is expected to be warm and luminous in jwst bands. with jwst planned for launch next year, we will prioritize our infrared predictions to prepare for potentially breakthrough jwst observations of agn outflows. we will also produce x-ray emission maps of hot wind gas observed by chandra. on larger scales, we will quantify the observational signatures of agn winds interacting with halo gas, which can be probed by quasar absorption lines, x-ray emission, and the sunyaev-zel'dovich effect. | the physics, observational signatures, and consequences of agn-driven galactic winds |
for a few million years after the gravitational collapse that led to their formation; young stellar systems remain surrounded by a circumstellar disk from which planets form. alma and vlt/sphere have provided spectacular images of the planet-forming disks on a scale of a few 10 au (garufi et al. 2017) and; even more recently; led to the direct detection of forming planets within the disk (benisty et al. 2021). in contrast; the exploration of the innermost disk regions (≲ 1 au) has remained quite challenging so far. moreover; the kepler satellite has revealed that the most ubiquitous planetary systems consist of compact strings of super-earth and mini-neptunes orbiting close to their host star (porb ≲ 100 days; e.g.; otegi et al. 2022;). it is; therefore; crucial to investigate the physics of the star-planet-inner disk interaction in young stars; not only for the role it plays in the early evolution of solar-type stars (e.g.; accretion/ejection; angular momentum; etc.) but also to determine the environmental conditions that prevail at the time of planetary formation. this is a necessary step towards understanding the formation of the plethora of compact inner low-mass planetary systems observed across the galaxy. ci tau is so far the only pre-main sequence star still accreting from its surrounding disk (classical t-tauri star) claimed to host a hot jupiter planet. however; the periodic radial velocity variation could result from magnetospheric accretion onto the star rather than from an orbiting body (donati et al. 2020). the most interesting aspect of ci tau regards its extreme magnetic field (3.7 kg) that disrupts the inner gaseous disk and generates accretion funnel flows down to the stellar surface. we propose to present our investigation about the inner region of ci tau; aiming at reconnecting the different spatial scales of the system down to a few stellar radii (≲ 0.1 au). method: we investigated this puzzling question using the long-baseline interferometry technique; the only way to probe the inner region of the system at sub-au precision. thanks to the high spectral resolution of vlti/gravity (r=4000); we are both sensitive to the emitting dusty part of the inner rim (k-band continuum); and the magnetosphere itself traced by the brγ emission line. results: in the continuum; we characterise the disk's inner rim; which appeared to be disconnected from the outer disk with a large misalignment both in inclination and position angle. we report an internal rim position at 0.20 ± 0.02 au; remarkably more significant than the theoretical sublimation radius of 0.03-0.06 au. such difference could infer the presence of a planet carving the inner part of the disk; as recently supported by hydrodynamical simulation (muley et al. 2021). the non-zero closure phases measured by gravity suggest an important asymmetry in the disk: the southwest side appears brighter than the northeast. such difference argues in favour of an inclined disk where the brilliant (and farthest) part is seen from the bottom (distant observer point of view). we confirmed such behaviours using radiative transfer modelling with radmc3d. in the brγ line (2.1661 μm), our model suggests a bright but smaller emitting region than the thermal emission with a radius of 0.05 ± 0.01 au (4 times smaller than the inner rim). such characteristic is strongly supported by the magnetosphere accreting models developed in our team and will be presented for comparison. conclusion: with gravity; we characterise the inner disk of ci tau with a sub-au precision; allowing a direct comparison with the standard yso's characteristic sizes such as sublimation; co-rotation or magnetic truncation radii. the existence of a larger than expected central cavity could be an observational signature of the well known 11.6 mj planet (ci tau b). the substantially detected misalignment between inner and outer disks constitutes a challenge for the modelling efforts and should be carefully investigated in the future. | a sub-au study of ci tau with gravity: the key to connect stars, disks and planets |
the dissipation of the kinetic energy of large-scale and wave-like tidal flows within the convective envelope of low-mass stars is one of the key physical mechanisms that shape the orbital and rotational dynamics of short-period exoplanetary systems. in the case of stable binary systems, they lead to the orbit circularisation and to the spins synchronisation and alignment; in the case of unstable systems they drive the spiraling of the planet towards the central star. in addition, stellar convective envelopes are (differentially) rotating, turbulent, and magnetized regions where an active dynamo action is sustained (e.g. brun & browning 2017 and references therein). therefore, as demonstrated by first theoretical works and numerical simulations, tidal flows and waves excitation, propagation, and dissipation can be impacted by stellar magnetic fields (e.g. wei 2017, lin & ogilvie 2018, wei 2018). for instance, the so-called dynamical tide is constituted of magneto-inertial waves (their restoring forces being the lorentz force and the coriolis acceleration) instead of inertial waves in the non-magnetized case. in the meanwhile, the amplitude and the geometry of dynamo-generated magnetic fields vary along the evolution of low-mass stars (e.g. vidotto et al. 2014, brun & browning 2017 and references therein). in this framework, the key question that should be answered is "for which stellar masses, rotation and evolution phases, do we need to take into account the action of magnetic fields on tidal waves excitation, propagation, and dissipation?". in this work, we identify the terms in mhd equations that should be computed to evaluate the impact of magnetic fields on tidal dissipation in the convective envelope of active rotating low-mass stars hosting planets. using scaling laws that provide the amplitude of dynamo-generated magnetic fields along the structural and rotational evolution of these stars (e.g. augustson, mathis & brun 2016) combined with detailed grids of rotating stellar models (e.g. amard et al. 2016), we demonstrate that a full mhd treatment of tidal waves excitation, propagation, and dissipation is required for all low-mass stars (from m to f-type stars) all along their evolution. consequences for the dynamical evolution of short-period exoplanetary systems are finally discussed. | do magnetic fields modify tidal dissipation in the convective envelope of low-mass stars along their evolution? |
in the age of the kepler space telescope and other exoplanet finding missions, a variety of exotic planets have been discovered. some of these planets have been found to be in binary star systems --- systems which have historically been overlooked in planet formation models. this is due to the single star scenario being simpler to model than binaries, as well our anthropocentric bias towards single stars like our sun. however, planet formation around binary stars in an important topic because a large fraction (50%) of stars form in binary systems. in this thesis i investigated the physics that influences the creation, stability, and survivability of discs around binary stars with the broad understanding that the longer the lifetime of a disc (around a single or binary star) the higher the likelihood of producing planets. the theoretical work of this thesis was conducted using the ideal magnetohydrodynamical numerical simulation program flash. i simulated the collapse of molecular cores until the formation of protostars and followed the early evolution of these systems. for the first theoretical project i investigated the influence that binarity had on the global evolution of a young stellar system. this included studying mechanisms such as accretion, jets and outflows, and dynamical interactions. i found that binary stars produce weaker outflows when considering the transport of mass, linear momentum, and angular momentum. for the second theoretical project i investigated the formation of discs in binary stars with the inclusion of turbulence in the initial conditions. i found that the turbulence helped to build large circumbinary discs which restructured the magnetic fields for efficient outflow launching, but too much turbulence may also disrupt this organisation of magnetic fields. given the environment where binary stars form (turbulent molecular cores), it appears that the formation of circumbinary discs should be common place, however circumstellar discs could also be destroyed quickly in these same environments. my observational work aimed to determine the typical survivability of discs around binary star systems. this work was carried out by using the wide field spectrograph (wifes) on the australian national university 2.3m telescope to search for radial velocity variation in disc-bearing members of the 11myr and 17myr old star-forming regions upper scorpius and upper centaurus-lupus. i found that the binary fraction of disc-bearing stars in these regions do not differ significantly from the field star binary fraction. i hypothesised that this is due to two competing factors: circumstellar discs are disrupted by companions and are dispersed quickly, but circumbinary discs are more common than equivalently sized discs around single stars. these results suggest that the typical lifetimes of discs in single and binary stars are comparable. overall, i found that in some scenarios binary stars may produce hostile environments for planet formation via the destruction of circumstellar discs, but the formation of large circumbinary discs is likely to be a common occurrence. this suggests that planet formation is as likely around binary stars as single stars. therefore, planet formation around binary stars needs to be considered to understand overall planet formation. | the formation, evolution, and survivability of discs around young binary stars |
the first multi-messenger observation of a binary neutron star merger has already lead to first constraints on the nuclear matter equation of state. to make the most out of the observational data, however, the theoretical modeling of the merger process needs to be improved further. among other ingredients, the evolution of the merger remnant within tens of milliseconds after the merger is essential, since it is connected to the kilonova as well as to the short gamma ray burst. key aspects linking this early phase to the observables of later stages are the fate of the remnant, the mass of the disk, dynamical matter ejection, and disk winds. these in turn are influenced by the interplay with the remnant and hence the delay before black hole formation. modeling the remnant lifetime and collapse is a difficult challenge for current methods. in this talk, i will present results of numerical simulations that highlight the complexity of the most basic hydrodynamic evolution without magnetic fields, and show novel visualizations of the three-dimensional remnant structure. | numerical inside view of hypermassive remnant models for gw170817 |
conservation of magnetic flux is associated with regions of the powerful magnetic fields (b ∽ 1013 g) near neutron stars' surface. the vector potential generated by moving electric charge q is uniformly distributed within a neutron star's surface (radius r). the evolution of the magnetic field of isolated neutron stars is studied and based on magnetic flux conservation; the multipolar magnetic fields for (l = 1; l = 2; l = 3; l = 4) have calculated. we developed the field line equations and simulated the magnetic field line geometry for the interaction between neutron stars' dipole-multipolar magnetic fields using the matlab software program. | magnetic dipole interaction with multipole magnetic field lines of neutron stars |
the evolution of protoplanetary disks is believed to be driven largely by viscosity. the ionization of the disk that gives rise to viscosity is caused by x-rays from the central star or by energetic particles released by shock waves travelling into the circumstellar medium. we have performed test-particle numerical simulations of gev-scale protons traversing a realistic magnetised wind of a young solar mass star with a superposed small-scale turbulence. the large-scale field is generated via an mhd model of a t tauri wind, whereas the isotropic (kolmogorov power spectrum) turbulent component is synthesised along the particles' trajectories. we have combined chandra observations of t tauri flares with solar flare scaling for describing the energetic particle spectrum. in contrast with previous models, we find that the disk ionization is dominated by x-rays except within narrow regions where the energetic particles are channelled onto the disk by the strongly tangled and turbulent field lines; the radial thickness of such regions broadens with the distance from the central star (5 stellar radii or more). in those regions, the disk ionization due to energetic particles can locally dominate the stellar x-rays, arguably, out to large distances (10, 100 au) from the star. | local protoplanetary disk ionisation by t tauri star energetic particles |
filaments are ubiquitous in the interstellar medium, yet their formation, internal structure, magnetic properties, and longevity have not been analysed in detail. in this thesis i report the results from a comprehensive numerical study that investigates the characteristics, formation, dynamics, and global evolution of filamentary structures arising from (magneto)hydrodynamic interactions between supersonic winds and interstellar clouds. here i improve on previous wind-cloud simulations by utilising higher numerical resolutions, sharper density contrasts, more complex magnetic field configurations, and more realistic systems with turbulent clouds. i use gas multi-tracking algorithms and state-of-the-art visualisation techniques to study the physical mechanisms acting upon wind-swept clouds. i find that material originally in the envelopes of the clouds is removed and transported downstream to form filamentary tails, while the cores of the clouds serve as footpoints and late-stage outer layers of these low-density tails. the evolution of filaments comprises four phases: 1) tail formation, 2) tail erosion, 3) footpoint dispersion, and 4) filament free floating. overall, wind-cloud interactions produce filaments with aspect ratios >10, lateral expansions 1-3 of the core radius, mixing fractions 10-30%, velocity dispersions 0.02-0.05 of the wind speed, and magnetic field amplifications by factors of 10-100. i find that the strength of magnetic fields regulates vorticity production: sinuous filamentary towers arise in non-magnetic environments, while strong magnetic fields inhibit small-scale kelvin-helmholtz perturbations at boundary layers making tails less turbulent. the orientation of magnetic fields also influences the morphology of filaments: magnetic field components aligned with the direction of the wind favour the formation of pressure-confined flux ropes inside the tails, whilst transverse components tend to form current sheets and favour the growth of rayleigh-taylor perturbations at the leading edge of the clouds. i also investigate how turbulence influences the formation of filaments by sequentially adding log-normal density profiles, gaussian velocity fields, and turbulent magnetic fields into the initial clouds. the porosity of turbulent density profiles aids the propagation of internal shocks through filament material, accelerating mixing and increasing the internal velocity dispersion. the inclusion of subsonically-turbulent velocity fields has little effect on the evolution, while supersonically-turbulent velocity fields accelerate the cloud expansion and subsequent break-up. line stretching and compression amplify the magnetic energy of turbulent filaments creating highly-magnetised knots and sub-filaments along their tails. in all models the magnetic energy enhancement saturates when the ratio of turbulent kinetic to turbulent magnetic energy densities is 5-10. at the end of this thesis i discuss the relevance of this work for the study of clouds and filaments in the galactic centre and provide my perspectives on potential future research in this field. using ray-tracing techniques i create synthetic emission maps of wind-swept clouds and compare them with radio observations of high-latitude h i clouds and non-thermal filaments in this region of the galaxy. i interpret these structures as remains of the interplay between outflows driven by localised star formation and dense clouds in the surrounding medium. the simulated morphology, lifespan, magnetic properties, and kinematics are consistent with those inferred from observations of these clouds and non-thermal filaments. | magnetohydrodynamics of wind-cloud interactions: filament formation in the interstellar medium |
although the origin of molecular cloud turbulence remains debated, one possibility is that stellar feedback injects enough energy to drive observed motions on parsec scales. to investigate this possibility, we use magnetohydrodynamic simulations where we vary the stellar mass-loss rates and magnetic field strength. we generate synthetic 12co(1-0) maps assuming that the simulation is at the distance of the nearby perseus molecular cloud. by comparing different initial conditions and evolutionary times, we are able to identify differences in our synthetic observations. using a variety of statistical techniques proposed in the literature, we quantify the differences by calculating the first, second, and higher order moment maps of the data cubes, analyzing the power spectrum, and convolving the data with gaussian wavelets. we find that many turbulent statistics, such as the spectral correlation function, principal component analysis, and delta-variance, are sensitive to changes in mass-loss rates and/or turbulent structure. this demonstrates that stellar feedback influences molecular cloud turbulence and may be characterized using certain statistical metrics. | quantifying the impact of stellar feedback on molecular clouds |
we present results for magnetohydrodynamical simulations of evolving neutron stars in the first moments of its lifetime. we study the poloidal field instability and how the magnetic field components change with time. we find that although our final field does not reach a stable equilibrium, it settles to a twisted torus geometry with a dominant poloidal component and a weaker toroidal field reaching 10 evolution times (t ∼ 450 ms), the toroidal field reduces to 1 | simulating magnetic field evolution in isolated neutron stars |
the activity of stars governs the environment of (exo) planetary systems with implications for habitability. star-planet interactions are primarily mediated via variable stellar radiation, particle fluxes and magnetized stellar winds. the nature of planetary magnetospheres and properties of stellar winds, in particular, determine the evolution of planetary atmospheres. based on simulations with a star-planet-interaction module we have developed – named cessi-spim – we perform a rigorous parameter space study by varying the magnetic field strength of both the plasma wind and planetary magnetosphere. either strengthening the stellar wind magnetic field or weakening the planetary magnetic field results in stellar magnetic field accumulation at the day-side similar to that of an imposed magnetosphere. we explore the formation of alfven wings across the planet at different levels of magnetic activity. we validate our simulations with observations of the sun-earth and sun-mars systems and demonstrate how such star-planet interactions may induce planetary atmospheric losses. our results show that the mass-loss rate and day-side reconnection point exhibit a strong negative correlation. we discuss the implications of our results for the interplay between star-planet systems and (exo)planetary habitability. | impact of evolving stellar activity on (exo)planetary environments |
collisions between giant molecular clouds (gmcs) have been proposed as a mechanism to compress gas to trigger the birth of massive star clusters and associations and thus be potentially relevant to the formation of most stars in galaxies. the spatial distributions of dense gas cores within protoclusters generated by such collisions may encode important diagnostic information about the process and the environmental properties of the parent clouds. to investigate this, we carried out 3d numerical simulations of magnetized turbulent gmcs. non-colliding cases were also considered. the projected 2d mass surface density structures of the clouds, including cases after applying simulated alma observations, were analyzed. in particular, dense cores were identified with the dendrogram method and the minimum spanning tree (mst) of the cores computed. we compared the results from clouds with different initial magnetic (b-) field strengths, ranging from 10 to 50 micro g. the number and mass fraction of dense cores are suppressed in the more strongly magnetized and non-colliding cases. we consider various mst statistics, including the q parameter, and how these evolve during the simulations and their potential ability to diagnose magnetic and collisional conditions. we also examine mass segregation, including detailed properties of the most massive cores, which relates to the question of whether or not massive stars tend to form in more central locations in protoclusters. finally, we study the properties of the filaments within and around the protoclusters, including their widths, mass per unit lengths, energy balance and magnetic field strengths and orientations. | cores and filaments in mhd simulations of massive protoclusters |
red supergiants (rsg) and asymptotic giant branch (agb) stars produce strong stellar winds, which are not well understood. these winds have a key role in the enrichment of the interstellar medium with chemical elements, which are the building blocks for the next generation of stars. the magnetic field in such stars coupled with the atmospheric dynamics may also affect the stellar wind, since it increases atmospheric velocities and higher temperatures in the chromosphere. the understanding of the stellar wind dynamics is crucial for unveiling the unknown mass-loss mechanism, their chemical composition, and their stellar parameters. the radiative-hydrodynamics code, co5bold, solves the equations of compressible hydrodynamics and non-local radiation transport. it is used to produce 3d grids of global ``star-in-a-box'' models. in this ongoing study, we used co5bold to perform simulations of rsg and agb so we analyze and better understand the mechanisms and rate of mass-loss of such stars related to convection and magnetic field. we could compare the differences between our agb and rsg models regarding density, velocity, pressure and b field, and found that magnetic fields could be part of the mass-loss mechanism in rsgs. | the role of convection and magnetic field in the mass loss of evolved cool stars |
this thesis focuses on bridging the gap between solar and stellar physics through the study of flares. solar flares are the most powerful explosions in the solar system and many stellar flares have been observed to be orders of magnitudes larger. however, is not yet known if these phenomena are formed through the same physical process. in this thesis we explore their common origins through a detailed case study of a solar flare and a robust statistical analysis of stellar flares with bespoke observations. using data from the swedish solar telescope, a detailed study of a solar flare associated with a filament eruption and jet was compared with advanced 3d mhd simulations. this amalgamation of observation and theory allows for a complete picture of the event including the pre-flare magnetic structure and the resulting kinematics of the jet post eruption. overall, this study aims to characterise the physical environment capturing many evolutionary properties of the event providing a unique perspective on eruptive phenomena on the sun. with regards to stellar flares, observational data from both k2 and tess are used to conduct a statistical analysis on flares from both low mass and solar-type stars. as a result of this, no relationship between the rotational phase of stellar flares and starspots is present. this was unexpected as there is a well-established relationship between solar flares and sunspots. this result yields potential implications for how the magnetic field in fully convective low mass stars is generated. possibly, this result implies the surface of these stars is more complex than the sun. furthermore, groups of rapidly fast rotating low mass and solar-type stars were discovered to exhibit very little flaring activity. this is unusual, as rotation is linked to a star's dynamo mechanism and so faster rotating stars are expected to show higher levels of activity. this research has raised new questions surrounding the underpinning mechanisms driving stellar flares. in an effort to address this the solar 3d mhd simulation is scaled up to replicate flare energies seen in the observed stellar flares. this comparative analysis allows for the exploration of the flare mechanism and potential magnetic structure on these stars, which will be a subject of future research, in order to explain such high energy flares. | solar and stellar flares and their connection |
i will present new results on magnetar-like transient events in neutron stars having low dipolar fields or generally catalogued as normal radio pulsars or central compact objects. i will then present simulations of magnetic field evolution that might explain the apparently puzzling behaviour of these objects. strong surface magnetic field might be an almost ubiquitous properties of pulsars, regardless their external dipolar magnetic field measured via their spin down properties. | magnetar-like emission in different neutron star classes |
bow shock formation, magnetic reconnection and plasmoid ejection are thought to be present in most planetary environments. the aim of this work is to show that the presence of this kind of structures in the vicinity of planets is not restrictive to magnetized bodies. the presence of a interplanetary magnetic field carried with the stellar wind is responsible of the formation of a planetary magnetotail, ejection of plasmoids and magnetic reconnection events, even though the planet-obstacle is completely unmagnetized. we study the interaction of this magnetized winds coming from cool ms stars, solar analogues, with non-magnetized terrestrial planets provided of an extended earth-like exosphere thought 2.5d numerical simulations carried out with pluto. in this work, we show a preliminary study of the impact of stellar winds on the evolution and stability of earth-like atmospheres/exospheres, in order to determine the position of the bow shock and its properties, and the position of the x point of magnetic reconnection in the case of non-magnetized rocky planets, in order to determine their emission and possible detection through these structures in the stellar winds. | numerical simulation of the interaction between planetary exospheres and the stellar wind |
energy and momentum coupling between spatially separate but magnetically linked plasmas is a key aspect of space physics. previous active plasma experiments (e.g., apex north star [pfaff et al., 2004]) showed the presence of parallel electric fields that are assumed to facilitate the cross-field "skidding" of the combined release and radiation effects satellite (crres) ionospheric barium release experiments [delamere et al., 2000]. the kinet-x mission is a rocket-based barium injection into earth's sunlit ionosphere, scheduled to launch in february 2021, designed to test our understanding of kinetic-scale transport of energy and momentum. the energy and momentum input will be constrained by ground-based optical observations. using multipoint ion and electron detectors, dc and ac vector electric fields, vector fluxgate magnetometer, and a langmuir probe located along the magnetic field of two separate releases, we seek to understand: 1) how momentum transport is affected by ion kinetic-scale physics, 2) how electromagnetic energy is converted into plasma kinetic and thermal energy, 3) the interplay between fluid- and kinetic-scales, and 4) how electrons are energized during the pickup process. we present hybrid simulations (kinetic ions and fluid electrons) of the barium cloud evolution. while electrons are not directly simulated in the hybrid model, we incorporate them as test particles. in addition, we compare the test-electron interaction with kinetic alfven waves evident in the hybrid simulations with the electron response seen in self-consistent gyrofluid-kinetic electron (gke) model simulations [damiano et al., 2003; damiano et al. 2015] for similar wave and plasma parameters. these computer simulations are used to make predictions for the kinet-x mission with particular emphasis on ion heating and electron energization. | kinetic-scale energy and momentum transport experiment (kinet-x): expectations for ion and electron heating |
plasma ions heating (especially minor heavy ions preferential heating) in fast solar wind and solar corona is an open question in space physics. however, alfvén waves have been always considered as a candidate of energy source for corona heating. in this paper, by using a two-dimensional (2-d) hybrid simulation model in a low beta electron-proton-alpha plasma system, we have investigated the relationships between plasma ions heating and power spectra evolution of density and magnetic field fluctuations excited from the parametric instabilities of initial pump alfvén waves with an incoherent spectrum at different propagation angles theta_k0b0 (an oblique angle between the initial pump wave vector k0 and the background magnetic field b0). it is found that, the wave-wave coupling as well as wave-particle interaction play key roles in ions heating, and an alfvén spectrum with small propagation angle (e.g. theta_k0b0=15degree) can most effectively heat alpha particles in perpendicular direction as well as in parallel direction for both proton and alpha particle than the case of a monochromatic alfvén wave or an alfvén spectrum with larger propagation angle. | parametric instabilities and particles heating of circularly polarized alfvén waves with an incoherent spectrum: two-dimensional hybrid simulations |
plasma is the most abundant form of visible matter in the universe. plasmas make up approximately 99 percent of stellar objects observed including billions of stars and nebulae. high-energy-density (hed) physics is a rapidly growing area of physics that encompasses several disciplines including plasma-, condensed-matter-, nuclear- and astro-physics. several important hed physics topics are currently active areas of research and include efforts to understand fundamental mechanisms driving magnetized plasma dynamics and radiative properties of magnetized plasmas. experimental investigations aim to provide data needed to more accurately model details of the plasma evolution such as how currents flow within the plasma and how current symmetries affect magnetic fields. plasma properties are usually determined from diagnostics that measure magnetic field strengths, photon emission and particle radiation, to name a few.in this dissertation, investigations of the temporal evolution of magnetically driven plasmas is presented. these studies were carried at the nevada terawatt facility using the zebra pulsed-power accelerator to magnetically compresses and confine a cylindrical plasma column. important parameters needed to characterize these laboratory plasmas include magnetic field strength and orientation, as well as electron number density and temperature. to fully understand the characteristics of a plasma, accurate experimental measurement of these parameters is essential. to determine the electron temperature, the boltzmann plot method was used. simulated spectra from prismspect where compared to experimental measurements in order to measure electron number density and temperature early in the pinched plasma formation. simultaneously, mach-zehnder laser interferometry was used to provide complementary measurements of the electron number density. a novel method, streaked zeeman-induced magnetic splitting (zims) optical spectroscopy, was developed to measure time- and space-resolved magnetic field strengths in these hot, dense plasmas. this technique has also been applied to diagnose the magnetic field strengths in laser ablation z-pinch experiments (laze), wherein a pulsed power driver pinches a laser ablated plume.to effectively utilize zims, ionic plasma species were chosen such that the zeeman splitting of different fine structure doublets occurs non-uniformly with increasing magnetic field strength in the plasma. a streak camera was then used to continuously observe spectroscopically resolved differential splitting from a well-defined spatial region in the plasma, and analysis of the measurements was used to determine non-directional magnetic field strengths with increasing plasma temperature and density. in parallel, we have developed a spectral line emission modeling code for zims. the initial line shape parameters input into the code were varied to simulate emission spectra observed, and the fitting parameters were adjusted iteratively until an acceptable fit was achieved. these results were then used to infer magnetic fields strengths as a function of time and space in the magnetized plasma implosion. zims results for a number of different laze target materials will be discussed. | investigations of plasma evolution in laser ablation z-pinch experiments using time-resolved optical spectroscopy |
massive stars are amongst the rarest but also most intriguing stars. their extreme, magnetised stellar winds induce, by wind-ism interaction, famous multi-wavelengths circumstellar gas nebulae of various morphologies, spanning from large-scale wind bubbles to stellar wind bow shocks, rings and bipolar shapes. we present two- and three-dimensional magneto-hydrodynamical (mhd) simulations of the circumstellar medium of such massive stars at different phase of their evolution. particularly, we investigate the stability properties of 3d mhd bow shock nebulae around the runaway red supergiant stars irc-10414 and betelgeuse. our results show that their astrospheres are stabilised by an organised, non-parallel ambient magnetic field. these findings suggest that betelgeuse's bar is of interstellar origin. last, we explore the circular aspect of the young nebula around the wolf-rayet stars. it is found that wolf-rayet nebulae are not affected by the ism gas distribution in which the stellar objects lie, even in the case of fast stellar motion: as testifies the ring-like surroundings of the milky way's fastest wolf-rayet star, wr124. the morphology of these nebulae is tightly related to their pre-wolf-rayet wind geometry and to their phase evolution transition properties, which can generate bipolar shapes. we will further discuss their diffuse projected emission by means of radiative transfer calculations and show that the projected diffuse emission can appear as bipolar structures as in ngc6888. | magnetised gas nebulae of evolved massive stars |
the merger of two carbon-oxygen white dwarfs has long been theorized to lead to a massive carbon-oxygen or oxygen-neon white dwarf, accretion-induced collapse to a neutron star, or a type ia supernova. determining which mergers lead to a particular outcome requires hydrodynamic simulations of the merging process. i give a brief overview of the current understanding of mergers and their end-products derived from simulations, and show how temperature, rather than density or mass, most strongly determines a merging binary's subsequent evolution. i then describe recent simulations that show mergers generate strong magnetic fields that could help drive a merger remnant to ignition. | the physics and end-products of merging co wd binaries |
we present the results of numerical magnetohydrodynamic (mhd) simulations of the isothermal stage of the collapse of magnetic rotating protostellar clouds of solar mass at various initial values of rotational and magnetic energies and with various degrees of nonuniformity. the simulations are carried out using the two-dimensional mhd code enlil. the simulations show that an increase in the initial degree of inhomogeneity of the cloud leads to the growth of the sizes of the elements of its hierarchical structure — oblate envelope and primary disk. the influence of magnetic barking also increases in the center of the cloud. magnetic barking transports from 40 to 90% of the total angular momentum of the cloud into the interstellar medium, depending on the initial ratio of the cloud's magnetic energy to the modulus of its gravitational energy, ɛ m = 0.2 ‑ 0.6. the efficiency of angular momentum transport weakly depends on the initial ratio of rotational and gravitational energies. at the end of the isothermal stage of the collapse of an inhomogeneous protostellar cloud, a "dead" zone is formed with the degree of ionization x ≤ 10 ‑12 . the "dead" zone radius is 90‑220 au for clouds with ɛ m = 0.2‑0.6. a comparison of the characteristic diffusion and dynamical times shows that ambipolar diffusion will lead to weakening of magnetic braking inside the "dead" zone within 1 ‑ 10 thousand years after the first core formation. | evolution of angular momentum during the collapse of magnetic rotating protostellar clouds |
we know that when two neutron stars the dense, compact cores of evolved stars collide, they produce signals that span the electromagnetic spectrum. but could these binaries also flare before they merge, as well?a broad range of signalsartists impression of the collision and merger of two neutron stars. [nsf/ligo/sonoma state university/a. simonnet]the discovery and follow-up of the gravitational-wave event gw170817, a collision of two neutron stars, provided the first direct evidence of the many forms of light that are emitted in these mergers. between the instant of collision and the months that followed, observatories around the world recorded everything from high-energy gamma rays to late-time radio emission.but emission might not be restricted to during and after the merger! a new study conducted by two researchers from the flatiron institute, elias r. most (also of goethe university frankfurt, germany) and alexander philippov, explores the possibility that neutron star binaries may also produce flares of emission in the time leading up to their final impact.this plot of the out-of-plane magnetic field density indicates the twist in flux tubes connecting the two neutron stars seen at the center of the plot. here, an electromagnetic flare is launched from the binary after a significant twist has built up due to relative rotation of the right star. [most philippov 2020]what about magnetic fields?in particular, most and philippov focus on how the magnetospheres of the two neutron stars the magnetized environment surrounding each body interact shortly before the objects collide.the authors conduct special-relativistic force-free simulations of orbiting pairs of neutron stars in which each star is threaded with the strong dipole magnetic field expected for these bodies. the simulations then track how the stars magnetic fields evolve, twist, and interact as the bodies orbit each other.a twisted fatemost and philippov find that dramatic releases of magnetic energy are a common outcome if the neutron stars orbit close enough to one another that their magnetospheres interact.the authors show that the brightness of the flare luminosity depends only on how far apart the neutron stars are in the simulation: the smaller the separation, the brighter the flare. this dependence demonstrates that the flaring events are driven primarily by the energy stored in the twisted tube of magnetic flux that forms connecting the two neutron stars.here, the twisted flux tube and resultant flaring is caused by orbital motion of 45 misaligned magnetic fields, rather than by one star spinning. the bottom panel shows a 3d visualization of the field line configuration at the time of flaring. [most philippov 2020]when the two neutron stars spin at different speeds, the magnetic field loop that forms between the stars becomes progressively more twisted until this stored rotational energy is abruptly ejected. and even if neither neutron star is spinning, the authors show that magnetic flux twist still builds up and releases as a result of the binarys orbital motion, assuming that the magnetic fields of the two stars are not aligned.look for radio cluesso can we observe these sudden releases of energy? most and philippov argue that we should be able to spot the drama in radio emission: a radio afterglow will be produced behind the magnetized bubble thats ejected from the twisted loop, and additional radio emission can be produced when the bubble collides with surrounding plasma.future work on this topic will explore the impacts of the neutron stars inspiral, and how the interactions of the magnetospheres change when the neutron stars carry unequal charge. the current study, however, indicates its worth keeping a radio eye out to see if we can spot signs of collisions to come!citationelectromagnetic precursors to gravitational-wave events: numerical simulations of flaring in pre-merger binary neutron star magnetospheres, elias r. most and alexander a. philippov 2020 apjl 893 l6. doi:10.3847/2041-8213/ab8196 | signs of collisions to come |
in order to explain the higher spin down rate of the intermittent pulsar psr b1931+24 in the radio-on state than that in the radio-off state, and to simulate the rotation evolution of the crab pulsar, the wind braking model in the annular acceleration gap considering both core and annular region with different acceleration potentials was established. for psr b1931+24, the magnetic field strength and magnetic inclination angle are calculated, and the theoretical braking index is predicted. for the crab pulsar, it is calculated for the magnetic field strength and magnetic inclination angle, evolution of the braking index with the period, and rotation evolution on the period-period derivative diagram. compared with the vacuum acceleration gap, outer acceleration gap, etc., the annular acceleration gap can also be applied to the wind braking model of pulsars. | wind braking model of pulsars in the case of annular acceleration gap |
an important phenomenon in a pulsing agb (asymptotic giant branch) star is the variability of its circumstellar envelope. the study of stimulated radiation called sio maser (microwave amplification by stimulated emission of radiation) in the inner edge of the envelope shows fluctuations in polarization during the stellar pulsation period which is caused by the unstable magnetic field around the agb star and needs to be investigated in order to understand it. we simulate the maser signal and polarization by using a computer code to describe the circumstellar envelope of the agb star for comparison with the observations. the methods, i) generate the energy transition of the stimulation matrix ii) simulate the first stimulation from an electric field background iii) convert the radiation to the frequency domain by using fourier transform iv) simulate the observation by amplifying radiation along a line of sight within the medium toward an observation position. at this stage, we set the tubular maser cloud with a constant magnetic field strength toward the observer. the program can simulate the maser signal and polarization, brighter when stimulating with a longer medium; the simulated polarization agrees well with expectations for this magnetic field orientation. further works need to be carried out by testing this program with many cases to explain the obtained data from actual observations. | maser polarization simulation in the circumstellar envelope of an evolving star |
the removal of the ism of disk galaxies through ram pressure stripping (rps) has been extensively studied in numerous simulations. these models show that this process has a significant impact on galaxy evolution (the truncation of the ism will lead to a decrease in the star formation and the galaxy will become redder).nevertheless, the role of the magnetic fields (mfs) on the dynamics of the gas in this process has been hardly studied, although the influence of magnetic fields on the large scale disk structure is well established. the presence of mfs produce a less compressible gas, thus increasing the scale height of the gas in the galaxy, that is, gas can be found farther away from the galactic potential well, which may lead to an easier removal of gas. we test this idea by performing a 3d mhd simulation of a disk galaxy that experiences rps under the wind-tunnel approximation. | mhd simulations of ram pressure stripping of a disk galaxy |
stable magnetic fields have been observed in intermediate-mass stars in the main-sequence (ap/bp stars) and in degenerate stars (white dwarfs and neutron stars). despite the important role that the magnetic field plays in the stellar evolution, its origin, internal structure and the way it can survive timescales as long as the star's lifetime are not still completely understood.in the past years, important steps have been made in this line of research. for example, purely toroidal and poloidal magnetic fields are known to develop instabilities at some point in the star. numerical simulations have shown that random initial magnetic fields in stably stratified stars can evolve into a roughly axisymmetric stable equilibrium configuration consisting of both toroidal and poloidal components of comparable strength in a twisted-torus shape. additionally, different studies have put rough upper and lower bounds on the ratio of the magnetic energy in the toroidal and poloidal components of the magnetic field. in order to contribute to the study of the stability of magnetic field configurations, we perform 3d-mhd simulations of the evolution of magnetic field configurations with the pencil code, a high-order-finite-difference code for compressible hydrodynamics flows. we first validate the use of the pencil code, confirming that a random magnetic field evolves, in a few alfven timescales, to an ordered magnetic configuration in a stratified star while it decays away in a barotropic one. then, we test the stability of different axially symmetric magnetic field configurations by following their dynamical evolution inside the star. we have evolved linked configurations of poloidal and toroidal magnetic fields under different initial conditions (i.e. the star's stratification and magnetic field strengths) and mapped the parameter space where the configuration evolves to a stable equilibrium. | stellar magnetic equilibria with the pencil code |
stellar feedback occurring at small-scales can significantly impact the evolution of galaxies at much larger scales. for example, an appropriate feedback mechanism, including thermal and radiative components, can help regulate star formation, particularly in low-mass galaxies. while feedback models are generally prevalent in numerical simulations, the magnetic component is often neglected. however, measurements of galaxies indicate the presence of fields with a strength on the order of µg. previous studies have demonstrated the formation of these fields through the amplification of a primordial magnetic field. here, we describe a self-consistent prescription where magnetic fields are injected in supernova injections, calibrated by observations of magnetic fields in supernova remnants. these fields will then become seeds that evolve by way of mixing and turbulence to result in galactic-scale magnetic fields. as a proof of concept, we apply this method to model the supernova of a single population iii star and trace the evolution of the injected magnetic field. future studies will apply this prescription to study not only the effects of magnetic fields on galaxy formation and evolution, but also the growth of the magnetized bubbles that form in the igm. | magnetic field seeding through supernova feedback |
the traditional paradigm for magnetic field lines changing connections ignores magnetic field line chaos and requires an extremely large current density, $j_{max}\propto r_m$, flowing in thin sheets of thickness $1/r_m$, where $r_m$ is the magnetic reynolds number. the time required for a general natural evolution to take a smooth magnetic field into such a state is rarely considered. natural evolutions generally cause magnetic field lines to become chaotic. a fast change in field line connections then arises on the timescale defined by the evolution multiplied by a $\ln(r_m)$ factor, and the required maximum current density scales as $\ln(r_m)$. even when simulations support the new paradigm based on chaos, they have been interpreted as supporting the old. how this could happen is an important example for plasma physics of kuhn's statements about the acceptance of paradigm change and on popper's views on the judgment of truth in science. | judgment of paradigms for magnetic reconnection in coronal loops |
ranging from dense plasmas above nuclear saturation in their interiors to strongly magnetized pair-plasmas in their magnetospheres, neutron stars feature some of the most extreme plasmas in the universe. the delicate interplay between strong gravity, nuclear and plasma physics makes the collision of two neutron stars an ideal playground to study matter in its most extreme form. in this talk, i will present recent advances in the state-of-the-art modeling of relativistic plasmas applicable to neutron star mergers and beyond. starting from hot and dense plasmas formed during the collision of two neutron stars, i will present novel insights into how these plasmas might be obtainable in ground-based experiments. i will then discuss why dissipative effects (e.g., weak-interaction driven bulk viscosity) might be crucial for understanding the post-merger evolution of a binary neutron star coalescence. going to plasmas at densities below saturation, i will comment on the importance of magnetic fields and relativistic turbulence during the collision. furthermore, i will present a systematic study of the emission of electromagnetic (em) flares from the inspiral that can drive powerful em precursors to gravitational wave events. the next-generation modeling of relativistic plasmas in these extreme regimes requires the consistent inclusion of dissipative effects into numerical magnetohydrodynamics (mhd) simulations. to this end, i will introduce a novel 14-moment based numerical approach to dissipative relativistic mhd. this 14-moment closure can seamlessly interpolate between the highly collisional limit found in neutron star mergers and heavy-ion collisions, and the weakly coupled braginskii-like limit of extended mhd appropriate for the study of accretion disks around supermassive black holes. going beyond these collisional limits, i will also provide an outlook on how to describe the collisionless dynamics of electron-ion/positron plasmas using dissipative two-fluid mhd. | simulating extreme plasmas in neutron star mergers and beyond |
black-hole binary (bhb) mergers are expected to be powerful sources of gravitational radiation at stellar and galactic scales. a typical astrophysical environment for these mergers will involve magnetized plasmas accreting onto each hole; the strong-field gravitational dynamics of the merger may churn this plasma in ways that produce characteristic electromagnetic radiation visible to high-energy em detectors on and above the earth. here we return to a cutting-edge grmhd simulation of equal-mass bhbs in a uniform plasma, originally performed with the whisky code. our new tool is the recently released illinoisgrmhd, a compact, highly-optimized ideal grmhd code that meshes with the einstein toolkit. we establish consistency of illinoisgrmhd results with the older whisky results, and investigate the robustness of these results to changes in initial configuration of the bhb and the plasma magnetic field, and discuss the interpretation of the ``jet-like'' features seen in the poynting flux post-merger. work supported in part by nasa grant 13-atp13-0077. | robust grmhd evolutions of merging black-hole binaries in magnetized plasma |
feedback processes from massive stars plays a critical role in their formation, destroy the molecular clouds in which they are born and shape the evolution of galaxies. in this talk i will discuss our recent 3d amr simulations that are the first to include the coupled feedback effects of protostellar outflows combined with protostellar heating and radiation pressure feedback and magnetic fields, in a single computation and their effects on the infalling dusty gas in the surrounding environs of the accreting core envelope. these simulations will address the detailed effects of feedback on the formation of high mass stars and massive clusters with implications for the imf. | the coupled effects of protostellar outflows, radiation feedback, magnetic fields and turbulence on the formation of massive stars and orion-like clusters |
we review theoretical models for the formation of dense cores in molecular clouds and their subsequent collapse. fragmentation of molecular clouds is affected by many ambient conditions including magnetic fields and background turbulence. we review the outcome of a wide range of simulations and focus in on the origin of elongated (filamentary) structures in molecular clouds that often have a large scale magnetic field aligned perpendicular to their long axis. fragmentation may occur in two distinct stages, with the second stage leading to the dense cores that collapse independently to form single or small multiple systems of stars. we review some models of core collapse including disk formation and evolution, accretion history, and the formation of companions. | molecular cloud fragmentation and core collapse |
a primary goal of the open-source spectre project is to simulate the disruption of neutron stars in compact-object binary systems. as a step towards this goal, we present the evolution of a relativistic magnetized accretion disk in a fixed gravitational field. the evolution uses a hybrid scheme of a discontinuous galerkin method with embedded finite-difference regions for shock capturing. | evolution of a magnetized accretion disk using spectre |
grb 170817a provided clear evidence of an intrinsic connection between short gamma-ray bursts (sgrbs) and binary neutron star (bns) mergers. the mechanism by which the latter are able to feed the former, however, remains poorly understood, and many theoretical and phenomenological models, combining observational data with results of numerical calculations, are currently under development with the aim of unveiling it. in this talk, i will present results of numerical simulations in which we modelled, for the first time, the three-dimensional evolution of an sgrb jet propagating through the post-merger environment of a fully general-relativistic (gr) hydrodynamic bns merger simulation, reaching space-time scales relevant for the gamma-ray emission, and providing self-consistent predictions for the jet's final structure and energetics. i will also discuss the ongoing work concerning the inclusion of magnetic fields in our jet simulations, directly importing magnetized initial data from gr magneto-hydrodynamic bns merger simulations. | simulating short gamma-ray burst jets with genuine surrounding environments |
the intrinsic magnetic field of a terrestrial planet is considered to be an important factor for the evolution of terrestrial atmospheres. this is in particular relevant for early stages of the solar system, in which the solar wind as well as the euv flux from the young sun were significantly stronger than at present-day. we therefore will present simulations of the paleo-magnetospheres of ancient earth and mars, which were performed for ∼4.1 billion years ago, i.e. the earth's late hadean eon and mars' early noachian. these simulations were performed with specifically adapted versions of the paraboloid magnetospheric model (pmm) of the skobeltsyn institute of nuclear physics of the moscow state university, which serves as iso-standard for the earth's magnetic field (see e.g. alexeev et al., 2003). one of the input parameters into our model is the ancient solar wind pressure. this is derived from a newly developed solar/stellar wind evolution model, which is strongly dependent on the initial rotation rate of the early sun (johnstone et al., 2015). another input parameter is the ancient magnetic dipole field. in case of earth this is derived from measurements of the paleomagnetic field strength by tarduno et al., 2015. these data from zircons are varying between 0.12 and 1.0 of today's magnetic field strength. for mars the ancient magnetic field is derived from the remanent magnetization in the martian crust as measured by the mars global surveyor mag/er experiment. these data together with dynamo theory are indicating an ancient martian dipole field strength in the range of 0.1 to 1.0 of the present-day terrestrial dipole field. for the earth our simulations show that the paleo-magnetosphere during the late hadean eon was significantly smaller than today, with a standoff-distance rs ranging from ∼3.4 to 8 re, depending on the input parameters. these results also have implications for the early terrestrial atmosphere. due to the significantly higher euv flux, the exobase of a nitrogen dominated atmosphere would most probably have been extended above the magnetopause, leading to enhanced atmospheric erosion, whereas a co2-dominated atmosphere would have prevented atmospheric loss in such a scenario. our simulations also show that the martian paleo-magnetosphere during the early noachian must have been comparable in size to the terrestrial paleo-magnetosphere, hence a co2-rich atmosphere should have been protected by the magnetic field from rapid atmospheric erosion until the cessation of the martian dipole field ∼4.0 billion years ago. finally, our results favor the idea that the young sun must have been a slow to moderate rotator. the solar wind and euv flux from a fast rotating sun would have been so intense, that most probably the ancient atmospheres of mars and earth would not have survived. acknowledgments. the authors acknowledge the support of the fwf nfn project "pathways to habitability: from disks to active stars, planets and life", in particular its related sub-projects s11604-n16, s11606-n16 and s11607-n16. this presentation is supported by the austrian science fund (fwf) and the us nsf (ear1015269 to jat). | on the paleo-magnetospheres of earth and mars |
this proposal is the continuation of a previous 3-year project that focused on modeling the nonthermal emission from magnetars and pulsars and testing the models against new observations, in particular by nustar. the proposed project develops in two directions: (1) first-principle simulations of the magnetospheric electron-positron discharge using our code aperture (based on the particle-in-cell method), which is specifically designed for this purpose. its performance is demonstrated by the first application to rotation-powered pulsars, and it can significantly advance our understanding of the magnetospheric activity of magnetars and pulsars. our simulations involve a detailed implementation of radiative processes, tracking the emission and propagation of gammarays and production of electron-positron pairs. the results will provide new theoretical foundation for interpreting emission from the twisted magnetospheres of neutron stars. they will clarify, in particular, the radiative mechanism of magnetar bursts and persistent emission. (2) investigation of magnetic field evolution inside neutron stars, which is ultimately responsible for driving the magnetospheric activity of magnetars and their surface heating. our recent results suggest two novel phenomena in the solid crust of an active magnetar: thermoplastic waves and hall-mediated avalanches. we propose to investigate scenarios for the global magnetic field evolution in the core and the crust, and its observables including (a) twisting of the external magnetosphere and the resulting nonthermal activity, (b) subsurface heating, and (c) sudden changes of the rotation rate. we will use our models and the rich accumulated data to disentangle the key dynamic processes inside magnetars. this analysis can constrain the magnetic fields hidden inside magnetars, the state of their core matter and its possible superfluidity. | activity of strongly magnetized neutron stars |
we present new fully general relativistic magnetohydrodynamic (grmhd) simulations of the merger of high-mass binary neutron star (bns) systems. we considered bnss that produce an hypermassive neutron star that promptly collapses to a spinning black hole (bh) surrounded by a magnetized accretion disk. we investigated whether such systems may launch relativistic jets and hence power short gamma-ray bursts. we considered the effects of different equations of state, different mass ratios, and different magnetic field configurations. for all cases we present a detailed study of the matter dynamics and of the magnetic field evolution, with particular attention to its global structure and possible emission of relativistic jets. | high-mass magnetized binary neutron star mergers and short gamma-ray bursts |
observations of lunar regolith composition have revealed the need for a source that differs in the abundance of light volatile elements (like h, he, c, n, and some noble gases) from the solar wind. mass supply from earth's atmosphere has been previously proposed as one possibility if the earth incurred phases of vanishing geomagnetic field on geological time scales that might increase the ratio of earth's atmosphere mass flux to solar wind flux that the moon encounters [ozima et al., 2005]. here we develop a framework for studying the relative escape from unmagnetized and magnetized evolution of a planetary atmosphere subject to the impinging stellar wind that combines 3-d mhd numerical simulations using the adaptive mesh refinement (amr) multi-physics code, astrobear*, with theoretical analyses. we specifically determine the orbit-averaged earth wind mass flux at the lunar surface for a range of plausible solar wind conditions to which the current geo-magnetosphere and paleoatmosphere (induced magnetosphere) may have been subject. using this, we quantify both the role the intrinsic geomagnetic field plays on the total escaping earth's atmosphere flux encountered by the moon and the relative fraction of upper versus lower atmosphere flux transported. we find that neither effect considerably changes the relative abundances of solar wind and earth atmosphere flux to the lunar surface. the earth's atmosphere flux remains small in both cases because the moon spends only a small fraction of its orbit within a cross-section that provides direct access to earth's atmosphere material. our model does not track specific chemical species and their energy states, primary factors controlling implantation into the regolith, and makes predictions only for the relative wind and atmospheric flux aggregate compositions rather than any particular element. we do not address the influence of the magnetic field in determining the composition distribution of specific elements from earth that arrive at the moon. similarly, our atmospheric model does not include detailed transport processes for individual ions. *http://astrobear.pas.rochester.edu/ references ozima, m., seki, k., terada, n. et al. terrestrial nitrogen and noble gases in lunar soils. nature 436, 655–659 (2005).https://doi.org/10.1038/nature03929 | investigating the influence of an unmagnetized versus magnetized earth on the relative solar wind and earth atmosphere contributions to lunar soil |
magnetized stellar plasma winds play a critical role in defining and shaping planetary magnetospheres. the dynamics of star planet interactions are different for planets with and without intrinsic large-scale magnetic fields. magnetohydrodynamic models provide the physical basis for understanding such interactions on the one hand. on the other hand, observations of such interactions in planets such as mars provide constraints to these simulations. we shall present results of the interactions of magnetized winds with a mars-like planet, with and without magnetospheres, and with varying martian magnetospheric field strength. we shall discuss the implications of these simulations for understanding how the evolution of the martian dynamo may have resulted in the current state of the martian atmosphere. | solar-stellar winds and the dynamics of star-planet interactions in mars-like planets |
the age-dependent activity of a star dictates the extent of its planetary impact. we study the interaction of the stellar wind produced by solar-like stars with the magnetosphere of earth-like planets using three dimensional (3d) magnetohydrodynamic (mhd) simulations. the numerical simulations reveal important features of star-planet interaction e.g. bow-shock, magnetopause, magnetotail, etc. interesting phenomena such as particle injection into the planetary atmosphere as well as atmospheric mass loss are also observed which are instrumental in determining the atmospheric retention by the planet. | the activity evolution of solar-like stars with age and its planetary impact |
the pipe nebula is a nearby filamentary shaped molecular cloud in which star formation is concentrated in the western end of the cloud known as b59, while the eastern part has no star formation. the morphology of the projected magnetic fields is well constrained through both extinction and emission dust polarization measurements, showing field lines perpendicular to the main axis of the molecular cloud. yet, field lines are much less dispersed in the non-star-forming part of the cloud compared to a less ordered b59. furthermore, kinematic studies of the eastern part show two velocity components, 3.5 km/s apart in projected velocity, which may be indicative of colliding flows. the eastern end of the pipe nebula thus constitutes a highly turbulent, strongly magnetized test case to be compared with numerical simulations. we present the first $^{12}co$(1-0), sub-arcmin resolution (32'' hpbw) large-scale (0.5x0.7 deg) map of the converging flow region, obtained with the iram-30m antenna. the result confirms the extremely dynamic nature of the region. the $^{12}co$(1-0), emission line covers 4.5 km/s with up to five velocity components connected in physical and velocity spaces. we performed longer integration towards dense cores identified based on dust extinction and emission, in $^{12}co$(1-0), (2-1), and isotopologues, as well as $hco^+$, $n_2h^+$, hcn, and cn. the non-detection rate of these species suggest that these targets are unlikely to be dense cores. this is also consistent with low visual extinctions based on co column density estimates. this suggests some limitation of automatic core detection methods based on dust in highly turbulent clouds, with consequences on the core mass function in such environments. the large area mapped in $^{12}co$ also offers the opportunity to explore the relative orientation of gaseous filaments with respect to the (projected) magnetic field lines. | challenging the dense core identification in a strongly dynamical, magnetized and turbulent region of the pipe nebula. |
the merger of two carbon-oxygen white dwarfs (co wds) can either create a more massive wd, lead to collapse into a neutron star, or explode spectacularly as a thermonuclear, or type ia, supernova (sn ia). it has traditionally been believed that sne ia result only from mergers at or above the chandrasekhar mass (mch), as these can subsequently become dense enough to trigger runaway fusion. recently, however, it has been proposed that the merged product, or "remnant", might instead subsequently ignite fusion from high temperatures. this opens the possibility of sne ia arising even from sub-mch mergers. to investigate this, i conducted a series of hydrodynamic simulations of the merging process. i first performed simulations spanning the range of possible mergers using the smoothed-particle hydrodynamics code gasoline, finding that remnant configurations are roughly homologous for mergers of wds with the same difference in mass deltam. in particular, "similar-mass" mergers with deltam < 0.1 msolar generate remnants that are heated throughout their dense cores, making them candidates for subsequent explosion. these results are challenged by my simulations of a 0.625-0.65 msolar merger using the moving-mesh code arepo. unlike in gasoline, the merger remnant in arepo not only has a relatively cold core, but one that is crescent-shaped and launches a one-armed spiral wave into its surroundings. i also insert weak magnetic fields into the wds in arepo, and find exponential field growth during their merger, leading to a > 10^10 g field within the remnant. further study is required to understand how these novel features alter post-merger evolution. lastly, i calculate the evolution of idealized co wds experiencing runaway nuclear burning in their centers, which ends either with an explosion or expansion into a carbon-burning star. i determine the minimum mass for an explosion to be mcrit ≈ 1.15 msolar, which can be reached by the dense cores of some sub-mch merger remnants. these remnants, however, are likely too underdense to explode, leaving only mergers with masses > mch that can. | illuminating mergers of carbon-oxygen white dwarfs and their possible link to thermonuclear supernovae |
we present recent improvements in general relativistic hydrodynamics simulations with the open source numerical relativity code spectre. major updates include support for tabulated equations of states, implementation of higher-order and positivity-preserving adaptive order finite difference schemes in our discontinuous galerkin-finite difference (dg-fd) hybrid solver, and integrating the dg-fd hybrid solver with spacetime evolution for fully general relativistic simulations. as a benchmark, we show a few recent tests including long-term (>100ms) stable simulations of isolated neutron star with strong magnetic field. this work was supported in part by the sherman fairchild foundation and by nsf grants no. phy-2011961, no. phy-2011968, and no. oac-1931266 at caltech, and nsf grants no. phy- 1912081 and no. oac-1931280 at cornell. | progress in grmhd simulations with spectre |
recent advances in numerical techniques and computational power have allowed us to simulate the pulsar magnetosphere from first principles using particle-in-cell techniques. these ab-initio simulations seem to indicate that pair creation through photon-photon collision at the light cylinder is required to sustain the pulsar engine. however for many rotation-powered pulsars, pair creation operates effectively only near the stellar surface where magnetic field is high. without efficient photon-photon pair conversion, how these ''weak pulsars'' fill their magnetospheres and produce radio emission is still an open question. by pushing towards a parameter regime that was not studied in detail before, we discovered a range of self-consistent solutions to the pulsar magnetosphere that do not require pair production near the light cylinder. depending on the electron-positron pairs produced, the pulsar transitions from a near-death state with little spin-down, through an highly time-dependent state where current is intermittent, to finally approaching a near force-free state with stable spin-down. we show the time evolution of all these different states, and attempt to compare these to the actual pulsar behaviors that we observe. | numerical experiments on magnetospheres of weak pulsars |
although a triangular vortex lattice is stable in a bulk type-ii superconductor, exotic vortex configurations are expected to appear in a small superconducting plate. theoretical calculations on vortex structures in a star-shaped superconducting plate have been given in our preceding work. in this work, we extended our theoretical studies to the case of having an artificial pin. we performed the ginzburg-landau (gl) calculations systematically to compare with the pin-free case by using the finite element method. we found that a vortex tends to accommodate preferentially in an aritificial pin in the star-shaped plate. we found a systematic evolution of vortex structure with increaseing magnetic field. we compare our theoretical calculations with vortices in a star-shaped mo80ge20 plate with an artificial pin and without an artificial pin obtained by a scanning squid microscope. we reconstructed the vortex image on the sample surface by using the inverse biot-savart law and the fourier transformation. | simulations of vortices in a star-shaped plate with an artificial pin |
magnetohydrodynamic turbulence is central to laboratory and astrophysical plasmas, and is invoked for interpreting many observed scalings. verifying predicted scaling law behaviour requires extreme-resolution direct numerical simulations (dns), with needed computing resources excluding systematic parameter surveys. we here present an analytic generator of realistically looking turbulent magnetic fields, that computes 3d ${\cal{o}}(1000^3)$ solenoidal vector fields in minutes to hours on desktops. our model is inspired by recent developments in 3d incompressible fluid turbulence theory, where a gaussian white noise vector subjected to a non-linear transformation results in an intermittent, multifractal random field. our $b\times c$ model has only few parameters that have clear geometric interpretations. we directly compare a (costly) dns with a swiftly $b\times c$-generated realization, in terms of its (i) characteristic sheet-like structures of current density, (ii) volume-filling aspects across current intensity, (iii) power-spectral behaviour, (iv) probability distribution functions of increments for magnetic field and current density, structure functions, spectra of exponents, and (v) partial variance of increments. the model even allows to mimic time-evolving magnetic and current density distributions and can be used for synthetic observations on 3d turbulent data cubes. | bxc: a swift generator for 3d magnetohydrodynamic turbulence |
buoyantly driven convection under the strong influence of rotation is a ubiquitous feature of geophysical and astrophysical fluid flows. it is known to be a principal ingredient in the generation of observable large-scale shear flows and magnetic fields within planetary and stellar interiors. the constraint of geostrophy, the dominant force balance between the pressure gradient force and the coriolis force, is known to be prevalent and responsible for introducing strong spatial anisotropy that greatly impacts the global circulations, and the energy and momentum transport properties of a rotating fluid. quasi-geostrophy, the flow evolution of a rotating fluid under geostrophic constraint, has been a well-developed staple of geophysical fluid dynamics for stably-stratified atmospheric and oceanic flows. the case for qg convectively unstably environments is a far more open research branch. in this talk, i will describe progress in understanding rotating convection through the development of asymptotically reduced models. the canonical paradigm of rapidly rotating rayleigh-benard thermal convection will be a specific focus. i will discuss how synergistic efforts asymptotic theory, laboratory and numerical simulations are advancing our understanding of rotating convective flows that include ever greater geophysical and astrophysical complexity. | rotationally constrained convection in extreme parameter regimes |
m dwarf habitable zones move inward with time over the star's extended pre-main sequence phase such that rocky planets that are currently orbiting in an m dwarf's habitable zone may have once orbited interior to it. before the habitable zone contracts to envelop the planet, the planet will likely be in a runaway greenhouse phase, losing water from its atmosphere while outgassing carbon dioxide to its atmosphere. once the habitable zone contracts to the planet's orbit, any water outgassed from the interior may condense to form liquid oceans, if the surface temperature and pressure allow liquid water to be stable. however, if the planet has outgassed enough carbon dioxide during its runaway greenhouse phase, surface temperatures will be too high for outgassed water to condense and the planet will not be habitable. to calculate the rate of carbon dioxide outgassing before the habitable zone reaches the planet, we add a geochemical model to the vplanet software package that self-consistently tracks water and carbon dioxide flows across a planet's mantle, crust, and atmosphere for a stagnant lid tectonic mode. our model simulates the interior thermal evolution of the planet (including the core) to calculate outgassing rates from magma production rates over time. we also simulate the evolution of the host star to calculate the rate of atmospheric escape of water during its runaway greenhouse phase. we validate our model by reproducing the 92 bars of carbon dioxide and 30 ppm of water vapor observed in venus' atmosphere today, as well as its lack of a magnetic field. we then apply this validated model to calculate the coupled atmosphere-interior evolution of the potentially habitable trappist-1 planets, assuming they possess stagnant lids and earth-like compositions. we identify the parameter space in which our simulated planets become habitable after their host star's pre-main sequence phase. we show that the habitability of a planet currently orbiting in the habitable zone around an m dwarf depends strongly on the initial carbon dioxide budget and the fraction of magma that erupts to the surface (extrusive volcanism) on that planet. while planets in the habitable zone with low initial carbon dioxide budgets or low fractions of extrusive volcanism can support liquid water, planets that have outgassed too much carbon dioxide during their runaway greenhouse phase will, despite being in the habitable zone, become venus-like worlds with thick carbon dioxide atmospheres and no liquid water. we investigate trappist-1e's potential habitability and demonstrate that, assuming an earth-like composition, it requires a carbon dioxide budget on the order of bars or an extrusive volcanism fraction of 0.001 to become habitable after the pre-main sequence phase. our model shows that trappist-1e's potential for habitability is severely limited unless it is either volatile poor or erupts very little magma to its surface. | carbon dioxide outgassing constrains the habitability of rocky planets after their host m dwarf's pre-main sequence phase |
magnetic fields play an essential role in the star formation process. however, due to challenges in directly observing magnetic fields, the morphology and evolution of magnetic fields within young cores are not well constrained. in order to discriminate between different star formation models, we have analyzed magnetohydrodynamic simulations of low-mass protostellar cores with outflows. using yt, a visualization tool, we have made projections in a number of different orientations that show the properties of the protostellar cores such as magnetic fields, outflows, and density, with the goal of comparing them with high-resolution, millimeter-wave dust polarization maps. simulations at resolutions and scales beyond those achievable in observations will provide insight into how magnetic fields change as a function of both time and spatial scale. future work will involve using a radiative transfer module from the artist package to make synthetic observations and compare them with carma, sma, and future alma data. | using synthetic observations to constrain the properties of magnetic fields in protostellar cores |
cosmic rays (crs) play an important role in galaxy formation and evolution: in the interstellar medium, their energy density is roughly in equipartition with magnetic and thermal energy densities of the gas, and their production may contribute to feedback regulating star formation. crs primarily exchange energy and momentum with the gas collisionlessly, through small-scale fluctuations in the magnetic field. these fluctuations may be either generated by pre-existing turbulence, or by the crs themselves through the streaming instability. in this work, we extend earlier models of winds driven by the cr streaming instability to include more realistic thermodynamics by incorporating cooling of the gas that can balance the alfvenic heating due to the crs. we use time-dependent spherically symmetric simulations to study the features of the winds, and compare our results with simplified steady-state analytic models. | realistic thermodynamics for cosmic-ray-driven galactic winds |
in strongly-magnetized neutron star crusts (b <= 1014 g), electrons are quantized into a moderate number of landau levels. this can dramatically change the thermodynamic (specific heat capacity, magnetization, differential magnetic susceptibility) and transport properties (electrical and thermal conductivity) of the crust. in a neutron star crust, magnetic field evolution is best described using the formalism of hall magnetohydrodynamics (mhd). we include for the first time the effects of landau quantization on both thermodynamic and transport properties of neutron star crusts in hall magnetothermal evolution simulations. comparing simulations including landau-quantization effects those excluding these effects, we assess their importance to the overall magneto-thermal history of neutron stars, in particular their impact on ohmic heating and evolution of the magnetic field topology. this research was supported by the int's u.s. department of energy grant no. de-fg02-00er41132. | numerical simulations of hall magnetohydrodynamics in neutron star crusts with landau-quantized electrons |
we examine the long-term evolution of accretion tori around the black hole remnants of compact object mergers involving at least one neutron star, to better understand the role of secular outflows in the creation of kilonovae and the synthesis of r-process elements. we modify the flash code to evolve magnetohydrodynamics in non-uniform 3d spherical coordinates, enabling efficient evolution of magnetic fields over large simulation domains. gravity is implemented as a pseudo-newtonian potential. we include neutrino evolution via an improved lightbulb/leakage scheme and take into account nuclear recombination of α-particles in the equation of state. with this new framework, we evolve post-merger systems of tori around black holes and examine the outflows. we find results broadly consistent with general relativistic simulations. magnetically driven outflows unbind a significant fraction of torus mass over a few seconds, with velocities ~ 0 . 1 c and average electron fractions favouring lanthanide-rich ejecta. ejected torus mass is negatively correlated with system compactness. the fraction of mass with ye > 0 . 25 is insufficient to explain the blue kilonova of gw170817 based on current kilonovae models. we acknowledge support from nserc of canada and from the university of alberta. | long term 3d-mhd simulations of neutron star merger accretion tori with realistic microphysics |
most studies of the 3d density structure of prestellar cores have used isotropic evolution models, which are projected into 2d for direct comparison with observation data. in this study, we introduced the anisotropic inverse abel transform method for reconstructing the 3d volume density profile of prestellar cores. we demonstrated that this approach could accurately preserve information about the realistic 3d structure with numerical simulation. this method has great advantages compared to the shape-assumption methods, particularly in high-density regions. we applied this method to the herschel 2d column density maps of three prestellar cores, and the results showed good agreement with theoretical models. by changing the direction of the inversion axis of symmetry, we confirmed that the density profiles of prestellar cores exhibit nearly cylindrical symmetry. | reconstructing the volume density profile of prestellar cores with the anisotropic inverse abel transform method |
the hubble tension between early universe and local measurements of h0 can be resolved by a brief episode of dark energy contributing about 10% of cosmic energy density at redshift z ~ 3500. new n-body simulations have shown that this early dark energy scenario predicts earlier structure formation, e.g. 50% more clusters than λcdm at redshift z ~ 1. galaxies were long thought to start as disks, but hst images show that most galaxies instead start prolate (pickle shaped). galaxy simulations can explain this as a consequence of the filamentary nature of the λcdm dark matter distribution. but comparisons between simulations and observations using novel machine learning methods reveal other potential challenges. earth may be a radioactively goldilocks planet, with just the right amount of radiogenic heating by th and u for plate tectonics and a magnetic field both of which may be necessary for the evolution of complex life. production of these elements in rare neutron star mergers implies incomplete mixing. a factor of 2 increase would have stopped earth's magnetic dynamo for hundreds of millions of years and also caused widespread vulcanism. a factor of 2 decrease could have stopped plate tectonics | julius edgar lilienfeld prize (2020): new challenges in cosmology, galaxy formation, and planets |
this review describes realistic evolution of magnetic field and rotation of the protostars, dynamics of outflows and jets, and the formation and evolution of protoplanetary disks. recent advances in the protostellar collapse simulations cover a huge dynamic range from molecular cloud core density to stellar density in a self-consistent manner and account for all the non-ideal magnetohydrodynamical effects, such as ohmic resistivity, ambipolar diffusion, and hall current. we explain the emergence of the first core, i.e., the quasi-hydrostatic object that consists of molecular gas, and the second core, i.e., the protostar. ohmic dissipation largely removes the magnetic flux from the center of a collapsing cloud core. a fast well-collimated bipolar jet along the rotation axis of the protostar is driven after the magnetic field is re-coupled with warm gas (∼103 k) around the protostar. the circumstellar disk is born in the "dead zone", a region that is de-coupled from the magnetic field, and the outer radius of the disk increases with that of the dead zone during the early accretion phase. the rapid increase of the disk size occurs after the depletion of the envelope of molecular cloud core. the effect of hall current may create two distinct populations of protoplanetary disks. | low-mass star formation: from molecular cloud cores to protostars and protoplanetary disks |
the first billion years of the universe are marked by a major transition: the epoch of reionisation. both the reionisation of the intergalactic medium and the regulation of galaxy growth are ruled by feedback mechanisms, but the nature and the interplay of these feedback processes remain undetermined. cosmic rays (crs) are ubiquitous and provide a pressure support similar to that of gas, turbulence and magnetic fields, and therefore could have played a significant role in galaxy evolution. in this work, we investigate how cosmic rays regulate the growth of high-redshift galaxies, and if they play a role in the reionisation process by affecting the escape of hydrogen ionising photons from galaxies. for this purpose, we perform and analyse two radiation-magneto-hydrodynamics \sphinx{} cosmological simulations, with and without cr feedback. we find that cr feedback contributes to regulate star formation, but strongly delays the reionisation of the intergalactic medium. by smoothing the galactic gas distribution, cr feedback prevents ionising photons from escaping the interstellar medium, at any galaxy mass. in order to reconcile cosmic rays with the reionisation of the universe, better understanding and constraining stellar feedback is required. | the impact of cosmic ray feedback on the reionisation of the universe |
simulations of the earth's magnetosphere obstacle, including the shape of the auroral oval and related field lines for early stages of the solar system are of particular importance for studying the evolution and mass loss of the earth's atmosphere. within this presentation, we will present simulations of the terrestrial paleo-magnetosphere of the earth for the late hadean, i.e. for ∼4.1 billion years ago. these were performed with an adapted version of the paraboloid magnetospheric model (pmm) of the skobeltsyn institute for nuclear physics of the moscow state university, which serves as an iso standard for the earth's magnetosphere (see e.g. alexeev et al., 2003). as an input parameter, the new measurements of the paleomagnetic field strength by tarduno et al., 2015, are taken. these data from zircons between 3.3 billion and 4.2 billion years old vary between 1.0 and 0.12 of today's equatorial field strength. available data at ∼4.1 billion years ago are among the lowest field strength values. another input into the adapted pmm is the solar wind pressure, which was derived from a newly developed solar/stellar wind evolution model (johnston et al., 2015a, b), which is strongly dependent on the rotation rate of the early sun. our simulations of the terrestrial paleo-magnetosphere with the adapted pmm show that for the most extreme case of a fast rotating sun and a paleomagnetic field strength with 0.12 of today's value, the stand-off distance of the magnetopause rs shrinks down from today's 10 re to 3.43 re. even for a slow rotating sun rs would be at only 4.27 re. taking the same magnetic field strength as that of today and a slow rotating sun leads to an rs of 8.23 re, which would be the least extreme case for the terrestrial atmosphere. another outcome of the modelling is that the auroral oval was significantly broader ∼4.1 billion years ago than today. as demonstrated by our calculations a good approach of the relationship between auroral oval size θpc (θpc as oval co-latitude) and magnetospheric subsolar distance rs is sin2θpc = re/rs. acknowledgments. the authors acknowledge the support of the fwf nfn project s116-n16 "pathways to habitability", in particular the related subproject s11606-n16 "magnetospheric electrodynamics of exoplanets". this publication is supported by the austrian science fund (fwf). references: alexeev, i. i., belenkaya, e. s., bobrovnikov, s. y., and kalegaev, v. v. (2003), modelling of the electromagnetic field in the interplanetary space and in the earth's magnetosphere, space sci. rev., 107, 7-26. johnstone, c.p., güdel, m., lüftinger, t., toth, g., and brott, i. (2015), stellar winds on the main-sequence: i. wind model, astron. astrophys., 577, id.a28. johnstone, c.p., güdel, m., brott, i., and lüftinger, t. (2015), stellar winds on the main-sequence: ii. the evolution of rotation and winds, astron. astrophys., 577, id.a27. tarduno, j. a., cottrell, r. d., davis, w. j., nimmo, f., and bono, r. k. (2015), a hadean to paleoarchean geodynamo recorded by single zircon crystals, science, 349, 521-524. | simulation of the earth's paleo-magnetosphere for the late hadean eon |
magnetic fields have only recently been included in theoretical simulations of high-mass star formation. the simulations show that magnetic fields can play a crucial role not only in the formation and dynamics of molecular outflows, but also in the evolution of circumstellar disks. therefore, new measurements of magnetic fields at milliarcsecond resolution close to massive young stellar objects (ysos) are fundamental for providing new input for numerical simulations and for understanding the formation process of massive stars. the polarized emission of 6.7 ghz ch3oh masers allows us to investigate the magnetic field close to the massive yso where the outflows and disks are formed. recently, we have detected with the evn ch3oh maser polarized emission towards 10 massive ysos. from a first statistical analysis we have found evidence that magnetic fields are primarily oriented along the molecular outflows. to improve our statistics we are carrying on a large observational evn campaign for a total of 19 sources, the preliminary results of the first seven sources are presented in this contribution. furthermore, we also describe our efforts to estimate the lande' g-factors of the ch3oh maser transition to determine the magnetic field strength from our zeeman-splitting measurements. | magnetic field measurements at milliarcsecond resolution around massive young stellar objects |
the first stars in the universe, known as population iii stars, formed just a few hundred million years after the big bang. while researchers expect that most population iii stars led bright, brief lives, those with masses less than 0.8 solar mass would still be shining faintly today. can magnetic fields in the early universe explain why weve yet to find these stars?producing population iii starsan infrared image of the orion star-forming region with magnetic field lines measured by the stratospheric observatory for infrared astronomy traced on top. [nasa/sofia/d. chuss, et al., and european southern observatory/m.mccaughrean, et al.]there are many possible explanations for why low-mass population iii stars are so elusive, but the simplest explanation might be that they never existed at all. its challenging to prove that something doesnt exist in the universe, but simulations give us a way to explore whether low-mass population iii stars could have formed in the conditions that existed when the first stars were born.previous research suggests that the early universe was suffused with a subtle magnetic field that was about a trillion times weaker than the fields measured in typical star-forming regions in the universe today. magnetic fields play an important role in shaping and sometimes suppressing star formation in the local universe, leading researchers to wonder if the weak magnetic fields in the early universe could have halted the formation of low-mass stars entirely.simulation results for the unmagnetized case (top row) and the case in which the magnetic field strength was 10-20 gauss (middle and bottom row). the top and middle rows show how the density of the gas varied between the two simulations at 0, 10, and 1,000 years of simulation time. the bottom row shows how the magnetic field strength was amplified and evolved over time in the magnetized case. click for high-resolution version. [hirano machida 2022]magnetic magnificationshingo hirano (university of tokyo and kyushu university, japan) and masahiro machida (kyushu university, japan) modeled the collapse of a cloud of gas under the conditions present in the early universe to understand how magnetic fields might have influenced the formation of the first stars.the team performed magnetohydrodynamic simulations of a gas cloud with weak initial magnetic field strengths of 0, 10-20, 10-15, and 10-10 gauss. in the unmagnetized case, the cloud fragmented into a massive (~200 solar masses) central protostar and a handful of smaller protostars, all of which persisted until the end of the simulation at 1,000 years.in the magnetized cases, on the other hand, the rapid rotation of the massive young star forming at the center of the cloud wound the magnetic field tighter and tighter, boosting the magnetic field strength up to 1,000 gauss. the strong magnetic field prevented the cloud from fragmenting further, and the few small protostars that started to form dissipated entirely, leaving only the massive protostar at the end of the simulation. this result suggests that even a weak magnetic field can be amplified enough to stop small protostars from forming, meaning that the first stars may have all formed alone.the number of protostellar fragments formed (top) and the overall mass of protostars formed (bottom) in all simulations. [adapted from hirano machida 2022]a turbulent twisthirano and machida caution that while their simulations suggest that the magnetic fields in the early universe can prevent the formation of low-mass stars, other factors may influence the formation of the first stars; if the star-forming gas is turbulent, for example, it might be more inclined to fragment into multiple stars. similarly, the slow process of diffusion could prevent the magnetic field from growing strong enough to play an important role.in future work, the authors plan to introduce turbulence into their simulations, test the effects of different rotation rates of star-forming clouds, and extend the simulation out to 100,000 years. in the meantime, the search for population iii stars continues!citationexponentially amplified magnetic field eliminates disk fragmentation around population iii protostars, shingo hirano and masahiro n. machida 2022 apjl 935 l16. doi:10.3847/2041-8213/ac85e0the post simulations suggest magnetic fields made the first stars form solo appeared first on aas nova. | simulations suggest magnetic fields made the first stars form solo |
durring their formation, most young stars are surrounded by a protoplanetary disc. the angular momentum evolution of these system is quite complex but still poorly understood despite a lot of effort and some recent breakthrough. observations indicate that stars with a disc tend to rotate more slowly even though they accrete angular momentum, and that during the first 10myr, young low-mass stars do not seem to spin-up while they are expected to contract. to tackle this long-standing problem, we present state-of-the-art stellar evolution models with accretion that include a self-consistent treatment of angular momentum thanks to the results of dynamical multi-d mhd simulations. we explore the observed range of several parameter, such as the accretion rate history, the composition and the thermodynamics of the accreted material as well as the large scale magnetic field strength of the star. we show that the observed spin rate distribution of very young stars can be explained by the complex interplay of the different processes. | stars and their disc - a short but complex story |
significant advances have been made over the past decade in the characterization of multiple protostar systems, enabled by the karl g. jansky very large array (vla), high-resolution infrared observations with the hubble space telescope, and ground-based facilities. to further understand the mechanism(s) of multiple star formation, a combination of statistics, high-angular resolution radio/millimeter continuum imaging, characterization of kinematic structure, magnetic fields via polarimetry, and comparison with numerical simulations are needed. thus, understanding the origin of stellar multiplicity in different regimes of companion separation will soon be within reach. however, to overcome challenges that studies in this field are now confronted with, a range of new capabilities are required: a new millimeter/centimeter wave facility with 10 mas resolution at {\lambda}=1 cm, space-based near to far-infrared observatories, continued development of low to high resolution spectroscopy on 3m to 10m class telescopes, and an elt-class telescope with near to mid-infrared imaging/spectroscopic capability. | astro2020 science white paper: the formation and evolution of multiple star systems |
we present a model for the seeding and evolution of magnetic fields in galaxies by supernovae (sn). sn explosions during galaxy assembly provide seed fields, which are subsequently amplified by compression, shear flows and random motions. our model explains the origin of μg magnetic fields within galactic structures. we implement our model in the mhd version of the cosmological simulation code gadget-3 and couple it with a multi-phase description of the interstellar medium. we perform simulations of milky way-like galactic halo formation and analyze the distribution and strength of the magnetic field. we investigate the intrinsic rotation measure (rm) evolution and find rm values exceeding 1000 rad/m2 at high redshifts and rm values around 10 rad/m2 at present-day. we compare our simulations to a limited set of observational data points and find encouraging similarities. in our model, galactic magnetic fields are a natural consequence of the very basic processes of star formation and galaxy assembly. | a supernova scenario for magnetic fields and rotation measures in galaxies |
we have performed numerical magnetohydrodynamic (mhd) simulations of two closed active regions (ar). the input magnetic field values were the coronal magnetic field computed as extrapolation coronal from observations of the photospheric magnetic field. the studied active regions, noaa ar12565 and ar12567, were registered as different bipolar region. our investigation, the 3d coronal extrapolations, as well as the numerical mhd experiments, revealed that actually they evolved together as a quadrupolar active region. the second region emerged later under the loops system of ar12565 and separated from this one. a natural current sheet formed between then and it plays an important role in the explosive events (flares and coronal mass ejections) occurrence. | numerical simulations of the evolution of solar active regions: the complex ar12565 and ar12567 |
recent high-cadence transient surveys and rapid follow-up observations have revealed that some massive stars dynamically lose their own mass within decades before supernovae (sne). such a mass-loss forms confined circumstellar medium (csm); a high density material distribution only in small radius (≲ 1015 cm with the mass-loss rate of 0.01 &sim 10-4 m_⊙yr-1). while the sn shock triggers particle acceleration and magnetic field amplification in the confined csm, synchrotron emission may be masked in centimeter wavelengths due to the free-free absorption; the millimeter range can however be a potential new window. we investigate the time evolution of synchrotron radiation from the system of red supergiant surrounded by the confined csm, relevant to typical type ii-p sne. we have revealed that synchrotron millimeter emission is generally detectable, and the signal can be used as a sensitive tracer of the nature of the confined csm; it traces different csm density parameter space than in the optical. furthermore, our simulations show that the confined csm efficiently produces secondary electrons and positrons through proton inelastic collisions, which can become main contributors to the synchrotron emission in several ten days since the sn. we predict that the signal is detectable by alma, and suggest that it will provide a robust evidence of the existence of the confined csm. | millimeter emission from supernovae in the very early phase: implications for dynamical mass loss of massive stars |
we use three-dimensional numerical magnetohydrodynamic simulations to investigate the quasi-equilibrium states of galactic disks regulated by star formation feedback. we incorporate effects from massive-star feedback via time-varying heating rates and supernova (sn) explosions, comparing momentum-only injection with thermal energy injection for sne. the latter sn prescription generates more realistic feedback, producing a hot interstellar medium (ism), but we show that the minimum required resolution (at least three grid zones for the sn remnant (snr) radius at shell formation epoch) is beyond the capability of most large-scale global galaxy simulations. our local model disks enable higher resolution, to follow the full snr evolution and all ism phases. the momentum-only approach for sne yields the correct turbulence level and star formation rate (sfr) in the warm and cold ism, and can be used when resolution is limited. we find that the ism disks in our simulations rapidly approach a quasi-steady state that satisfies vertical dynamical equilibrium. the sfr surface density self-adjusts to provide the total momentum flux (pressure) in the vertical direction that matches the weight of the gas. the final (time-averaged) state is insensitive to initial conditions and vertical boundary conditions. we quantify feedback efficiency by measuring ``feedback yields’’ defined by the ratio between different pressure components and the sfr surface density. for both magnetized and unmagnetized models, the turbulent and thermal yields are the same, and the ratio of turbulent to thermal yield is about 3. in magnetized models, turbulent magnetic fields are rapidly generated by the small-scale turbulence dynamo, and saturate at a level corresponding to the equipartition with the turbulent kinetic energy. the presence of magnetic fields enhances the total feedback yield and therefore reduces the sfr, since the same vertical support can be supplied at a smaller sfr. since additional vertical support can be provided by mean magnetic fields in the azimuthal direction, the sfrs are even more reduced in strongly magnetized models. | feedback regulated turbulence, magnetic fields, and star formation rates in galactic disks |
thanks to the observational and simulation works, the importance of the nonideal magnetohydrodynamic (mhd) effects, i.e., hall effect, ohmic resistivity, and ambipolar diffusion, have been well established at various stages of cloud evolution. to get a comparison between the hall effect with other effects, we aim to model the time evolution of a rotating filamentary molecular cloud during the isothermal/polytropic collapse phase in the presence of the hall drift. three components of the velocity vector are investigated when the angular momentum is fully coupled with the magnetic field at large radii of a filament. for this purpose, the nonideal mhd equations in the self-similar formalism are considered at large radii of a molecular cloud where the magnetic field evolution is affected by the hall drift. then, the connection between the self-similar approach with the observational data from the filamentary clouds is examined to get a realistic model. due to the existence of hall drift, the significant changes on the rotation of the cloud can be seen when the cloud switches from the isothermal collapse phase to the polytropic collapse phase. also, the results of this model are useful in the study of the multiple star formation process as well as the initial conditions for driving the outflows during the collapse of the filamentary clouds. finally, we found that there are some conditions for the comparability of the hall effect with the ambipolar diffusion in the outer regions of the clouds. | the importance of hall effect in the self-similar collapse of a filamentary cloud |
magnetic field plays an important role in star formation and galaxy evolution. previous studies discussed about the origin of magnetic field and its effect to the environment. with the recent advancement of supercomputers, adding the magnetic field to a cosmological hydrodynamic simulations only become feasible. in this proceeding, we present the results of high-resolution magneto-hydrodynamic simulation with gizmo and compare our simulation result with the previous literature and the observations. | magneto-hydrodynamic simulations on galaxy modeling |
black holes embody one of the few, simple, solutions to the einstein field equations that describe our modern understanding of gravitation. in isolation they are small, dark, and elusive. however, when a gas cloud or star wanders too close, they light up our universe in a way no other cosmic object can. the processes of magnetohydrodynamics which describe the accretion inflow and outflows of plasma around black holes are highly coupled and nonlinear and so require numerical experiments for elucidation. these processes are at the heart of astrophysics since black holes, once they somehow reach super-massive status, influence the evolution of the largest structures in the universe. it has been my goal, with the body of work comprising this thesis, to explore the ways in which the influence of black holes on their surroundings differs from the predictions of standard accretion models. i have especially focused on how magnetization of the greater black hole environment can impact accretion systems. | magnetohydrodynamic simulations of black hole accretion |
periods of solar wind where the interplanetary magnetic field (imf) is pointing northward are traditionally considered not to be geo-effective, or having little interaction with the earth's magnetosphere and ionosphere. we simulate and present an example of extreme northward imf stellar wind impacting an earth-like, closely orbiting exoplanet. in this regime magnetic reconnection occurs above the magnetic poles, allowing for stellar wind to penetrate and convect through the magnetosphere. this results in the loss of planetary plasma via reconnected field lines and charge exchange with the captured stellar wind population, which may have long-term implications on how the planetary environment evolves. we compare this scenario to that where the same stellar wind event is incident on the earth-like planet at 1au. | earth-like exoplanet response to extreme northward imf stellar wind |
the explosion energy of supernovae is believed to be a major energy source to drive and maintain turbulent motions in the interstellar gas. the interaction of supernova remnants with the interstellar medium plays a crucial role in shaping the statistics of interstellar turbulence, and has important effects on physical properties of molecular clouds. to investigate supernova-driven turbulence in molecular clouds and the implications for star formation, we conducted a large-scale mhd simulation, keeping track of the evolution of supernova remnants and their interactions with the interstellar gas in a region of 250 pc. the simulation accounts for the effects of gas heating and cooling, the magnetic fields and self-gravity, and the explosion energy of supernovae is injected as thermal energy at randomly selected locations in the simulation box. we analyzed the dense molecular clouds formed in our simulation, and showed that their properties, including the mass-size, velocity-size relations, mass and size probability distributions, and magnetic field-density relation, are all consistent with observational results, suggesting that the dynamics and structure of molecular clouds are the natural result of supernova-driven turbulence. we also found that, at the scale of molecular clouds, turbulent motions contain more power in solenoidal modes than in compressive modes. this suggests that the effective driving force for interstellar turbulence is largely solenoidal, in contrast to the recenthypothesis that supernova driving is purely compressive. the physical reason is that, as a supernova remnant impacts the ambient interstellar gas, the baroclinic effect arises immediately, which preferentially converts compressive motions to solenoidal modes throughout the evolution of the remnant in the interstellar medium. the implications of our results concerning the statistics of supernova-driven turbulence in molecular clouds on theoretical modeling of star formation will be discussed. | the impact of supernova remnants on interstellar turbulence and star formation |
the magnetohydrodynamics of stars and planetary cores is usually dominated by the overwhelming importance of rotation compared to other forces. under these conditions the fluid motions are characterized by a strong invariance along the rotation axis. in the presence of a background magnetic field, magnetohydrodynamic oscillations can be triggered. among these, of particular interest are the torsional waves, azimuthal perturbations of the fluid that are axisymmetric and invariant along the vertical direction. their periods depend solely on the intensity of the magnetic field component aligned with the radial direction of propagation. as the detection of the fundamental period could constrain the magnetic field intensity in the earth's outer core there is a long history of attempted detection of torsional waves from geomagnetic data. there is however a fundamental lack of knowledge concerning the propagation and reflection properties of these waves, as observational studies suggests behaviors that are different from theoretical expectations. in particular, recent findings (gillet et al., 2011) suggest the lack of reflection at the equator and at the rotation axis. through numerical simulation and analytical techniques we analyze the temporal evolution of diffusionless torsional waves in spherical geometry, with particular attention on the reflection at the equator and the pseudo-reflection at the rotation axis. we develop a novel analytical solution to the torsional wave eigenvalue problem whose behavior at the boundaries helps us to illustrate the meaning of the boundary conditions. furthermore we find that for any acceptable magnetic background field, reflections at both boundaries are allowed and we illustrate how the wkbj approximation is an efficient tool for investigating them. | propagation and reflection of diffusionless torsional waves in a sphere |
dispersionless and dispersed particle injections associated with substorms have been studied for many years based on observations acquired primarily at geosynchronous orbit. a general picture that has emerged is that particles are energized and rapidly transported/organized behind an "injection boundary" that penetrates closer to earth in some magnetic local time sector (e.g. the so-called double-spiral injection boundary model). while this picture provides a very good description of injections at geosynchronous orbit, with the launch of the van allen probes mission, we are now able to explore the evolution of injection signatures well inside of geosynchronous orbit at multiple locations as well. we find that the injection boundary model also appears to reproduce a number of complicated types of dispersion patterns observed in the van allen probes particle data. the dispersion patterns are found to depend dramatically on orbital configuration and timing of onset relative to the phasing of the spacecraft in their orbits. in addition to observational results, we present results of simulated dispersion patterns obtained from the injection boundary model using guiding center particle tracing in two different field configurations: 1) a simplistic dipole magnetic field with volland-stern electric field, and 2) ram/scb running in the space weather modeling framework. | multi-point observations and modeling of particle injections during substorms |
the orbital evolution of cataclysmic variables with periods above the "period gap" (>3 hrs) is governed by angular momentum loss via the magnetized wind of the unevolved secondary star. the usual prescription to study such systems takes into account only the magnetic field of the secondary and assumes its field is dipolar. it has been shown that introduction of the white dwarf and its magnetic field can significantly impact the wind’s structure, leading to a change in angular momentum loss rate and evolutionary timescale by an order of magnitude. furthermore, the complexity of the magnetic field can drastically alter stellar spin-down rates. we explore the effects of orbital separation and magnetic field configuration on mass and angular momentum loss rates through 3-d magnetohydrodynamic simulations. we present the results of a study of cataclysmic variable orbital evolution including these new ingredients. | realistic mhd modelling of cataclysmic variable spin-down |
created at the base of the convective envelope by a nonlinear dynamo process, the large scale magnetic field of a star evolves with its rotational history. beyond the photosphere, magnetic processes heat the corona above one million kelvin hence driving a magnetized wind responsible for the braking of main sequence stars. hence a feedback loop tie those processes. development of zeeman-doppler imaging through spectropolarimetry allows to precisely describe the surface magnetic field of a large sample of stars. thus the study of the coronal structure and magnetic field with age, magnetochoronology, has developed to extend and complete gyrochronology. we propose a study of the corona and the wind of a sample of k-type stars of different age to follow the evolution of its properties from 20 myr to 8 gyr thanks to a set of 3d mhd simulations with the pluto code constrained by spectropolarimetric maps of the surface magnetic field obtained by the bcool consortium. to perform those simulations we developed a coherent framework to assess various stellar parameters such as the equilibrium coronal temperature driving the wind. those assumptions have consequences on uv emissions, wind terminal speed and mass loss that impact planetary systems that could potentially host life. | coronal magnetic field and wind of an aging k-type star |
the galactic magnetic fields are considered as one of the key components regulating star formation, but their actual role on the dense cores formation and evolution remains today an open question.dust polarized continuum emission is particularly well suited to probe the dense and cold medium and study the magnetic field structure. such observations also provide tight constraints to better understand the efficiency of the dust alignment along the magnetic field lines, which in turn relate on our grasp to properly interpret the b-field properties.with the planck all-sky survey of dust submillimeter emission in intensity and polarization, we can investigate the intermediate scales, between that of molecular cloud and of prestellar cores, and perform a statistical analysis on the polarization properties of cold clumps.combined with the iras map at 100microns, the planck survey has allowed to build the first all-sky catalogue of galactic cold clumps (pgcc, planck 2015 results xxviii 2015). the corresponding 13188 sources cover a broad range in physical properties, and correspond to different evolutionary stages, from cold and starless clumps, nearby cores, to young protostellar objects still embedded in their cold surrounding cloud.i will present the main results of our polarization analysis obtained on different samples of sources from the pgcc catalogue, based on the 353ghz polarized emission measured with planck. the statistical properties are derived from a stacking method, using optimized estimators for the polarization fraction and angle parameters. these properties are determined and compared according to the nature of the sources (starless or ysos), their size or density range. finally, i will present a comparison of our results with predictions from mhd simulations of clumps including radiative transfer and the dust radiative torque alignment mechanism. | statistical properties of the polarized emission of planck galactic cold clumps |
the combined feedback of supernova explosions and stellar winds from associations of massive stars has a dramatic impact on their environment: large amounts of energy coming from the ejecta create dense shocks around the associations, compressing the surrounding ism and triggering the formation of molecular clouds and new stars. in this work we employ high-resolution, three-dimensional simulations of this process with the mhd code ramses to explore the effects of self-gravity and magnetic fields on the structure of the shells. two superbubbles expand and collide in a turbulent diffuse medium. in the expansion phase rich dense structure appears on the surface of the shocks due to hydrodynamic and hydromagnetic instabilities. although gravity seems to play a minor role in the formation and evolution of these dense clumps, magnetic fields completely alter both the expansion of the superbubble and the morphology of the dense gas, slowing the expansion down and causing the appearance of large-scale filaments. the collision does not help increase the amount of cold gas, but rather destroys a lot of the pre-existing dense structures. finally, we compare clouds formed in these simulations with observations of a molecular cloud crushed between two superbubbles. | formation of cold clumps and filaments around superbubbles |
solar energetic particle (sep) phenomena represent one of the major components of space weather. often, but not exclusively associated with coronal mass ejections (cmes), they pose a significant scientific as well as practical interest. as these particles originate at such explosive events, they have energies up to several gev. sep may cause disruptions in operations of space instruments and spacecrafts and are dangerous to astronauts. for this reason, studies of sep events and predictions of their impact are of great importance. the motion and acceleration of sep, though kinetic in nature, is governed by interplanetary magnetic field (imf) and its disturbances. therefore, a consistent and accurate simulation and predictive tool must include a realistic mhd model of imf. at the same time, transport of sep is essentially one-dimensional as at high energies particles are tied to magnetic field lines. this allows building a model that can effectively map active regions on the solar surface onto various regions of the solar system thus predicting the affected regions of the at any distance from the sun. we present the first attempt to construct a model that employs coupling of mhd and kinetic models. the former describes the evolution of imf disturbed by cme, while the latter simulates particles moving along the field lines extracted from mhd model. the first results are provided. | field-aligned transport and acceleration of solar energetic particles |
massive infrared dark clouds (irdcs) are believed to be the precursors to star clusters and massive stars (e.g. bergin & tafalla 2007). the supersonic, turbulent nature of molecular clouds in the presence of magnetic fields poses a great challenge in understanding the structure and dynamics of magnetized molecular clouds and the star formation therein. using the high-order radiation-magneto-hydrodynamic adaptive mesh refinement (amr) code orion2 (li et al. 2012), we perform a large-scale driven-turbulence simulation to reveal the 3d filamentary structure and dynamical state of a highly supersonic (thermal mach number = 10) and strongly magnetized (plasma β=0.02) massive infrared dark molecular cloud. with the high resolution afforded by amr, we follow the dynamical evolution of the cloud in order to understand the roles of strong magnetic fields, turbulence, and self-gravity in shaping the cloud and in the formation of dense cores. | structure and dynamics of magnetized dark molecular clouds |
understanding the processes related to the formation and evolution of molecular clouds is essential to our understanding of the interstellar medium (ism) at large and of star formation. high galactic latitude clouds are ideal laboratories for studying the physics of the ism as only turbulence, magnetic fields and the interstellar radiation field come into play. using clues from uv h2 absorption lines and by comparing iras dust emission to hi column density from aperture synthesis observations obtained using the drao interferometer, we have probed the morphology and dynamics of 14 potential molecular sites (totaling 151 square degrees), in the hopes of identifying molecular clouds at different stages of evolution. seven sites have confirmed molecular clouds. most are new, four of which have been observed in co using the onsala 20m telescope. the hi line shows varying degrees of velocity shears very probably related to the age of the molecular site. our newobservations will be presented. simulations of turbulent hi fields have recently been acquired andwill be compared to our observations. | aspects of hi behaviour in the birth of molecular clouds |
magnetic fields play a key role in the processes leading to the formation of stars and planets. analytical models and mhd numerical simulations of the evolution of star-forming cores show that the magnetic field is critical for transporting angular momentum during the protostellar phase and sets the conditions for strongly anisotropic accretion. pre-main-sequence phases, in which central protostars feed from surrounding planet-forming accretion discs, are especially crucial for understanding how worlds like our solar system are born. about 20 herbig ae/be stars have been reported to have globally organized magnetic fields. our analysis of their magnetic fields based on observations obtained with harpspol at eso's 3.6m telescope supports the idea that the low detection rate of magnetic fields in these stars can be explained by the weakness of these fields: only a few stars have magnetic fields stronger than 200, and half of the sample possesses magnetic fields of ~100 g, whereas their lower mass t tauri counterparts possess kg magnetic fields. studies of the magnetic field structure combined with the determination of the chemical composition of herbig ae/be stars are extremely important because they enable us to improve our insight into how the magnetic fields in these stars are generated and how they interact with their environment, including their impact on the planet formation process and the planet-disk interaction. | magnetic fields in herbig ae/be stars |
this work is dedicated to the numerical simulations of the dynamics of toroidal magnetic flux tubes (mfts) in the accretion disk of a young t tauri star. the equations of mft dynamics take into account the buoyancy and drag forces, the magnetic field of the disk, and the tensions of the internal magnetic field of the mft. the case of efficient heat exchange with the surrounding gas is considered. the structure of the accretion disk is simulated using the magnetohydrodynamic (mhd) model of the accretion disks developed by dudorov and khaibrakhmanov. the equation of state for a polytropic gas is used to model the vertical structure of the disk. simulations show that mfts with cross-section radius of 0.1h, where h is the disk scale height, rise almost vertically to the disk surface at a speed of up to 7 km s ‑1 . thin mfts with cross-section radius of 0.001h float up with velocities up to 20 km s ‑1 and contract towards the axis of rotation. during evolution, mfts can expand to sizes comparable to the accretion disk scale height and form an inhomogeneous outflowing magnetized disk corona. floating of mfts is an effective mechanism for removing excess magnetic flux from the inner regions of the disk, where the ionization fraction is large, and the magnetic field is frozen into gas. the mft concentration near the disk rotation axis can affect the generation of jet outflows and cause the observed jet inhomogeneities. | formation and dynamics of magnetic flux tubes in the accretion disks of young stars |
magnetic helicity, as one of the few conserved quantities in magneto-hydrodynamics, is often invoked as the principle driving the generation and structuring of magnetic fields in a variety of environments, from dynamo models in stars and planets, to post-disruption reconfigurations of tokamak's plasmas. most particularly magnetic helicity has raised the interest of solar physicists, since helicity is suspected to represent a key quantity for the understanding of solar flares and the generation of coronal mass ejections. in recent years, several methods of estimation of magnetic helicity have been proposed and already applied to observations and numerical simulations. however, no systematic comparison of accuracy, mutual consistency, and reliability of such methods has ever been performed. we present the results of the first benchmark of several finite-volume methods in estimating magnetic helicity in 3d test models. in addition to finite volume methods, two additional methods are also included that estimate magnetic helicity based either on the field line's twist, or on the field's values on one boundary and an inferred minimal volume connectivity. the employed model tests range from solutions of the force-free equations to 3d magneto-hydrodynamical numerical simulations. almost all methods are found to produce the same value of magnetic helicity within few percent in all tests. however, methods show differences in the sensitivity to numerical resolution and to errors in the solenoidal property of input fields. our benchmark of finite volume methods allows to determine the reliability and precision of estimations of magnetic helicity in practical cases. as a next step, finite volume methods are used to test estimation methods that are based on the flux of helicity through one boundary, in particular for applications to observation-based models of coronal magnetic fields. the ultimate goal is to assess if and how can helicity be meaningfully used as a diagnostic of the evolution of magnetic fields in the solar atmosphere. | magnetic helicity estimations in models and observations of the solar magnetic field |
it is not fully clear how the magnetic field acts during the first stages of star formation. a possible way to clarify its role is to observe the polarized light coming from masers and thermal dust emission. by measuring linear polarization angles and zeeman splitting of different maser species it is possible to study the magnetic field morphology and strength in different parts of the protostar. polarized emission of thermal dust has also been used extensively to probe the magnetic field at the onset of star formation. in this thesis we study the magnetic field properties of two well-known sources: the massive protostar iras18089-1732, showing a hot core chem- istry and a disc-outflow system, and the high-mass star forming complex g9.62+0.19, presenting several cores at different evolutionary stages. we also investigate the polarization properties of selected methanol masers, con- sidering newly-calculated methanol g-factors and hyperfine components. we compare our results with previous maser observations and we evaluate the contribution of preferred hyperfine pumping and non-zeeman effects. we make use of merlin and alma observations and we analyse the polarized emission by 6.7 ghz methanol masers and thermal dust. simulations were run using the radiative transfer code champ for different magnetic field values, hyperfine components and pumping efficiencies. we observe that the large scale field probed by dust continuum emission is consistent with the small scale magnetic field probed by masers. moreover, in the g9.62+0.19 complex we resolved several cores showing polarized emission. we propose an evolutionary sequence of magnetic field in this complex, where the less evolved stellar embryo exhibits a magnetic field stronger than the more evolved one. from our simulations, we find that preferred hyperfine pumping can explain some high levels of linear and circular polarization. we also notice that non-zeeman effects need to be considered in magnetic field studies. in conclusion, our work indicates that there is a link between the magnetic field at different scales. more masers observations will help in evaluating the relevance of non-zeeman effects and obtain good estimates of magnetic fields close to the protostar. future multi-wavelength and multi-scale observations, aimed at detecting polarized light from masers, thermal dust and thermal molecular lines, will help to constrain magnetic field properties around massive protostars. | magnetic fields around massive protostars as traced by masers and dust emission |
the physical mechanisms involved in the evolution of young stellar objects (ysos) are a complex subject still under debate. classical t tauri stars (ctts) are solar-like stars that are not only accreting mass from their circumstellar disks, but are also ejecting part of it in different shapes of outflows. at the moment, the star-disk interaction seems to be responsible for the angular momentum extraction. besides being held by strong magnetic fields, it includes also accretion and outflow processes. the aim of this study is to characterize the dynamics of accretion and outflow regions under both observations and magnetohydrodynamic (mhd) numerical simulations of ctts. | unveiling yso dynamics through observations and simulations. |
turbulence, magnetic fields and gravity driven flows are important for the formation of new stars. although magnetic fields have been proven to be important in the formation of stars, only a few works have been done combining magnetic field and kinematic information. such studies are important to analyze both gravity and gas dynamics and be able to compare them with the magnetic field. in this thesis we will combine dust polarization studies with kinematic analysis towards different star-forming regions. we aim to study the physical properties at core scales (<0.1 pc) from molecular line and dust emission, and study the role of the magnetic field in their dynamic evolution. for this, we will use millimeter and submillimeter observational data taken towards low- and high- mass star-forming regions in different environments and evolutionary states. the first project is the study of the physical, chemical and magnetic properties of the pre-stellar core fest1-457 in the pipe nebula. we studied the emission of the molecular line n2h+(1-0) which is a good tracer of dense gas and therefore describes well the structure of the core. in addition, we detected more than 15 molecular lines and found a clear chemical spatial differentiation for molecules with nitrogen, oxygen and sulfur. using the artist radiative transfer code (brinch & hogerheijde 2010, padovani et al., 2011, 2012, jørgensen et al., 2014), we simulated the emission of the different molecules detected and estimated their abundance. in addition, we estimated the magnetic field properties of the core (using the chandrasekhar-fermi approximation) from polarization data previously obtained by alves et al., (2014). finally, we found interesting correlations between the polarization properties and the chemistry in the region. the second project is the study of a high-mass star-forming region called ngc6334v. ngc6334v is in a more advanced evolutionary state and in an environment surrounded by other massive star-forming regions. during the project we studied the magnetic field from the polarized emission of the dust and also the kinematics of the gas from the molecular line emission of the different tracers of dense gas. from the molecular emission of the gas tracing the envelope of the dense core, we see two different velocity structures separated by 2 km/s and converging towards the potential well in the region. in addition, the magnetic field also presents a bimodal pattern following the distribution of the two velocity structures. finally, we compared the observational results with 3d magnetohydrodynamic simulations of star-forming regions dominated by gravity. the last project is the study of a lower-mass star-forming region, l1287. from the data obtained with the sma, the dust continuum structure shows six main dense cores with masses between 0.4 and 4 solar masses. the dense gas tracer dcn(3- 2) shows two velocity structures separated by 2-3 km/s, converging towards the highest-density region, the young stellar object iras 00338+6312, in a similar scenario to the one observed in the higher-mass case of ngc6334v. finally, the studies of the pre-stellar core fest1-457 and the massive region ngc6334v, show how the magnetic field has been overcome by gravity and is not enough to avoid the gravitational collapse. in addition, ngc6334v and the lower- mass region l1287 present very similar scenarios with the material converging from large scales ( 0.1 pc) to the potential wells of both regions at smaller scales ( 0.02 pc) through two dense gas flows separated by 2-3 km/s. in a similar scenario, fest1-457 is located just in the region where two dense gas structures separated by 3 km/s appear to converge. | collapse scenarios in magnetized star-forming regions |
magnetars are neutron stars of extreme magnetization, the most magnetized objects known in the universe. they are among the most topical sources in high energy astrophysics, being discovered in 1979 when their emission of intense flares of nonthermal hard x rays was first detected. pulsed modulation of the signal in a single giant flare forged the interpretation of a neutron star origin. yet in the intervening 35 years, we still do not know precisely what emission mechanisms are acting in their magnetospheres to produce such signals. nor do we know how the much more common `regular' bursts from magnetars arise. notwithstanding, the observational database for magnetars has expanded dramatically over the last 2 decades, and we now know of nearly 30 confirmed magnetars. last year elicited the exciting discoveries of the detection of a fast radio burst from a galactic magnetar during an x-ray bursting epoch, and the detection of a giant flare from a magnetar in the nearby sculptor galaxy. nasa's fermi mission featured in both these discoveries, and has enhanced magnetar science through the observation of prolific bursting activity from select sources using the gamma-ray burst monitor (gbm). the gbm has clearly demonstrated that magnetar spectra are non-thermal with a quasi-exponential cutoff, a so-called comptonized form. the intensities of magnetar bursts and giant flares strongly suggests that their emission zones are highly optically thick to compton scattering. this program will perform detailed numerical computations of the establishment of hard x-ray spectra of transient magnetar signals using a monte carlo approach. we plan to develop state of the art models for both burst and giant flare emission signatures using monte carlo simulations of radiation transport and particle dynamics in magnetar magnetospheres, assembled using several existing well-tested codes. the dominant process will be compton scattering, with polarizations of photons being tracked, since they are integral to determining the scattering probabilities. the compton cross section will include the details of cyclotronic resonances that are sampled at higher altitudes and enhance overall opacity. it is planned to first model an evolving comptonizing cloud that expands in closed field regions of the magnetosphere, adding the important radiation processes of magnetic photon splitting, and magnetic one-photon pair creation for giant flares. given an impulsive injection of energy at either equatorial or polar locales, the transport of radiation and energy exchange between pairs and photons will be tracked, using observations to provide diagnostics on the lepton injection/acceleration site. a prime objective is to determine what physical processes limit the emission to less than a few hundred kev in energy. polarization characteristics will be ascertained to serve as a guide for science agendas of planned future hard x-ray/soft gamma-ray polarimeters such as leap and amego. the simulation will be adapted to define a dynamical outflow model for the spectral shape and pulsation properties of the initial spike and temporal tail of magnetar giant flares. the outflow zone will be fed radiation from an equatorial fireball zone. one goal here is to ascertain whether a super-eddington polar outflow can explain the observed pulsation amplitude in giant flare tails, and if so, how it can constrain the geometry and energetics of these rare events. another agenda is to calibrate the pair density so as to assess how it loads the outer magnetosphere with plasma, an influence that might account for the alteration of magnetar spin-down rates observed subsequent to giant flares. | modeling hard x-ray bursts and giant flares from magnetars |
interplanetary coronal mass ejections (icmes) originate from the eruption of complex magnetic structures occurring in our stars atmosphere. they propagate in the interplanetary medium, where they can be probed by spacecraft. icmes are known to generate geomagnetic storms that can disturb our technologies on earth, this is why they are a subject of interest. studying icmes could, therefore, allow us to predict and lower their impact in our technology. we present the results of the propagation simulation of a set of titov-demoulin flux ropes (titov et al. 2014) with different magnetic fields and sizes at the initiation. this is done with the 3d mhd module of the pluto code. our grid starts at the low corona and goes up to 2 astronomical units. this allows us to study the effect of the magnetic field intensity or the size of the flux rope at the initiation on its properties during the propagation, highlighting then the physical processes happening during their journey in the inner heliosphere. the evolution of the magnetic field of the flux rope during the propagation agrees with evolution laws deduced from in situ observations. we also simulate in situ profiles that spacecraft would have measured at mercury and at earth, and we compare with the results of janvier et al. 2019 and regnault et al. 2020. we find a good match between simulated in situ profiles and typical profiles obtained in these studies. the magnetic components of the simulated flux rope match well with what we are expecting from theory (lundquist et al. 1950). this simulation helps us to have a better understanding of the physical mechanisms that happen during propagation of an icme. | 3d modelling of titov-demoulin modified flux ropes propagation in the solar wind |
when a supermassive black hole (smbh) feeds on its host galaxy, it launches energetic mechanical outflows and emits radiation. such active galactic nuclei (agn) can launch relativistic jets that blow away and heat up the ambient gas via a process known as agn feedback. this feedback is key to understanding galaxy evolution and star formation. however, following the complex dynamics of gas accretion is currently computationally prohibitive, due to the disparate length and time scales between the smbh and its galaxy. attempting to bridge this gap, we have performed state-of-the-art long-duration general relativistic magnetohydrodynamic (grmhd) simulations that follow the accretion of magnetized gas from the smbh's sphere of influence (bondi radius rb) down to the event horizon (rg). we vary the ambient gas degree of rotation, the smbh spin, and the bondi radius, with the latter reaching the largest scale separation to date rb/rg = 1000, in a single grmhd simulation. the infall of rotating gas self-consistently forms an accretion disk, and the smbh launches magnetized relativistic jets. under the pressure of the infalling gas, the jets intermittently turn on and off, erratically wobble, and inflate pairs of cavities in different directions. this morphology resembles the rare x-shaped radio galaxies (xrgs), first time created in a grmhd simulation, without any special initial conditions. stable jets are produced when the smbh accretes enough magnetic flux that it transitions to a magnetically arrested disk (mad) state, characterized by particularly strong jets that drill through the ambient gas and propagate well outside the bondi sphere. we reliably measured the fraction of the bondi sphere gas that reaches the central smbh, which agrees with the observationally inferred value. for a given bondi radius, the maximum accretion rate suppression is weakly affected by the size of the accretion disk. surprisingly, at very late times, the sense of disk rotation and magnetic fields continuously flip, quenching the jets. thereafter, the system exits the mad state and the newly formed jets become erratic again, which has not been seen before in the presence of unlimited large-scale magnetic flux. | from inside out: bridging the bondi and event horizon scales using 3d grmhd simulations |
the dynamical evolution of short-period low-mass binary stars (m < 1.5 msun, p < 10 days) is strongly influenced by tidal dissipation. despite its fundamental role in binary evolution, constraining the strength of tidal dissipation, typically parameterized by the tidal quality factor q, has remained discrepant by orders of magnitude in the existing literature. new observational constraints from time-series photometry (i.e. kepler, k2, tess), as well as advancements in theoretical models that incorporate more realistic rheology of stellar structure are invigorating new optimism for the field. two paths have emerged to leverage the new data sets to constrain q: modeling individual systems or population inference. we examine the challenges and advantages of various model validation approaches by using numerical simulations of equilibrium tide model combined with stellar evolution and magnetic braking to predict binary dynamical evolution over myr-gyr timescales. we use sensitivity analysis to examine to what degree each unknown model input (the initial conditions and tidal q) influences observables (the final orbital and rotation states). these techniques allow us to analyze the coupled nonlinear effects of the 18-dimensional phase space and systematically assess the limitations due to both inherent model degeneracies, as well as observational uncertainties. our results show that even under the simplest and most tractable models of tides, the path towards validating q is ill-posed: inherent degeneracies between tidal q and model initial conditions severely limit the prospects of constraining tidal q for individual binary systems, even when considering the strongest possible constraints (i.e. binaries with precise masses and ages). alternatively we show that evolution states over a wide range of initial conditions tend to converge towards trajectories along a low-dimensional (orbital period, rotation period, eccentricity) manifold. this analysis suggests that a population approach based on equilibrium analysis may be a promising path forward for validating tidal theories. | prospects of constraining tidal dissipation in low-mass binary stars |
star formation has important implications for the formation of planetary systems and the evolution of galaxies, but due to the complex interplay of gravity, turbulence, magnetic fields, as well as feedback from newly formed stars, star formation is still poorly understood. the evolution of the gas density probability distribution function (⍴-pdf) forms the basis of several star formation models, as it can be directly related to the star formation rate (sfr). we investigate the effect of initial conditions on the shape of the ⍴-pdf in two runs from the starforge simulation suite, these calculations follow molecular clouds as they evolve, while resolving individual stars and including all feedback physics. we analyze two runs with different initial conditions: one is a periodic box with driven turbulence (box), while the other is an isolated cloud without turbulent driving (sphere), the two common representations of molecular clouds in simulations. we fit the shape of the ⍴-pdf at various times, using least squares fitting. we find that the shape of the ⍴-pdf in both simulations is best described by a log-normal (ln) plus a power-law (pl) function at early times. however, as the sfr peaks, the ⍴-pdf for the sphere run becomes well-fit by a wide ln function only, but the box run remains well-fit by a ln+pl function throughout. | effect of initial conditions on the gas density distribution in a simulated star-forming cloud with stellar feedback |
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