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we calculate the evolution of cloud cores embedded in different envelopes to investigate environmental effects on the mass accretion rate on to protostars. as the initial state, we neglect the magnetic field and cloud rotation, and adopt star-forming cores composed of two parts: a centrally condensed core and an outer envelope. the inner core has a critical bonnor-ebert density profile and is enclosed by the outer envelope. we prepare 15 star-forming cores with different outer envelope densities and gravitational radii, within which the gas flows into the collapsing core, and calculate their evolution until ~2 × 105 yr after protostar formation. the mass accretion rate decreases as the core is depleted when the outer envelope density is low. in contrast, the mass accretion rate is temporarily enhanced when the outer envelope density is high and the resultant protostellar mass exceeds the initial mass of the centrally condensed core. some recent observations indicate that the mass of pre-stellar cores is too small to reproduce the stellar mass distribution. our simulations show that the mass inflow from outside the core contributes greatly to protostellar mass growth when the core is embedded in a high-density envelope, which could explain the recent observations. | environmental effects of star-forming cores on mass accretion rate |
understanding the role galactic-scale winds play in the formation and evolution of galaxies is a fundamental goal of current astrophysical research. galactic winds, driven by energetic feedback processes associated with supernovae, regulate the baryonic content, star formation rates, and stellar masses of galaxies. to understand the properties of galaxies throughout cosmic time, astrophysical theory must endeavor to model the hydrodynamic processes that govern how gas is ejected from galaxies. this project aims to 1) simulate galactic outflows with numerical models that allow for supersonic wind velocities, 2) quantify the importance of radiative cooling for the multiphase structure of observed galactic outflows, 3) determine the mass and energy coupling of ism gas to supernova-driven outflows, 4) explore the physics that influence the development and structure of galactic winds (conduction, magnetic fields, etc.), and 5) compare with observations of galactic outflows from nasa facilities (hst, chandra, etc.) and make predictions for forthcoming observations with james webb space telescope. the simulations will be performed using cholla, the massively-parallelized hydrodynamical simulation code by schneider & robertson (2015, 2017) that leverages graphics processor units to achieve state-of-the-art performance on the world's fastest supercomputers. by simultaneously resolving small scales over large volumes using fixed spatial resolution, the cholla simulations can accurately track feedback-driven outflows from their origins in the interstellar medium to large galactocentric radii where radiative cooling becomes efficient. the proposed research will build on the insight developed in controlled simulations of shock-cloud interactions on small scales (schneider & robertson 2017). the proposed simulations will greatly extend our physical understanding of galactic outflows by meeting a wide range of scientific goals. they will improve theoretical models for the mass and momentum loading of interstellar gas into baryonic outflows, and better characterize the multiphase structure of supernova-driven galactic winds. in addition, they have the potential to prove the validity of recently developed key analytic theories regarding the large-scale cooling of these winds. such theories are critical for explaining the observations of large reservoirs of gas in galaxy halos observed with nasa great observatories and likely for understanding forthcoming integral field unit spectroscopic observations of galactic winds with james webb space telescope. | resolving the physics of galactic winds using the gpu-accelerated cholla code |
most massive stars are in binaries. binarity can drastically alter their evolution, which can result in the formation of x-ray binaries or binary black hole systems, the dominant sources of gravitational waves. while single star formation is increasingly well understood, the detailed physics of binary/multiple massive star formation has received less attention. in particular, a good understanding of the combined influence of birth environmental conditions such as rotation, turbulence, magnetic fields, on (massive) stellar multiplicity is still lacking. in this proceeding, we summarize recent numerical efforts to clarify these points, using radiation-magneto-hydrodynamical simulations of massive pre-stellar core collapse with the ramses code, including the relevant physics to identify several fragmentation processes. we find magnetic fields to limit disk fragmentation in several ways: they remove angular momentum via outflows and magnetic braking, thereby limiting (and/or delaying) disk growth, self-gravitational effects and fragmentation. in binaries linked by a gaseous filament, magnetic pressure impedes gas concentration in the filament, thus suppressing fragmentation via spiral arm-filament, a fragmentation channel ubiquitous in hydrodynamical runs. on the opposite, core rotation and turbulence carry angular momentum, favouring disk growth and fragmentation. moreover, turbulence reduces magnetic braking and outflows, again favoring disk fragmentation. we conclude that the environmental conditions (rotation, turbulence, magnetic fields) are keys to understand (massive) stellar multiplicity. | how do (massive) binary stars form? |
the unified model for agn stats that seyfert 1 or 2 types are the same type of object harbouring a luminous accretion disk surrounded by a thick torus, but seen under different viewing angles. this model has been successfully tested for many years by several observers, bringing some evolutions to the initial model, but on the torus, one of the most important pieces, we still lack information because of its limited extension and of the high contrast required. we took advantage of sphere's extreme contrast and high angular resolution to propose a near infrared (nir) polarimetric observation of the archetypal seyfert 2 galaxy ngc 1068 in h and ks broad bands, revealing a clear double hourglass shape of centro-symmetric pattern and a central pattern diverging from these. we continued these measurements in narrow bands (in the nir) and at shorter wavelengths (r band) in order to study the wavelength dependency of the measured polarisation, which we are currently analysing. if the features are wavelength-dependent, this will bring new constraints on the properties of scatterers. polarimetry is a powerful tool as it gives access to more information than spectro- scopy or imaging alone. in particular, indications on the geometry of the scatterers, the orientation of the magnetic fields or the physical conditions of matter can be re- vealed thanks to the additional parameters measured (the polarisation degree and the polarisation position angle for linear polarisation). the counterpart of polarimetric measurements is that analyses of data is not straightforward. the use of numerical simulations, and especially radiative transfer codes is necessary to fully understand the observations. the first part describes a radiative transfer code, montagn, with the aim of reproducing the features that we observed, thus including full processing of polarisation. this code allowed us to bring some constraining results on the optical depth of the structures. the torus' optical depth was constrained to 20 in the ks band as a minimum. our investigations on the densities in the ionisation cone are consistent with densities of 2.0 × 109 m-3, in the range of previously estimated values for this agn. the last part of the phd work was dedicated to ssc (also known as young massive clusters). these clusters correspond to the more massive example of star forming clusters, often about 10 myr and still embedded in a dusty shell. these clusters exhibit extreme observed star formation rates. we took advantage of an instrumental run of the multi-object ao demonstrator canary instrument, installed at the wil- liam hershel telescope in 2013 to obtain images in h and ks bands at higher resolution than previously achievable. data on the galaxy iras 21101+5810 were reduced and analysed, constraining the age of the clusters between 10 and 100 myr and the ex- tinction to about av ≍ 3. photometry was obtained thanks to a new algorithm to estimate the galaxy background, bringing improvements on the fitting of the clusters' luminosity distributions. | extragalactic observations with adaptive optics: polarisation in active galactic nuclei and study of super stellar clusters |
our closest neighbour star, proxima centauri, has been shown to have at least one orbiting planet, proxima cen b. this planet has a minimum mass close to earth's mass and is likely rocky in its composition. unlike the sun, proxima centauri possesses a very strong magnetic field of at least a few hundred gauss, which may have been even higher in the past. although at present the star has a slow rotation period of ∼83 days, it may have rotated much faster in the past. if the stellar rotation axis and the dipole axis are inclined with respect to each other, the planet experiences constant change of the magnetic flux caused by stellar rotation and orbital motion. due to finite conductivity of the planetary interiors, this leads to generation of eddy currents and electromagnetic induction heating of the planetary mantle. here we study the electromagnetic induction heating in the interior of proxima cen b over time, and its effect on the evolution of interior, surface and atmosphere of the planet. we consider both the present-day parameters of the star as well as stellar evolution, and study the possible induction heating in the past. we calculate different evolutionary tracks for proxima centauri and investigate the evolution of its rotation period and magnetic field. we calculate the heating in planetary interiors over time for different magnetic fields and rotation evolutions of the star. we implement the local magnetic induction heating effect in our mantle convection simulations depending on the composition, the mantle electrical conductivity, and the inclination between stellar rotation axis and dipole axis. our results show that the induction heating may have been substantial in the past, leading to formation of local magma oceans in the planetary mantle and to extreme volcanism during substantial periods of planetary history. this implies a very different evolution history of proxima cen b in comparison to the earth and raises important questions about its claimed habitability potential. | induction heating of the interior of proxima cen b |
young stars such as protostars and pre-main-sequence stars evolve via the interaction with the surrounding accretion disks. it is believed that stellar and disk magnetic fields play important roles in shaping the accretion structure and exchanging the angular momentum between the stars and the disks. however, because of the complexity of gas dynamics around the stars, the star-disk interaction remains poorly understood, which makes the construction of the stellar evolution models difficult. to reveal the interaction processes, we have been performing 3d magnetohydrodynamic simulations of accretion onto a young star with different stellar magnetic fields. in the case of a weakly magnetized, magnetosphere-free star, we found that failed disk wind becomes supersonic, high-latitude accretion flows onto the star (takasao et al. 2018). this result may explain the reason why herbig ae/be stars show fast accretion. in a different model with stronger disk fields, we showed that the star can produce recurrent explosions via magnetic reconnection (takasao et al. 2019). we consider that the mechanism is relevant to protostellar flares in class-0/i protostars. in addition to the above two models, we have been investigating the magnetospheric accretion which is very relevant to classical t-tauri stars. in this talk, we will introduce our 3d modeling and discuss how the star-disk interaction changes depending on the stellar and disk field strengths. | 3d mhd simulations of an accreting young star |
we present simulations of the merger of binary neutron star systems calculated with full general relativity and incorporating the global magnetic field structure for the stars evolved with resistive magnetohydrodynamics. we also incorporate the effects of neutrino transport and tabular equations of state to describe the degenerate matter. we gratefully acknowledge the support of nasa through the astrophysics theory program grant nnx13ah01g. | merger of magnetized binary neutron stars |
during the late 1990's the mars global surveyor mag/er experiment detected crustal remanent magnetization at mars indicating an ancient internal magnetic dynamo. the location of this remanent magnetization and in particular its absence at the large martian impact craters like hellas suggests a cessation of the dynamo during the early naochian epoch, i.e. ~ 4.1 to 4 billion years ago. the strength of the remanent magnetization together with dynamo theory are indicating an ancient dipole field strength lying in the range of ~0.1 and ~1.0 of the present-day dipole field of the earth, making the martian paleo-magnetosphere comparable with the terrestrial paleo-magnetosphere. this also has implication for the early martian atmosphere.in this poster we will present simulations of the paleo-magnetosphere of mars for the early naochian, just before cessation (i.e. for ~4.1 to ~4.0 billion years ago). these were performed with an adapted version of the paraboloid magnetospheric model (pmm) of the skobeltsyn institute of nuclear physics of the moscow state university, which serves as an iso standard for the magnetosphere. here the ancient magnetic field was assumed to be a dipole field (with dipole tilt ψ=0). the ancient solar wind ram pressure as important input parameter was derived from a newly developed solar/stellar wind evolution model, which is strongly dependent on the rotation rate of the early sun. these simulations show that for the most extreme case of a fast rotating sun and a paleomagnetic field strength of 0.1 of the present-day earth value, the martian magnetopause was located at ~5.5 rm (i.e. ~2.9 re) above the martian surface. assuming a strong dipole field (i.e. 1.0 of present-day earth) and a slow rotating sun - our least extreme case - would lead to a standoff-distance of rs~16 rm (i.e. ~8.5 re).our simulations also have implications for the early martian atmosphere, which will be demonstrated within this poster. these first results on the erosion of the early martian atmosphere take into account the paleo-magnetosphere, the enhanced euv-flux and solar wind conditions during the early naochian epoch. | the martian paleo-magnetosphere during the early naochian and its implication for the early martian atmosphere |
in the near future, next-generation telescopes, covering most of the electromagnetic spectrum, will provide a view into the very earliest stages of galaxy formation. to accurately interpret these future observations, accurate and high-resolution simulations of the first stars and galaxies are vital. this proposal is centered on the formation of the first galaxies in the universe and their observational signatures in preparation for these future observatories. this proposal has two overall goals: 1. to simulate the formation and evolution of a statistically significant sample of galaxies during the first billion years of the universe, including all relevant astrophysics while resolving individual molecular clouds, in various cosmological environments. these simulations will utilize a sophisticated physical model of star and black hole formation and feedback, including radiation transport and magnetic fields, which will lead to the most realistic and resolved predictions for the early universe; 2. to predict the observational features of the first galaxies throughout the electromagnetic spectrum, allowing for optimal extraction of galaxy and dark matter halo properties from their photometry, imaging, and spectra; the proposed research plan addresses a timely and relevant issue to theoretically prepare for the interpretation of future observations of the first galaxies in the universe. a suite of adaptive mesh refinement simulations will be used to follow the formation and evolution of thousands of galaxies observable with the james webb space telescope (jwst) that will be launched during the second year of this project. the simulations will have also tracked the formation and death of over 100,000 massive metal-free stars. currently, there is a gap of two orders of magnitude in stellar mass between the smallest observed z > 6 galaxy and the largest simulated galaxy from "first principles", capturing its entire star formation history. this project will eliminate this gap between simulations and observations of the first galaxies, providing predictions for next-generation observations coming online throughout the next decade. the proposed activities present the graduate students involved in the project with opportunities to gain expertise in numerical algorithms, high performance computing, and software engineering. with this experience, the students will be in a powerful position to face the challenging job market. the computational tools produced by this project will be made freely available and incorporated into their respective frameworks to preserve their sustainability. | kinetic modeling of radiative turbulence in relativistic astrophysical plasmas: particle acceleration and high-energy flares |
magnetic fields are speculated to play a significant role in early star formation, in particular, in the collapse dynamics at formation to influence the imf, which may be imprinted in the local metal-poor population. these fields may arise by the amplification of primordial fields during the formation of the first stars (population iii) as well as their feedback. we study the former using cosmological magneto-hydrodynamic (mhd) simulations following the evolution of the magnetic field given a uniform primordial field from cosmological initial conditions to the formation of a single pop iii star and 2 myr after its supernova. we find that a seed field of b = 10-15 g can be maximally amplified by 6 orders of magnitude at the density peak and by a factor of 100 around the shell of the supernova shock. these stars then enrich their surroundings, setting the stage for the formation of the first metal-poor stars. we also explored the collapse dynamics of metal-poor mini-halos by running simulations with varying lyman-werner background strength and metallicity. we produce a fit for the minimum mass for collapse as a function of the two parameters. furthermore, pop iii stars provide a significant fraction of ionizing photons for reionization at high redshift (z > 10). we modify existing semi-numeric methods to include pop iii stars as ionizing sources. we find that the characteristic hii bubble sizes at all redshifts is decreased in comparison with models that only consider atomic-cooling halos and calculate an optical depth, τe = 0.0569, consistent with the latest results from planck. the resulting ionization fields from this method can then be used to efficiently model the ionizing uv background in numerical simulations. these results are essential to building a full mhd simulation of the first galaxies. | magnetizing the universe during the epoch of reionization |
magnetic fields and self-gravity are likely to play important and interconnected roles in the evolution of protostellar disks around newly formed stars. we propose to use a combination of numerical simulations and analytic models to understand how net magnetic fields evolve during a self-gravitating disk phase, and whether conditions in such disks allow for the collisional growth of icy particles as a pre-requisite for early planet formation. we will specifically seek to answer three main questions: (1) how do self-gravitating protostellar disks threaded by net magnetic fields evolve? to address this question we will develop simulations that include self-gravity and either ideal magnetohydrodynamics (mhd) or non-ideal mhd including ambipolar diffusion. we will quantify the effect of net fields on angular momentum transport in self-gravitating disks and develop a physical understanding of net flux transport that can be used in long-term evolutionary models. (2) how does the net flux of protostellar disks evolve over long time scales? we will develop simplified numerical models that build on prior work by lubow, papaloizou & pringle (1994) and guilet & ogilvie (2014). (3) how is energy dissipated in self-gravitating disks? we will use idealized numerical experiments to quantify the balance between dissipation in large-scale spiral structures versus small-scale turbulence. we will use the results to better understand whether conditions in early protostellar disks allow for the initial growth of icy solid particles to larger sizes. the proposed study is a theoretical investigation into how basic physical processes - disk self-gravity and magnetic flux transport - affect the evolution of protostellar disks during the formation of stars and planetary systems. the expected results are relevant to the direct imaging of disk structure with space astrophysics missions, and to the interpretation of exoplanet populations via theoretical models that start with the collisional growth of small particles. | magnetic fields and self-gravity in early protostellar disks |
i discuss how hydrodynamical simulations enable exploring nonlinear couplings amongst different physical processes in galaxy formation. i focus on three topics: the effect of baryonic processes on dark matter halos, how stellar feedback leads to 'self-regulated' galaxy formation, and the interplay amongst stellar feedback, magnetic fields, and cosmic rays. invited talk presented at the conference galaxy evolution across time, 12-16 june 2017, paris, france | galaxy evolution from hydrodynamical simulations: what have we learned about the physical processes governing galaxy evolution? |
the origin of large-scale magnetic fields in most astrophysical systems like the sun, stars and galaxies remains a challenging open problem. dynamo action in the underlying turbulent fluid is thought to be responsible for the emergence of coherent magnetic fields. due to the enormity of magnetic reynolds numbers in these astrophysical systems, current theoretical models of the turbulent dynamo struggle to generate large-scale field on fast dynamic timescales. the conservation properties of magnetic helicity can constrain the nonlinear evolution of the dynamo. we have performed direct numerical simulations of the turbulent dynamo to investigate if employing open boundaries relaxes the constraint imposed by magnetic helicity conservation. we find that in the open systems a net magnetic flux (or system-scale fields) of significant strength arises. however, the type of open boundary we employ does not alleviate the magnetic reynolds number (in the range explored) dependence in the nonlinear evolution of the large-scale fields. finally, simulations performed across different magnetic prandtl numbers indicate that the behavior of the magnetic helicity evolution is affected by flow properties as well. european research council. | generation of coherent magnetic fields in periodic (closed) and non-periodic (open) domains |
with energies a thousand times greater than an average solar flare, stellar superflares can strip away atmospheres and endanger life on planetary surfaces. these violent outbursts often go hand in hand with coronal mass ejections, shaping the evolution of planets near and far.a bit of solar system historyastronauts on the international space station captured this view of the aurora over canada. [nasa]in 1859, a solar storm launched a coronal mass ejection toward earth. named after one of the british astronomers who dutifully recorded the associated solar flare, the carrington event is one of the most powerful solar storms to impact the earth in recorded history.luckily for us, our middle-aged sun rarely throws tantrums that explosive; if it happened today, the carrington event would fry spacecraft electronics and power grids, causing trillions of dollars of damage.in the suns wilder youth, however, superflares were likely common. studying younger sun-like stars, like 700-million-year-old 1cet, can help us understand what the sun was like billions of years ago and what the planets orbiting young stars may be subjected to today.simulation of the coronal mass ejection magnetic flux rope propagating away from the star, as seen from the stars north pole. [lynch et al. 2019]simulating a global superflarea team of astronomers led by benjamin lynch (university of california-berkeley) used three-dimensional magnetohydrodynamic models to study the eruption of a superflare and a coronal mass ejection from the young sun-like star 1cet.they started with observations of 1cets surface magnetic field, then modeled how the stellar wind drags the magnetic field outward. by setting the stars surface in motion, they caused the field lines to become twisted and tangled, building up the magnetic energy.the magnetic energy gradually increased until reconnection kicked in and launched a coiled rope of magnetic flux into space. the eruption of the modeled coronal mass ejection lasted 10 hours and released a whopping 3 1026 joules about the same amount of energy estimated to have been released by the sun in the 1859 carrington event.radial cuts showing the speed (left) and density (right) of the coronal mass ejection. [adapted from lynch et al. 2019]an effect to considerlynch and coauthors note that their model captures the most extreme superflare that 1cet can release based on past observations of the stars surface magnetic flux. however, because starspots arent resolved in those observations, its possible that they could contain even more magnetic flux than expected, leading to even more energetic outbursts.the authors hope that models of stellar superflares and coronal mass ejections can be used to understand how stellar activity affects planetary systems. they pointed to the importance of taking into consideration the effects of flares and coronal mass ejections on the habitability of exoplanets, not all of which are bad: when energetic particles from these events enter a planets atmosphere, they can generate compounds like hydrogen cyanide, which may play a role in the formation of the building blocks of life.citationmodeling a carrington-scale stellar superflare and coronal mass ejection from k1cet, benjamin j. lynch et al 2019apj88097. doi:10.3847/1538-4357/ab287e | launching a stellar superflare |
in recent omega laser experiments we have created narrowly collimated mg plasma jets by using 20 omega beams from one hemisphere to form a hollow ring pattern on a flat ch target, and characterized the properties of these jets as a function ring radius d and target composition (pure ch vs. 2 percent fe-doped ch). the strong mg poloidal magnetic field of these jets is created via the biermann battery (grad pe\ xgrad ne) mechanism by the collisions of individual laser blow offs and further compressed by the on-axis flow. the magnetic field gets stronger, more ordered and persists to greater distances from the target as d is increased from 0 to 1200 microns. here we discuss the formation and evolution of magnetized high-beta shocks created by the collision of two such mg plasma jets, and the effects of changing the ring radius, target separation and composition. results from 2d and 3d flash code simulations, and designs for future omega experiments, will be presented. we will highlight the effects of electron thermal conduction on the shock structure and evolution. potential applications of high-beta magnetized shocks to young stellar object outflows will also be discussed. work supported by doe nnsa nluf. | collision of two magnetized jets created by hollow ring lasers |
molecular clouds, the birthplaces of stars in galaxies throughout the universe, are complicated and dynamic environments. a new series of simulations has explored how these clouds form, grow, and collapse over their lifetimes.this composite image shows part of the taurus molecular cloud. [eso/apex (mpifr/eso/oso)/a. hacar et al./digitized sky survey]stellar birthplacesmolecular clouds form out of the matter in between stars, evolving through constant interactions with their turbulent environments. these interactions taking the form of accretion flows and surface forces, while gravity, turbulence, and magnetic fields interplay are thought to drive the properties and evolution of the clouds.our understanding of the details of this process, however, remains fuzzy. how does mass accretion affect these clouds as they evolve? what happens when nearby supernova explosions blast the outsides of the clouds? what makes the clouds churn, producing the motion within them that prevents them from collapsing? the answers to these questions can tellus about the gas distributed throughout galaxies, revealing information about the environments in which stars form.a still from the simulation results showing the broader population of molecular clouds that formed in the authors simulations, as well as zoom-in panels of three low-mass clouds tracked in high resolution. [ibez-meja et al. 2017]models of turbulencein a new study led by juan ibez-meja (mpi garching and universities of heidelberg and cologne in germany, and american museum of natural history), scientists have now explored these questions using a series of three-dimensional simulations of a population of molecular clouds forming and evolving in the turbulent interstellar medium.the simulations take into account a whole host of physics, including the effects of nearby supernova explosions, self-gravitation, magnetic fields, diffuse heating, and radiative cooling. after looking at the behavior of the broader population of clouds, the authors zoom in and explore three clouds in high-resolution to learn more about the details.watching clouds evolveibez-meja and collaborators find that mass accretion occurring after the molecular clouds form plays an important role in the clouds evolution, increasing the mass available to form stars and carrying kinetic energy into the cloud. the accretion process is driven both by background turbulent flows and gravitational attraction as the cloud draws in the gas in its nearby environment.plots of the cloud mass and radius (top) and mass accretion rate (bottom) for one of the three zoomed-in clouds, shown as a function of time over the 10-myr simulation. [adapted from ibez-meja et al. 2017]the simulations show that nearby supernovae have two opposing effects on a cloud. on one hand, the blast waves from supernovae compress the envelope of the cloud, increasing the instantaneous rate of accretion. on the other hand, the blast waves disrupt parts of the envelope and erode mass from the clouds surface, decreasing accretion overall. these events ensure that the mass accretion rate of molecular clouds is non-uniform, regularly punctuated by sporadic increases and decreases as the clouds are battered by nearby explosions.lastly, ibez-meja and collaborators show that mass accretion alone isnt enough to power the turbulent internal motions we observe inside molecular clouds. instead, they conclude, the cloud motions must be primarily powered by gravitational potential energy being converted into kinetic energy as the cloud contracts.the authors simulations therefore show that molecular clouds exist in a state of precarious balance, prevented from collapsing by internal turbulence driven by interactions with their environment and by their own contraction. these results give us an intriguing glimpse into the complex environments in which stars are born.bonuscheck out the animated figure below, which displays how the clouds in the authors simulations evolve over the span of 10 million years.http://cdn.iopscience.com/images/0004-637x/850/1/62/full/apjaa93fef1_video.mp4citationjuan c. ibez-meja et al 2017 apj 850 62. doi:10.3847/1538-4357/aa93fe | star-forming clouds feed, churn, and fall |
neutron stars contain the strongest magnetic fields known in the universe. using numerical simulations restricted to axially symmetric geometry, we study the long-term evolution of the magnetic field in the interior of an isolated neutron star under the effect of ambipolar diffusion, i.e. the drift of the magnetic field and the charged particles relative to the neutrons. we model the stellar interior as an electrically neutral fluid composed of neutrons, protons and electrons; these species can be converted into each other by weak interactions (beta decays), suffer binary collisions, and be affected by each other's macroscopic electromagnetic fields. we show that, in the restricted case of pure ambipolar diffusion, neglecting weak interactions, the magnetic fields evolves towards a stable mhd equilibria configuration, in the timescales analytically expected. | magnetic field evolution in neutron stars |
modern theories of structure formation unequivocally predict that density perturbations seeded in the big bang collapse to produce``halos'' of dark matter filled with hot, virialized gas. the physics of this hot halo gas fundamentally determines the mass-scale of galaxies, and likely plays a critical role in their subsequent evolution. since this virialized halo gas is typically invisible, however, cosmological simulations have largely overlooked it, understandably focusing on more observable properties of galaxies such as their ism content and star formation histories. however, as new observational techniques begin to probe the diffuse gas in galaxy halos, they are finding results inconsistent with predictions from cosmological simulations. though halo gas is fundamental to galaxy formation, it cannot be explained with current models; halo gas thus represents the new frontier in testing and advancing our models of galaxy formation. one particularly surprising development has been the near-ubiquitous finding that galaxy halos are full of tiny, dense clouds of neutral gas. in a recent paper (mccourt et al 2016), we show that these unexpected observations imply that galaxies contain an enormous number of tiny cloudlets, dispersed throughout the halo like the water droplets in a fog. we detail a new hydrodynamical process, which we call ``shattering,'' that explains the tiny characteristic size for these cloudlets. while we can explain many observable properties of this cold gas (such as its broad line-width and tiny volume-filling fraction), we treated the amount of cold gas as a free parameter; this is fundamentally determined by galaxy formation rather than gas dynamics. this proposal extends the work of mccourt et al (2016) by focusing on the origin of the cold gas in galaxy halos. since cold gas represents the fuel for star formation and feedback in galaxies, this question is crucial for studies of galaxy evolution. we consider two possibilities: 1) that cool cgm gas is expelled from the galaxy disk in large-scale outflows, or 2) that it is produced in-situ by thermal instability. in both cases, we focus on observational tests of our model, and on methods to incorporate our results into future cosmological simulations via a sub-grid model. additional science results will include understanding the unexplained entrainment of cold gas in galactic winds, as well as understanding the surprisingly strong magnetic fields seen in galaxy halos at low redshift, which likely dominate over thermal pressure in halo gas. to our knowledge, no models currently exist for either of these results. the work outlined in this proposal focuses on recent observations which cannot yet be reproduced in cosmological simulations. as part of our proposed work, we will produce a sub-grid model for unresolved cold clouds in hydrodynamics, and will determine the resolution needed to reproduce these effects in future cosmological simulations. our work is timely and represents the necessary next step in advancing our theories of the cgm. | the origin and survival of cold gas in hot halos |
in this paper, we present the numerical simulation for the steady and unsteady incompressible viscous magnetohydrodynamic (mhd) rotating flow over a rotating sphere near the equator and investigate the effect of the ratio of the angular velocities of the sphere and the fluid, the suction/injection and magnetic field on the flow characteristics. the continuity equations for this problem give rise to a system of nonlinear partial differential equations which have been solved with two radial basis function (rbf) methods: the mixed ritz and kansa method and the rbf-hfd method. the results will be useful in the temporal evolution of rotating magnetic stars. also the convergence of the numerical solutions has been explicitly studied by means of the residual function which confirms that rbfs methods give a simple and accurate solution for this problem. | an improvement to the unsteady mhd rotating flow over a rotating sphere near the equator via two radial basis function schemes |
we carried out high resolution simulations of weakly-magnetized core-collapse supernovae in two-dimensional axisymmetry in order to see the influence of the magnetic field and rotation on the explosion. we found that the magnetic field amplified by magnetorotational instability (mri) has a great positive impact on the explosion by enhancing the neutrino heating, provided that the progenitor has large angular momentum close to the highest value found in stellar evolution calculations. we also found that even for progenitors neither involving strong magnetic flux nor large angular momentum, the magnetic field is greatly amplified by the convection aand rotation, and this leads to the boost of the explosion again by enhancing the neutrino heating. | magnetically assisted explosions of weakly magnetized stars |
the computational advances of the past several decades have allowed theoretical astrophysics to proceed at a dramatic pace. numerical simulations can now simulate the formation of individual molecules all the way up to the evolution of the entire universe. observational astrophysics is producing data at a prodigious rate, and sophisticated analysis techniques of large data sets continue to be developed. it is now possible for terabytes of data to be effectively turned into stunning astrophysical results. this is especially true for the field of star and planet formation. theorists are now simulating the formation of individual planets and stars, and observing facilities are finally capturing snapshots of these processes within the milky way galaxy and other galaxies. while a coherent theory remains incomplete, great strides have been made toward this goal. this dissertation discusses several projects that develop models of star and planet forma- tion. this work spans large spatial and temporal scales: from the au-scale of protoplanetary disks all the way up to the parsec-scale of star-forming clouds, and taking place in both contemporary environments like the milky way galaxy and primordial environments at redshifts of z 20. particularly, i show that planet formation need not proceed in incremental stages, where planets grow from millimeter-sized dust grains all the way up to planets, but instead can proceed directly from small dust grains to large kilometer-sized boulders. the requirements for this model to operate effectively are supported by observations. additionally, i draw suspicion toward one model for how you form high mass stars (stars with masses exceeding 8 msun), which postulates that high-mass stars are built up from the gradual accretion of mass from the cloud onto low-mass stars. i show that magnetic fields in star forming clouds thwart this transfer of mass, and instead it is likely that high mass stars are created from the gravitational collapse of large clouds. this work also provides a sub-grid model for computational codes that employ sink particles accreting from magnetized gas. finally, i analyze the role that radiation plays in determining the final masses of the first stars to ever form in the universe. these stars formed in starkly different environments than stars form in today, and the role of the direct radiation from these stars turns out to be a crucial component of primordial star formation theory. these projects use a variety of computational tools, including the use of spectral hydrodynamics codes, magneto-hydrodynamics grid codes that employ adaptive mesh refinement techniques, and long characteristic ray tracing methods. i develop and describe a long characteristic ray tracing method for modeling hydrogen-ionizing radiation from stars. additionally, i have developed monte carlo routines that convert hydrodynamic data used in smoothed particle hydrodynamics codes for use in grid-based codes. both of these advances will find use beyond simulations of star and planet formation and benefit the astronomical community at large. | star and planet formation throughout cosmic history |
this thesis presents investigations in four areas of theoretical astrophysics: the production of sterile neutrino dark matter in the early universe, the evolution of small-scale baryon perturbations during the epoch of cosmological recombination, the effect of primordial magnetic fields on the redshifted 21-cm emission from the pre-reionization era, and the nonlinear stability of tidally deformed neutron stars. in the first part of the thesis, we study the asymmetry-driven resonant production of 7 kev-scale sterile neutrino dark matter in the primordial universe at temperatures t >~ 100 mev. we report final dm phase space densities that are robust to uncertainties in the nature of the quark-hadron transition. we give transfer functions for cosmological density fluctuations that are useful for n-body simulations. we also provide a public code for the production calculation. in the second part of the thesis, we study the instability of small-scale baryon pressure sound waves during cosmological recombination. we show that for relevant wavenumbers, inhomogenous recombination is driven by the transport of ionizing continuum and lyman-alpha photons. we find a maximum growth factor less than ≈ 1.2 in 107 random realizations of initial conditions. the low growth factors are due to the relatively short duration of the recombination epoch. in the third part of the thesis, we propose a method of measuring weak magnetic fields, of order 10--19 g (or 10--21 g if scaled to the present day), with large coherence lengths in the inter galactic medium prior to and during the epoch of cosmic reionization. the method utilizes the larmor precession of spin-polarized neutral hydrogen in the triplet state of the hyperfine transition. we perform detailed calculations of the microphysics behind this effect, and take into account all the processes that affect the hyperfine transition, including radiative decays, collisions, and optical pumping by lyman-alpha photons. in the final part of the thesis, we study the non-linear effects of tidal deformations of neutron stars (ns) in a compact binary. we compute the largest three- and four-mode couplings among the tidal mode and high-order p- and g-modes of similar radial wavenumber. we demonstrate the near-exact cancellation of their effects, and resolve the question of the stability of the tidally deformed ns to leading order. this result is significant for the extraction of binary parameters from gravitational wave observations. | the astrophysics of strongly interacting systems |
the expansion of hot, dense plasma (100 ev, 1018 cm-3) into vacuum occupied by a strong magnetic field (β =pkinetic /pmag ~ 1) along the expansion axis is a seemingly elementary physics problem, yet it is one that has scarcely been investigated. as well as being a fundamental problem in plasma physics, understanding such a situation is important to provide an explanation of large-scale jets observed in the formation of young stellar objects (yso). additionally, the ability to manipulate such a situation (e.g. to optimize x-ray emission) may be essential to the feasibility of recently proposed inertial confinement fusion (icf) schemes with an imposed magnetic field. to investigate these situations, a cf2 foil is irradiated with the elfie laser (1013 w/cm2, 0.6 ns) in an external axial magnetic field of 20 t. as the plasma expands radially it is restricted by magnetic pressure that creates a cavity with a shock at the expansion edge. this shock redirects flow back on axis and creates a strong, stationary, conical shock that collimates the flow into a jet traveling over 1000 km/s and extending many centimeters. the effect of episodic heating (e.g. from variable mass ejection in a yso, or pulse shaping in icf) was investigated by irradiating the target with a precursor laser (1012 w/cm2, 0.6 ns) at 9 to 19 ns prior to the main pulse. the addition of this relatively small addition of energy (<20% of the main pulse energy) changed the dynamics of the expansion dramatically by increasing the strength of the conical shock, reducing the forward expansion of the cavity and dramatically increasing emission. we also present mhd simulations that reproduce the experimental observables and help to understand dynamics of jet and cavity formation. prepared by llnl under contract de-ac52-07na27344. presently at lawrence livermore national laboratory. | laboratory study of the shaping and evolution of magnetized episodic plasma jets |
high resolution observations of young stellar object (yso) jets show them to be composed of many small-scale knots or clumps. 2-d and 3-d numerical simulations were conducted with the code astrobear to study how such clumps interact and create morphologies and kinematic patterns seen in emission line observations. two main classes of simulations were used in this study: outflows of spherical, over-dense clumps, and pulsed jets in which the pulsations create clumps within the jet. such flows lead to the formation of bow shocks which then interact with each other as faster material overtakes slower material. we show that much of the spatial structure apparent in emission line images of jets arises from the dynamics and interactions of these bow shocks. the simulations show a variety of time-dependent features, including bright knots associated with mach stems where the shocks intersect, a "frothy" emission structure that arises from the presence of the non-linear thin shell instability (ntsi) along the surfaces of the bow shocks, and the merging and fragmentation of clumps. simulations with magnetic fields show how the field affects the dynamics of yso jets and the emission they produce. this work contributes to the ultimate goal of one day being able to observationally estimate the strength of the magnetic field within these jets. the simulations use a new non-equilibrium cooling method to produce synthetic emission maps in h? and [s ii]. these are directly compared with multi-epoch hubble space telescope (hst) observations of herbig-haro (hh) jets. there is excellent agreement between features seen in the simulations and the observations in terms of both proper motion and morphologies. thus, yso jets may be dominated by heterogeneous structures, and interactions between these structures and the shocks they produce can account for many details of yso jet evolution. | outflows from young stellar objects: bringing numerical simulations closer to observations of herbig-haro objects |
with the recent discovery of gravitational waves from the merger of two black holes, its especially important to understand the electromagnetic signals resulting from mergers of compact objects. new simulations successfully follow a merger of two neutron stars that produces a short burst of energy via a jet consistent with short gamma-ray burst (sgrb) detections.still from the authors simulation showing the two neutron stars, and their magnetic fields, before merger. [adapted from ruiz et al. 2016]challenging systemwe have long suspected that sgrbs are produced by the mergers of compact objects, but this model has been difficult to prove. one major hitch is that modeling the process of merger and sgrb launch is very difficult, due to the fact that these extreme systems involve magnetic fields, fluids and full general relativity.traditionally, simulations are only able to track such mergers over short periods of time. but in a recent study, milton ruiz (university of illinois at urbana-champaign and industrial university of santander, colombia) and coauthors ryan lang, vasileios paschalidis and stuart shapiro have modeled a binary neutron star system all the way through the process of inspiral, merger, and the launch of a jet.a merger timelinehow does this happen? lets walk through one of the teams simulations, in which dipole magnetic field lines thread through the interior of each neutron star and extend beyond its surface(like magnetic fields found in pulsars). in this example, the two neutron stars each have a mass of 1.625 solar masses.simulation start (0 ms)loss of energy via gravitational waves cause the neutron stars to inspiral.merger (3.5 ms)the neutron stars are stretched by tidal effects and make contact. their merger produces a hypermassive neutron star that is supported against collapse by its differential (nonuniform) rotation.delayed collapse into a black hole (21.5 ms)once the differential rotation is redistributed by magnetic fields and partially radiated away in gravitational waves, the hypermassive neutron star loses its support and collapses to a black hole.plasma velocities turn around (51.5 ms)initially the plasma was falling inward, but as the disk of neutron-star debris is accreted onto the black hole, energy is released. this turns the plasma near the black hole poles around and flings it outward.magnetic field forms a helical funnel (62.5 ms)the fields near the poles of the black hole amplify as they are wound around, creating a funnel that provides the wall of the jet.jet outflow extends to heights greater than 445 km (64.5 ms)the disk is all accreted and, since the fuel is exhausted, the outflow shuts off (within 100ms)neutron-star successplot showing the gravitational wave signature for one of the authors simulations. the moments of merger of the neutron stars and collapse to a black hole are marked. [adapted from ruiz et al. 2016]these simulations show that no initial black hole is needed to launch outflows; a merger of two neutron stars can result in an sgrb-like jet. another interesting result is that the magnetic field configuration doesnt affect the formation of a jet: neutron stars with magnetic fields confined to their interiors launch jets as effectively as those with pulsar-like magnetic fields. the accretion timescale for both cases is consistent with the duration of an sgrb.while this simulation models milliseconds of real time, its enormously computationally challenging and takes months to simulate. the successes of this simulation represent exciting advances in numerical relativity, as well as in our understanding of the electromagnetic counterparts that may accompany gravitational waves.bonuscheck out this awesome video of the authors simulations. the colors differentiate the plasma density and the white lines depict the pulsar-like magnetic field that initially threads the two merging neutron stars. watch as the neutron stars evolve through the different stages outlined above, eventually forming a black hole and launching a powerful jet.[simulations and visualization by m. ruiz, r. lang, v. paschalidis, s. shapiro and the illinois relativity group reu team: s. connelly, c. fan, a. khan, and p. wongsutthikoson]citationmilton ruiz et al 2016 apj 824 l6. doi:10.3847/2041-8205/824/1/l6 | jets from merging neutron stars |
the x-ray spectra of central compact objects (ccos) with weak magnetic fields, and quiescent magnetars with strong magnetic fields, are very similar despite the four orders of magnitude difference in their dipole magnetic field strengths. for cco pulsars, the apparently localized surface emission indicated by their large pulsed modulation is difficult to reconcile with a surface b-field of only 3.e10 g. recent work suggests that magnetar-strength b-fields might lie buried in ccos by accretion during the formation of the ns. we propose to model the x-ray pulse profiles and phase-resolved spectra of ccos to infer their surface temperature distributions, which must be controlled by their magnetic field geometry. having measured the spin-down rates for all three cco pulsars, we are now in a position to model their phase-dependent spectra using nearly 2 million seconds of x-ray data available in the archive. we seek funding to refine and generalize our modeling of these data sets, expanding upon our methods that were previously applied to the transient anomalous pulsar axp j1810-197 and the cco psr j0821- 4300 in puppis a. by incorporating new theoretical results on the magnetic field structure of nss and its evolution, we will infer possible magnetic field topologies. magnetothermal simulations suggest that the observed properties of ccos may be explained by strong toroidal field components in the ns crust. probing the internal and external magnetic fields will provide important insight into the populations and evolution of young neutron stars, and the possible unification of their different classes. | probing the magnetic structure of young neutron stars |
we propose to model magnetized gas as it flows into galaxy disks in milky way-like and redshift 2 environments in order to understand the pc to kpc scale physics that control a crucial link in galaxy evolution: how do galaxies get the gas which sustains star formation over cosmic time? uv observations with the cosmic origins spectrograph (cos) on hst have demonstrated that star-forming galaxies have baryonic halos much more massive than the galaxies themselves; these halos are most likely a link in the evolution of galaxies as cosmological filaments feed ongoing star formation in galactic disks. however, the galaxy formation simulations that support this hypothesis do not resolve the parsec-scale hydrodynamic processes which determine if and how the gas in the halo can reach the disk. to address this theoretical disconnect, we will conduct magnetohydrodynamic simulations in which these clouds fall under the galactic potential into a state-of-the-art simulation of the three-phase interstellar medium in the galactic disk. we will leverage recent hst and radio observations of accreting clouds around the milky way to set the initial conditions of the gas, including magnetic fields and metallicity. our results will connect the hst metallicity measurements directly to the impact of gaseous galactic halos and infall on galaxy evolution and the star formation history of the universe. | gaseous infall and star formation from redshift 2 to the milky way |
atmospheric escape is capable of shaping a planet's atmospheric composition and total mass, and thus the planet's long-term habitability. loss to space of atmospheric particles has played a key role in the atmospheric evolution of both mars and venus. intrinsic planetary magnetic fields like the earth's have long been thought to shield planets from atmospheric erosion via stellar winds; however, recent arguments have suggested that a magnetic field will increase the interaction area with the solar wind, collecting correspondingly more energy that can be used to drive increased escape. using a set of global three-dimensional hybrid plasma simulations validated via observations at mars and venus, we find that neither of these paradigms are complete descriptions. rather than solely inhibiting or driving ion escape, there is a value of magnetic field strength associated with maximum ion outflow. for weaker magnetic fields, ion escape is enhanced due to shielding of the southern hemisphere from ``misaligned" ion pickup forces. for stronger magnetic fields ion escape decreases due to trapping associated with closed magnetic field lines. the peak escape rate occurs where the intrinsic magnetosphere (caused by the planetary magnetic field) reaches the induced magnetosphere (caused by ionospheric conductivity). as the size of the intrinsic magnetosphere is determined by pressure balance between the incoming solar wind and the planetary magnetic field, the magnetic field associated with peak escape is critically dependent on the solar wind pressure. where possible we have fit power laws for the variation of fundamental parameters (escape rate, escape power, polar cap opening angle and effective interaction area) with magnetic field, and assessed upper and lower limits for the relationships. such power laws can be used in generalized studies of atmospheric escape and potential habitability to better characterize a wide variety of systems. | do magnetic fields prevent atmospheric escape? |
with the observation of a kilonova signal following the detection of the binary neutron star merger gw170817, the need for an extended duration simulation of the post-merger environment has become important in order to determine the effects of the stability of the remnant by secular processes on any observed signals. in order to extend simulation times in the spectral einstein code (spec), we have developed a modification to spec's multipatch coordinate transformation implementation to allow for easily running a post-merger simulation in axisymmetry without the need to explicitly rewrite evolved equations in an axisymmetric form. several tests using simple equilibrium systems with magnetic fields, neutrino transport, and viscosity have shown the viability of this method and demonstrated a significant decrease in required computational resources versus a full 3d simulation. nsf. | an implementation of axisymmetry in numerical relativity using a multipatch scheme |
the advance in both instrumentation and numerical simulation techniques in the past decade have unfolded the importance of magnetic fields in regulating filamentary clouds formation, evolution, and physical properties of star-forming processes in molecular clouds. in particular, there are emerging interests on the effects of cloud-field orientations on both morphological and kinematics conditions in molecular clouds. here we present the analysis of the spatial mass distribution and mass distributions in 12 nearby molecular clouds (d < 500pc) column density maps. we show that molecular clouds with long axis orientations align with the magnetic field direction have more even mass distribution, a lower fraction of mass resides in higher column density and a higher column density probability density function(n-pdf) transition density. these findings are consistent with the recently proposed model that magnetic fields perpendicular to clouds will possess a higher magnetic flux, and thus can hinder the cloud fragmentation more efficiently from mass accumulations, and a higher density barrier for the cloud to fragment. | links between magnetic fields and molecular cloud fragmentation: bimodal mass distribution and density structures. |
we explore the effect of galactic evolution on the rotation of giant molecular clouds (gmcs) in isolated magnetized galaxy simulation. in this model, without prominent structures, we have extracted about 1000 isolated clouds. the properties (mass, size, velocity dispersion) and scaling relations of these clouds consistent with that found for the milky way and nearby galaxies. by making an analysis of the velocity field of each isolated gmc we found that clouds itself has a substantial linear velocity gradient - ranging from 0.01 to 0.1 km s^{-1} pc^{-1} which is a function of galactocentric distance. | velocity gradients of giant molecular clouds at galactic scales |
the process of star formation is fundamental to understand planet formation and galaxy evolution. almost all stars form in clusters embedded in massive filaments. we present an analysis of the gas kinematics of the integral shaped filament in orion a using different molecular transitions (12co, 13co, nh3, n2h+) to trace different gas densities. we describe the velocity structure along the filament with intensity-weighted position-velocity diagrams and compare it with the mass distribution of stars and gas. we find a north to south velocity gradient that terminates with a blue-shifted velocity peak seen in all tracers at ~ 7km/s, close to the center of the orion nebula cluster (onc). the blue peak has been previously proposed to originate to originate either from the gravitational collapse of the onc or the wave-like nature of the gas filament. we find that this blue peak appears offset form the peak of the mass distribution and position of the onc, suggesting that the action of forces other than gravity, such as magnetic fields, shape the observed velocity structure and wave-like structure. we characterize the global kinematics of the filament with velocity ridgelines of each tracer. we find that in the northern portion of the filament the ridgelines agree within ~0.5km/s, but observe significant deviations in the cluster region. we show that near the center of the cluster, the gas velocities depend on the molecular tracer. we describe the linewidths of the four tracers with velocity dispersion radial profiles, following the center of the filament. we find that the non-thermal velocity dispersions strongly depend on the tracer, that is, the higher density gas (nh3, n2h+) is kinematically colder than the lower density gas (12co, 13co). we compute the specific kinetic energy profile from the linewidths and compare with the gravitational potential profiles of the filament and cluster. we find that the cloud is gravitationally bound as velocities are insufficient to surpass the gravitational potential. this implies that either the cloud is undergoing gravitational collapse or that other forces, such as magnetic fields and rotation, are required to reproduce observed star formation efficiencies. we report, for the first time, the presence of two velocity components present in the full 12co position-velocity diagram, suggesting that the filament is rotating with an angular velocity of omega = 1.4/myr. the velocity structure of 12co presents velocity fluctuations in the form of 6 velocity peaks with regular separations (~0.44 pc spacing in projection in declination). comparison of these peaks positions to young stellar objects and protostar catalogs reveals that 3/6 peaks cannot be ruled out by outflows, while the other 3 appear unassociated to known (proto)stellar sources. the separation of the peaks has a characteristic timescale of ~1 myr. our analysis suggests that the cluster formation scenario is more complex than previously thought. we suggest that the action of magnetic fields impacts the kinematic features of the filament. this hypothesis can be tested with magnetic field observations and cluster formation simulations. | velocity structure of the orion a integral shaped filament |
the interaction between galaxies and the surrounding gas plays a key role in galaxy formation and evolution. feedback processes driven by star formation and active galactic nuclei facilitate the exchange of mass and energy between the galaxy and the circumgalactic medium through inflowing and outflowing gas. these outflows have a significant impact on the star formation rate and metallicity of the galaxy. observations of outflows have provided evidence that these outflows are multi-phase in nature, identifying both low energy ions such as mg ii and c iii and high energy ions such as o vi. the underlying physics maintaining the two phases as well as the ionization mechanism for these phases remains unclear. in order to better understand galactic outflows, hydrodynamic simulations are used to study the evolution of wind-cloud interactions. in this work, i carried out a suite of magnetohydrodynamic simulations to characterize the influence of magnetic fields on the evolution and lifetime of cold clouds. i found magnetic fields either provided little improvement to cloud stability over other influences such as radiative cooling or accelerated cloud disruption by pushing cloud material in the direction orthogonal to the wind and magnetic fields. to investigate the ionization mechanism of the material within outflows i first considered estimating the column densities of various ions within wind-cloud simulations with the post-processing tool trident. under the assumption of ionization equilibrium, the simulations did not reproduce the observed absorption profiles demonstrating the need for a more detailed treatment of the ionization processes. i then performed a new set of simulations with the non-equilibrium chemistry solver, maihem. the column densities produced in the non-equilibrium model alter the evolution of the cloud and highlight the increased ionization along the boundary of the cloud. | evolution, disruption, and composition of galactic outflows around starburst galaxies |
stable magnetic fields have been observed in stars of a wide range of ages, from intermediate-mass stars in the main-sequence up to degenerate stars. what is the equilibrium configuration of the magnetic field inside these stars and which conditions allow it to remain stable over the star's lifetime are still open questions. it has been formally demonstrated that purely toroidal or poloidal magnetic fields develop instabilities at some point in the star. previous numerical works 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 map out the parameter space under which magnetic field configurations remain stable, we have performed 3d-magnetohydrodynamic simulations of the evolution of magnetic field in stably stratified and barotropic stars with the pencil code, a high-order finite-difference code for compressible hydrodynamic flows with magnetic fields. 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 identified the parameter space under which the configuration evolves to a stable equilibrium. | stellar magnetic equilibria with the pencil code |
type-i superluminous supernovae (slsne) are an exotic class of core-collapse sn (ccsn) that can be up to 100 times brighter and more slowly-evolving than normal ccsne. slsne represent the end-stages of the most massive stripped stars, and are thought to be powered by the spin-down energy of a millisecond magnetar. studying them and measuring their physical parameters can help us to better understand stellar mass-loss, evolution, and explosions. moreover, thanks to their high luminosities, slsne can be seen up to greater distances, allowing us to explore how stellar physics evolves as a function of redshift. the high latitude time domain survey (hltds) will provide us with an exquisite dataset that will discover 100s of slsne. here, we focus on the question of which sets of filters and cadences will allow us to best characterize the physical parameters of these slsne. we simulate a set of slsne at redshifts ranging from z = 0.1 to z = 5.0, using six different sets of filters, and cadences ranging from 5 to 100 days. we then fit these simulated light curves to attempt to recover the input parameter values for their ejecta mass, ejecta velocity, magnetic field strength, and magnetar spin period. we find that four filters are sufficient to accurately characterize slsne at redshifts below $z = 3$, and that cadences faster than 20 days are required to obtain measurements with an uncertainty below 10\%, although a cadence of 70 days is still acceptable under certain conditions. finally, we find that the nominal survey strategy will not be able to properly characterize the most distant slsne at $z = 5$. we find that the addition of 60-day cadence observations for 4 years to the nominal hltds survey can greatly improve the prospect of characterizing these most extreme and distant sne, with only an 8\% increase to the time commitment of the survey. | roman ccs white paper: characterizing superluminous supernovae with roman |
we explore the collapsar scenario for long gamma-ray bursts by performing axisymmetric neutrino-radiation magnetohydrodynamics simulations in full general relativity for the first time. in this paper, we pay particular attention to the outflow energy and the evolution of the black-hole spin. we show that for a strong magnetic field with an aligned field configuration initially given, a jet is launched by magnetohydrodynamical effects before the formation of a disk and a torus, and after the jet launch, the matter accretion onto the black hole is halted by the strong magnetic pressure, leading to the spin-down of the black hole due to the blandford-znajek mechanism. the spin-down timescale depends strongly on the magnetic-field strength initially given because the magnetic-field strength on the black-hole horizon, which is determined by the mass infall rate at the jet launch, depends strongly on the initial condition, although the total jet-outflow energy appears to be huge $>10^{53}$ erg depending only weakly on the initial field strength and configuration. for the models in which the magnetic-field configuration is not suitable for quick jet launch, a torus is formed and after a long-term magnetic-field amplification, a jet can be launched. for this case, the matter accretion onto the black hole continues even after the jet launch and black-hole spin-down is not found. we also find that the jet launch is often accompanied with the powerful explosion of the entire star with the explosion energy of order $10^{52}$ erg by magnetohydrodynamical effects. we discuss an issue of the overproduced energy for the early-jet-launch models. | outflow energy and black-hole spin evolution in collapsar scenarios |
we use high-resolution mhd simulations of isolated disk galaxies to investigate the co-evolution of magnetic fields with a self-regulated, star-forming interstellar medium (ism). the simulations are conducted using the ramses amr code on the standard agora initial condition, with gas cooling, star formation and feedback. we run galaxies with a variety of initial magnetic field strengths. the fields grow rapidly and achieve approximate saturation within 500 myr, but at different levels. the galaxies reach a quasi-steady state, with slowly declining star formation due to both gas consumption and increases in the field strength at intermediate ism densities. we connect this behaviour to differences in the gas properties and overall structure of the galaxies. in particular, strong fields limit feedback bubbles. different cases support the ism using varying combinations of magnetic pressure, turbulence and thermal energy. magnetic support is closely linked to stellar feedback in the case of initially weak fields but not for initially strong fields. the spatial distribution of these supports is also different in each case, and this is reflected in the stability of the gas disk. we relate this back to the overall distribution of star formation in each case. we conclude that a weak initial field can grow to produce a realistic model of a local disk galaxy, but starting with typical field strengths will not. | regulating star formation in a magnetized disk galaxy |
the angular momentum of molecular cloud cores plays a key role in the star formation process. however, the evolution of the angular momentum of molecular cloud cores formed in magnetized molecular filaments is still unclear. in this paper, we perform three-dimensional magnetohydrodynamics simulations to reveal the effect of the magnetic field on the evolution of the angular momentum of molecular cloud cores formed through filament fragmentation. as a result, we find that the angular momentum decreases by 30% and 50% at the mass scale of 1msun in the case of weak and strong magnetic field, respectively. by analyzing the torques exerted on fluid elements, we identify the magnetic tension as the dominant process for angular momentum transfer for mass scales < 3msun for the strong magnetic field case. this critical mass scale can be understood semi-analytically as the timescale of magnetic braking. we show that the anisotropy of the angular momentum transfer due to the presence of strong magnetic field changes the resultant angular momentum of the core only by a factor of two. we also find that the distribution of the angle between the rotation axis and the magnetic field does not show strong alignment even just before the first core formation. our results also indicate that the variety of the angular momentum of the cores are inherited from the difference of the phase of the initial turbulent velocity field. the variety could contribute to the diversity in size and other properties of protoplanetary disks recently reported by observations. | evolution of the angular momentum of molecular cloud cores in magnetized molecular filaments |
the cosmological 21-cm signal from neutral hydrogen, which is considered as a promising tool, is being used to observe and study the cosmic dawn (cd) and epoch of reionization (eor). a significant part of this thesis focuses on the semi-analytical modeling of the global hi 21-cm signal from cd considering several physical processes. further, it investigates the nature of galaxies that dominate during cd and eor using current available observations. in our work, we study the redshift evolution of the primordial magnetic field (pmf) during the dark ages and cosmic dawn, and prospects of constraining it in light of edges 21-cm signal in the `colder igm' background. we find that the igm heating rate due to the pmf enhances compared to the standard scenario. however, pmf is an unlikely candidate for explaining the rise of the edges absorption signal at lower redshift. we further consider, in detail, the heating of the igm owing to cosmic ray protons generated by the supernovae from both early pop~iii and pop~ii stars. we show that the edges signal can be well fitted by the cosmic ray heating along with the lyman-$\alpha$ coupling and the dark matter-baryon interaction. we, further, explore the conditions by which the edges detection is consistent with current reionization and post-reionization observations. by coupling a physically motivated source model derived from radiative transfer hydrodynamic simulations of reionization to a mcmc sampler, we find that high contribution from low-mass halos along with high photon escape fractions are required to simultaneously reproduce the existing constraints. with the extreme effort in building more advanced and sophisticated telescopes, the future 21-cm signal detection would be able to provide better constraints on the amplitude of pmf and the efficiencies on cosmic ray protons, and consequently on early star formation rates. | unveiling the cosmic dawn and epoch of reionization using cosmic 21-cm signal |
we study the dynamics of stellar wind from one of the bodies in the binary system, where the other body interacts only gravitationally. we focus on following three issues: (i) we explore the origin of observed periodic variations in maser intensity; (ii) we address the nature of bipolar molecular outflows; and (iii) we show generation of baroclinicity in the same model setup. from direct numerical simulations and further numerical modelling, we find that the maser intensity along a given line of sight varies periodically due to periodic modulation of material density. this modulation period is of the order of the binary period. another feature of this model is that the velocity structure of the flow remains unchanged with time in late stages of wind evolution. therefore the location of the masing spot along the chosen sightline stays at the same spatial location, thus naturally explaining the observational fact. this also gives an appearance of bipolar nature in the standard position-velocity diagram, as has been observed in a number of molecular outflows. remarkably, we also find the generation of baroclinicity in the flow around binary system, offering another site where the seed magnetic fields could possibly be generated due to the biermann battery mechanisms, within galaxies. | dynamics of stellar wind in a roche potential: implications for (i) outflows & periodicities relevant to astronomical masers, and (ii) generation of baroclinicity |
rotating plasmas are found in astrophysical phenomena such as magnetized neutron stars and pulsar electrospheres, as well as in laboratory experiments such as pure electron/ion plasmas trapped in penning-malmberg (pm) traps and fusion plasmas. in many of these cases, external velocity shear or velocity strain flows affect the dynamics of the rotating plasmas. non-neutral (pure electron/ion) plasmas are routinely investigated to study the dynamics of the inviscid fluid vortices due to the isomorphism between the 2d inviscid incompressible euler equations and the governing dynamics of the electron plasmas at mass-less limit (m_e→0). in the earlier experimental and numerical investigations[1], evolution of a 2d vortex under external non-axisymmetric quadrupole strain has been studied using low density electron plasmas in the pm trap, confined due to an axial magnetic field and a radial electric field created by the 8-segment trap[1]. in the present study, we have numerically investigated pure electron plasmas under experiment-like conditions using 2d2v particle-in-cell (pic) simulations with an existing pic code pec2pic[2], developed and maintained at the institute for plasma research, india. in the first part, we present results that qualitatively and quantitatively benchmark the code against the experiments[1] and show that in agreement with previous work, there is a critical value of the quadrupole strain (relative to the vorticity of the plasma) beyond which the low density electron plasma vortex is destroyed[1]. in the second part, we have explored the dynamics of high density electron plasma vortices under the effect of external velocity strain flow. we have also investigated the vortex dynamics in the presence of other strain flow geometries and time-varying strains. we report several new findings. references: [1] n. c. hurst et al. physical review letters 117, 235001 (2016). [2] m. sengupta and r. ganesh, physics of plasmas 21, 022116 (2014). | electron plasma vortices under external strain - a study using 2d particle-in-cell simulations |
the injection of energy and momentum into the interstellar medium by young massive stars' intense radiation fields and their fast, radiatively driven winds can have a profound influence on their formation and environment. massive star forming regions are rare and highly obscured, making the early moments of their formation difficult to observe. instead, we must turn to theory to elucidate the physics involved in the formation of massive stars and massive star clusters (mscs), which can host thousands of massive stars. in my thesis, i developed analytical and numerical techniques to study the formation of massive stars and how stellar wind feedback affects the dynamics of gas that surrounds mscs. to estimate the initial rotation rates of massive stars at birth, i developed a protostellar angular momentum evolution model for accreting protostars to determine if magnetic torques can spin down massive stars during their formation. i found that magnetic torques are insufficient to spin down massive stars due to their short formation times and high accretion rates. radiation pressure is likely the dominate feedback mechanism regulating massive star formation. therefore detailed simulation of the formation of massive stars requires an accurate treatment of radiation. for this purpose, i developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. with this new tool, i performed a suite of three-dimensional adaptive mesh refinement radiation-hydrodynamic simulations of the formation of massive stars from collapsing massive pre-stellar cores. i found that mass is channeled to the massive star via dense infalling filaments that are uninhibited by radiation pressure and gravitational and rayleigh-taylor instabilities. to determine the importance of stellar wind feedback in young mscs, i used observations to constrain a range of kinetic energy loss channels for the hot gas produced by the shock- heating of stellar winds to explain the low x-ray luminosities observed in hii regions. i demonstrated that the energy injected by stellar winds is not a significant contributor to stellar feedback in young mscs. | the destructive birth of massive stars & massive star clusters |
understanding how the baryonic physics affects the formation and evolution of galax- ies is one of the most critical questions in modern astronomy. significant progress in understanding stellar feedback and modeling them explicitly in simulations have made it possible to reproduce a wide range of observed galaxy properties. however, there are still various pieces of missing physics and uncertainties in galaxies of different mass range.in this thesis, | will explore these missing pieces in baryonic physics on top of the feedback in realistic environments (fire) stellar feedback in the cosmological hydrodynamic zoom-in simulations (fire-2 suite) and isolated galaxy simulations. these high-resolution simulations with fire physics capture multi-phase realistic interstellar medium (ism) with gas cooling down to 10k, and star formations in dense clumps in giant molecular clouds. they are, therefore, an ideal tool for investigating the missing pieces in baryonic physics.in the first part of the thesis, chapter 2, i will focus on the discrete effects of stellar feedback like individual supernovae, hypernovae, and initial mass function (imf) sampling in dwarfs (109 ~ 1010 m⊙). these discrete processes of stellar feedback can have maximum effects on the small galaxies without being averaged out. i will show that the discretization of supernovae (sne) is absolutely necessary, while the effects from imf sampling and hypernovae (hne) is not apparent, due to the strong clustering nature of star formation.in the second part of the thesis, chapter 3 - 4, i will focus on fluid microphysics, exploring their effects on galaxy properties and their interplay with stellar feedback in sub-l* galaxies. i will demonstrate that, once the stellar feedback is explicitly implemented as fire stellar feedback model, fluid microphysics such as magnetic fields, conduction, and viscosity only have minor effects on the galaxy properties like star formation rate (sfr), phase structure, or outflows. stellar feedback also strongly alters the amplifications and morphology of the magnetic fields, resulting in much more randomly-oriented field lines. however, despite the stellar feedback, the amplification of magnetic fields in ism gas is primarily dominated by flux-freezing compression.in the final part of my thesis, i focus on the massive cluster ellipticals of 1014 ~ 1014 m⊙, where the physical mechanisms that regulate the observation-inferred cool-ing flows are highly uncertain — the classic "cooling flow problem". i showed that solutions in the literature not associated with an active galactic nucleus (agn), including stellar feedback, the cosmic ray from stellar feedback, magnetic fields, conduction, and morphological quenching, cannot possibly quench the galaxies, mostly because of the insufficient energy and the limited size of the affected region. after ruling out the non-agn feedback solutions to the cooling flow problem, i will go into the most accessible, and perhaps promising solution: "agn feedback", ex- ploring the generic classes of agn feedback models proposed in the literature. tam going to show that enhancing turbulence and injecting cosmic ray are probably the most important aspects of agn feedback in galaxy quenching. since they provide non-thermal pressure support that stably suppresses the core density, they can stably reduce the cooling flows without overheating the galactic cores. | stellar feedback, agn feedback and fluid microphysics in galaxy evolution |
galaxy clusters are harsh environments for their constituent galaxies. a variety of physical processes effective in these dense environments transform gas-rich, spiral, star-forming galaxies to elliptical or spheroidal galaxies with very little gas and therefore minimal star formation. the consequences of these processes are well understood observationally. galaxies in progressively denser environments have systematically declining star formation rates and gas content. however, a theoretical understanding of of where, when, and how these processes act, and the interplay between the various galaxy transformation mechanisms in clusters remains elusive. in this dissertation, i use numerical simulations of cluster mergers as well as galaxies evolving in quiescent environments to develop a theoretical framework to understand some of the physics of galaxy transformation in cluster environments. galaxies can be transformed in smaller groups before they are accreted by their eventual massive cluster environments, an effect termed `pre-processing'. galaxy cluster mergers themselves can accelerate many galaxy transformation mechanisms, including tidal and ram pressure stripping of galaxies and galaxy-galaxy collisions and mergers that result in reassemblies of galaxies' stars and gas. observationally, cluster mergers have distinct velocity and phase-space signatures depending on the observer's line of sight with respect to the merger direction. using dark matter only as well as hydrodynamic simulations of cluster mergers with random ensembles of particles tagged with galaxy models, i quantify the effects of cluster mergers on galaxy evolution before, during, and after the mergers. based on my theoretical predictions of the dynamical signatures of these mergers in combination with galaxy transformation signatures, one can observationally identify remnants of mergers and quantify the effect of the environment on galaxies in dense group and cluster environments. the presence of long-lived, hot x-ray emitting coronae observed in a large fraction of group and cluster galaxies is not well-understood. these coronae are not fully stripped by ram pressure and tidal forces that are efficient in these environments. theoretically, this is a fascinating and challenging problem that involves understanding and simulating the multitude of physical processes in these dense environments that can remove or replenish galaxies' hot coronae. to solve this problem, i have developed and implemented a robust simulation technique where i simulate the evolution of a realistic cluster environment with a population of galaxies and their gas. with this technique, it is possible to isolate and quantify the importance of the various cluster physical processes for coronal survival. to date, i have performed hydrodynamic simulations of galaxies being ram pressure stripped in quiescent group and cluster environments. using these simulations, i have characterized the physics of ram pressure stripping and investigated the survival of these coronae in the presence of tidal and ram pressure stripping. i have also generated synthetic x-ray observations of these simulated systems to compare with observed coronae. i have also performed magnetohydrodynamic simulations of galaxies evolving in a magnetized intracluster medium plasma to isolate the effect of magnetic fields on coronal evolution, as well the effect of orbiting galaxies in amplifying magnetic fields. this work is an important step towards understanding the effect of cluster environments on galactic gas, and consequently, their long term evolution and impact on star formation rates. | simulating the dynamical evolution of galaxies in group and cluster environments |
during common envelope evolution, an initially weak magnetic field may undergo amplification by interacting with spiral density waves and turbulence generated in the stellar envelope by the inspiralling companion. using 3d magnetohydrodynamical simulations on adaptively refined spherical grids with excised central regions, we studied the amplification of magnetic fields and their effect on the envelope structure, dynamics, and the orbital evolution of the binary during the post-dynamical inspiral phase. about $95\%$ of magnetic energy amplification arises from magnetic field stretching, folding, and winding due to differential rotation and turbulence while compression against magnetic pressure accounts for the remaining $\sim 5\%$. magnetic energy production peaks at a scale of $3a_\text{b}$, where $a_\text{b}$ is the semimajor axis of the central binary's orbit. because the magnetic energy production declines at large radial scales, the conditions are not favorable for the formation of magnetically collimated bipolar jet-like outflows unless they are generated on small scales near the individual cores, which we did not resolve. magnetic fields have a negligible impact on binary orbit evolution, mean kinetic energy, and the disk-like morphology of angular momentum transport, but turbulent maxwell stress can dominate reynolds stress when accretion onto the central binary is allowed, leading to an $\alpha$-disk parameter of $\simeq 0.034$. finally, we discovered accretion streams arising from the stabilizing effect of the magnetic tension from the toroidal field about the orbital plane, which prevents overdensities from being destroyed by turbulence and enables them to accumulate mass and eventually migrate toward the binary. | post-dynamical inspiral phase of common envelope evolution. the role of magnetic fields |
charged particles are constantly accelerated to non-thermal energies by the reconnecting magnetic field in the solar atmosphere. our understanding of the interactions between the particles and their environment can benefit from three-dimensional atmospheric simulations accounting for non-thermal particle beams. in a previous publication, we presented the first results from such a simulation. however, the original treatment of beam propagation ignores potentially important phenomena. here, we present a more general beam propagation model incorporating magnetic gradient forces, the return current, acceleration by the ambient electric field, and temperature-dependent collision rates. neglecting collisional velocity randomisation makes the model sufficiently lightweight to simulate millions of beams. we investigate how each new physical effect changes beam energy transport in a three-dimensional atmosphere. we applied the method of characteristics to the steady-state continuity equation for electron flux to derive ordinary differential equations for the mean evolution of energy, pitch angle, and flux with distance. for each beam, we solved these numerically for a range of initial energies to obtain the evolving flux spectrum, from which we computed the energy deposited into the ambient plasma. magnetic gradient forces significantly influence the deposition of beam energy. the strong magnetic field convergence leads to a small coronal deposition peak followed by a heavy dip caused by the onset of magnetic mirroring. mirrored electrons carry away 5 to 10% of the injected beam energy on average. the transition region peak produced by the remaining energetic electrons occurs slightly deeper than in a uniform magnetic field. an initial diverging magnetic field enhances the subsequent impact of mirroring. the other new physical effects are less significant for the studied atmospheric conditions. | accelerated particle beams in a 3d simulation of the quiet sun. effects of advanced beam propagation modelling |
coronal mass ejections (cmes) are solar eruptions of plasma and magnetic fields that significantly impact space weather, causing disruptions in technological systems and potential damage to power grids when directed towards earth. traditional coronagraphs along the sun-earth line struggle to precisely track the early evolution of earth-directed cmes. coronal dimmings, localized reductions in extreme-ultraviolet (euv) and soft x-ray emissions, are key indicators of cmes in the low corona, resulting from mass loss and expansion during the eruption. this study introduces a novel method, direcd (dimming inferred estimate of cme direction), to estimate the early propagation direction of cmes based on the expansion of coronal dimmings. the approach involves 3d simulations of cmes using a geometric cone model, exploring parameters like width, height, source location, and deflection from the radial direction. the dominant direction of dimming evolution is then determined, and an inverse problem is solved to reconstruct an ensemble of cme cones at various heights, widths, and deflections. by comparing the cme orthogonal projections onto the solar sphere with the dimming geometry, the 3d cme direction is derived.validated through case studies on october 1, 2011, and september 6, 2011, the direcd method reveals the early propagation directions of cmes. the cme on october 1, 2011, predominantly expands towards the south-east, while the cme on september 6, 2011, inclines towards the north-west. these findings align with previous studies using multi-viewpoint coronagraphic observations. the study demonstrates the utility of coronal dimming information for early cme direction estimation, providing valuable data for space weather forecasting and mitigating potential adverse impacts on earth before observation in coronographs' field-of-view. | coronal dimmings as indicators of early cme propagation direction |
in this thesis, the origin of large-scale structures in hot star winds, believed to be responsible for the presence of discrete absorption components (dacs) in the absorption troughs of ultraviolet resonance lines, is constrained using both observations and numerical simulations. these structures are understood as arising from bright regions on the stellar surface, although their physical cause remains unknown. first, we use high quality circular spectropolarimetric observations of 13 well-studied ob stars to evaluate the potential role of dipolar magnetic fields in producing dacs. we perform longitudinal field measurements and place limits on the field strength using bayesian inference, assuming that it is dipolar. no magnetic field was detected within this sample. the derived constraints statistically refute any significant dynamical influence from a magnetic dipole on the wind for all of these stars, ruling out such fields as a cause for dacs. second, we perform numerical simulations using bright spots constrained by broadband optical photometric observations. we calculate hydrodynamical wind models using three sets of spot sizes and strengths. co-rotating interaction regions are yielded in each model, and radiative transfer shows that the properties of the variations in the uv resonance lines synthesized from these models are consistent with those found in observed uv spectra, establishing the first consistent link between uv spectroscopic line profile variability and photometric variations and thus supporting the bright spot paradigm (bsp). finally, we develop and apply a phenomenological model to quantify the measurable effects co-rotating bright spots would have on broadband optical photometry and on the profiles of photospheric lines in optical spectra. this model can be used to evaluate the existence of these spots, and, in the event of their detection, characterize them. furthermore, a tentative spot evolution model is presented. a preliminary analysis of its output, compared to the observed photometric variations of ξ persei, suggests the possible existence of "active longitudes" on the surface of this star. future work will expand the range of observational diagnostics that can be interpreted within the bsp, and link phenomenology (bright spots) to physical processes (magnetic spots or non-radial pulsations). | investigating the potential magnetic origin of wind variability in ob stars |
simulations from the scales of isolated galaxies to clouds have been instrumental in informing us about molecular cloud formation and evolution. simulations are able to investigate the roles of gravity, feedback, turbulence, heating and cooling, and magnetic fields on the physics of the interstellar medium, and star formation. compared to simulations of individual clouds, galactic and sub-galactic scale simulations can include larger galactic scale processes such as spiral arms, bars, and larger supernovae bubbles, which may influence star formation. simulations show cloud properties and lifetimes in broad agreement with observations. gravity and spiral arms are required to produce more massive gmcs, whilst stellar feedback, likely photoionisation, leads to relatively short cloud lifetimes. on larger scales, supernovae may be more dominant in driving the structure and dynamics, but photoionisation may still have a role. in terms of the dynamics, feedback is probably the main driver of velocity dispersions, but large scale processes such as gravity and spiral arms may also be significant. magnetic fields are generally found to decrease star formation on galaxy or cloud scales, and simulations are ongoing to study whether clouds are sub or supercritical on different scales in galaxy scale simulations. simulations on subgalactic scales, or zoom in simulations, allow better resolution of feedback processes, filamentary structure within clouds, and the study of stellar clusters. | 2a results: galaxy to cloud scales |
over a hundred millisecond radio pulsars (msps) have been observed in globular clusters (gcs), motivating theoretical studies of the formation and evolution of these sources through stellar evolution coupled to stellar dynamics. here we study msps in gcs using realistic n-body simulations with our cluster monte carlo code. we show that neutron stars (nss) formed in electron-capture supernovae can be spun up through mass transfer to form msps. both ns formation and spin-up through accretion are greatly enhanced through dynamical interaction processes. we find that our models for average gcs at the present day with masses ≍ 2 × 105m⊙ can produce up to 10 - 20 msps, while a very massive gc model with mass ≍ 106m⊙ can produce close to 100. we show that the number of msps is anti-correlated with the total number of stellar-mass black holes (bhs) retained in the host cluster. as a result, the number of msps in a gc could be used to place constraints on its bh population. some intrinsic properties of msp systems in our models (such as the magnetic fields and spin periods) are in good overall agreement with observations. | modeling pulsars in dense star clusters |
recent advanced simulations of protoplanetary disks allow us to search for observational constraints to identify the magnetic field activity in protoplanetary disks. with our 3d radiation non-ideal magneto-hydrodynamical (mhd) models including irradiation from an herbig type star we are able to model the thermal and dynamical evolution in a so far never reached detail (flock et al. 2017). the activity of the magneto-rotational instability in the inner hot ionized regions comes along with a magnetic dynamo. the oscillations in the mean toroidal magnetic field with a timescale of 10 local orbits can slightly bend the inner dust rim and so the irradiation surface. this causes a clear variability pattern in the near infrared (nir) emission at the dust inner rim surface. another way to identify the presence of magnetic fields are to search for polarization signatures. using 3d non-ideal mhd simulations of the outer disk regions (flock et al. 2015) we calculate synthetic images of the intrinsically polarized continuum emitted by aspherical grain aligned with the dominantly toroidal magnetic field (bertrang et al. 2017). our results show a clear radial polarization pattern for face-on observed disk, similar to recent observations by ohashi et al. (2018). additionally, we are even able to see the change of the polarization pattern inside the vortex as the poloidal magnetic field dominates therein. | how to identify magnetic field activity in young circumstellar disks |
the amplification of magnetic fields via dynamo mechanisms is a fundamental process that not only shapes the dynamics of stars, but also deeply affects the evolution of compact objects from their formation throughout their activity. understanding how magnetic fields are dissipated and amplified in the environment surrounding neutron stars and black holes can thus disclose the connection between the properties of the central object and the multi-messenger emission related to the accretion process. we present results from recent studies aimed at quantifying the properties of dynamo-generated magnetic fields in proto-neutron stars and their potential impact on the explosion and the neutrino and gravitational waves signals emitted at the formation of the compact object. we also show how mean-field dynamos can modify the dynamics of accreting black holes, as weak magnetic seeds are amplified to strong large-scale fields that can reshape the properties of the accretion flow. finally, we present some recent models of resistive relativistic jets and high-order numerical schemes for (relativistic) magnetohydrodynamics (rmhd) that showcase the importance of reducing the numerical dissipation of magnetic fields observed in simulations, thus enhancing the robustness of their predictions. | numerical modeling of dynamos in compact objects: magnetic field amplification and dissipation |
streamers and pseudostreamers structure the corona at the largest scales, as seen in both eclipse and coronagraph white-light images. their inverted-goblet appearance encloses broad coronal loops at the sun and tapers to a narrow radial stalk away from the star. the streamer associated with the global solar dipole magnetic field is long-lived, predominantly contains a single arcade of nested loops within it, and separates opposite-polarity interplanetary magnetic fields with the heliospheric current sheet anchored at its apex. pseudostreamers, on the other hand, are transient, enclose double arcades of nested loops, and separate like-polarity fields with a dense plasma sheet. we use numerical magnetohydrodynamic simulations to calculate, for the first time, the formation of pseudostreamers in response to photospheric magnetic-field evolution. convective transport of a minority-polarity flux concentration, initially positioned under one side of a streamer, through the streamer boundary into the adjacent, pre-existing coronal hole forms the pseudostreamer. interchange magnetic reconnection at the overlying coronal null point(s) governs the development of the pseudostreamer above - and of a new, satellite coronal hole behind - the moving minority polarity. the reconnection dynamics liberate coronal-loop plasma that can escape into the heliosphere along so-called separatrix-web ("s-web") arcs, which reach far from the heliospheric current sheet and the solar equatorial plane, and can explain the origin of high-latitude slow solar wind. we describe the implications of our results for in-situ and remote-sensing observations of the corona and heliosphere as obtained, most recently, by parker solar probe and solar orbiter. | the dynamic formation of pseudostreamers |
my thesis work has focused on understanding the physics of star formation in galaxies and its role in shaping the galaxies and the universe. i have conducted a series of multiscale simulations of star cluster formation in isolated turbulent giant molecular clouds (gmcs) using ramses, a state-of-the-art radiation-magneto-hydrodynamic code. while resolving the formation of individual stars, we have pushed the parameters (mass and density) of the gmcs simulated to well above the limit explored in the literature. i find physically motivated scaling relationships for the timescale and efficiency of star formation regulated by photoionization feedback, a type of stellar feedback that i have shown to be efficient at dispersing dense molecular clouds before the onset of supernova explosions. i find that star formation in gmcs is a purely stochastic process: instantaneous star formation follows a universal mass probability distribution, suggesting a definite answer to the open question - what is the chronological order of low- and high-mass star formation? in another work, i published the first study of the escape of ionizing photons from resolved stars in molecular clouds into the intercloud gas. i concluded that the sources of photons responsible for the epoch of reionization, one of the most important yet poorly understood stages in cosmic evolution, must have been very compact star clusters forming in dense environments different from today's galaxies. in follow-up work, i use a novel adaptive-mesh-refinement method to simulate the formation and fragmentation of prestellar cores and resolve the evolution of gmcs to circumstellar disk scales, achieving an unprecedented dynamic range of 18 orders of magnitude in volume. i have shown that massive stars form from the filamentary collapse of dense cores and grow to masses several times bigger than the core mass due to accretion from larger scales via circumstellar disks, suggesting a competitive accretion scenario of high-mass star formation, a problem that is not well understood. magnetic braking fails to prevent the formation of keplerian disks even in cores with mass-to-flux ratios close to critical. this is due to the extremely turbulent nature of the magnetic field that is many times weaker at braking the disk rotation than in the perfectly aligned case, proposing a solution to the magnetic braking catastrophe. | multiscale radiation-mhd simulations of dense star clusters |
context. the main driving forces supplying energy to the interstellar medium (ism) are supernova explosions and stellar winds. such localized sources are assimilable to curl-free velocity fields as a first approximation. they need to be combined with other physical processes to replicate real galactic environments, such as the presence of turbulence and a dynamo-sustained magnetic field in the ism.aims: this work is focused on the effect of an irrotational forcing on a magnetized flow in the presence of rotation, baroclinicity, shear, or a combination of any of the three. it follows an earlier analysis with a similar focus, namely, subsonic spherical expansion waves in hydrodynamic simulations. by including magnetic field in the model, we can evaluate the occurrence of dynamo on both small and large scales. we aim to identify the minimum ingredients needed to trigger a dynamo instability as well as the relation between dynamo and the growth of vorticity.methods: we used the pencil code to run resistive magnetohydrodynamic direct numerical simulations, exploring the ranges of values of several physical and numerical parameters of interest. we explored reynolds numbers up to a few hundreds. we analyzed the temporal evolution of vorticity, kinetic, and magnetic energy, as well as their features in fourier space.results: we report the absence of a small-scale dynamo in all cases where only rotation is included, regardless of the given equation of state and rotation rate. conversely, the inclusion of a background sinusoidal shearing profile leads to an hydrodynamic instability that produces an exponential growth of the vorticity at all scales, starting from small ones. this is know as vorticity dynamo. the onset of this instability occurs after a rather long temporal evolution of several thousand turbulent turnover times. the vorticity dynamo in turn drives an exponential growth of the magnetic field, first at small scales, followed by large ones. the instability is then saturated and the magnetic field approximately reaches equipartition with the turbulent kinetic energy. during the saturation phase, we can observe a winding of the magnetic field in the direction of the shearing flow. by varying the intensity of the shear, we see that the growth rates of this instability change. the inclusion of the baroclinic term has the main effect of delaying the onset of the vorticity dynamo, but then leads to a more rapid growth.conclusions: our work demonstrates how even purely irrotational forcing may lead to dynamo action in the presence of shear, thus amplifying the field to an equipartition level. at the same time, we confirm that purely irrotational forcing alone does not lead to any growth in terms of the vorticity, nor the magnetic field. this picture does not change in the presence of rotation or baroclinicity, at least up to a resolution of 2563 mesh points. to further generalize such a conclusion, we will need to explore how this setup works both at higher magnetic reynolds numbers and with different prescriptions of the irrotational forcing. | vorticity and magnetic dynamo from subsonic expansion waves |
we investigate the thermal, kinematic, and magnetic structure of small-scale heating events in an emerging flux region (efr). we use high-resolution multiline observations (including ca ii 8542 å, ca ii k, and the fe i 6301 å line pair) of an efr located close to the disk center from the crisp and chromis instruments at the swedish 1 m solar telescope. we perform non-lte inversions of multiple spectral lines to infer the temperature, velocity, and magnetic field structure of the heating events. additionally, we use the data-driven coronal global evolutionary model to simulate the evolution of the 3d magnetic field configuration above the events and understand their dynamics. furthermore, we analyze the differential emission measure to gain insights into the heating of the coronal plasma in the efr. our analysis reveals the presence of numerous small-scale heating events in the efr, primarily located at polarity inversion lines of bipolar structures. these events not only heat the lower atmosphere but also significantly heat the corona. the data-driven simulations, along with the observed enhancement of currents and poynting flux, suggest that magnetic reconnection in the lower atmosphere is likely responsible for the observed heating at these sites. | solar atmospheric heating due to small-scale events in an emerging flux region |
the complete orbital and spin period evolutions of the double neutron star (ns) system psr j0737-3039 are simulated from birth to coalescence, which include the two observed radio pulsars classified as primary ns psr j0737-3039a and companion ns psr j0737-3039b. by employing the characteristic age of psr j0737-3039b to constrain the true age of the double pulsar system, the initial orbital period and initial binary separation are obtained as 2.89 h and 1.44 × 106 km, respectively, and the coalescence age or the lifetime from the birth to merger of psr j0737-3039 is obtained to be 1.38 × 108 yr. at the last minute of coalescence, corresponding to the gravitational wave frequency changing from 20 hz to 1180 hz, we present the binary separation of psr j0737-3039 to be from 442 km to 30 km, while the spin periods of psr j0737-3039a and psr j0737-3039b are 27.10 ms and 4.63 s, respectively. from the standard radio pulsar emission model, before the system merged, the primary ns could still be observed by a radio telescope, but the companion ns had crossed the death line in the pulsar magnetic-field versus period (b - p) diagram at which point it is usually considered to cease life as a pulsar. this is the first time that the whole life evolutionary simulation of the orbit and spin periods for a double ns system is presented, which provides useful information for observing a primary ns at the coalescence stage. | simulation of the orbit and spin period evolution of the double pulsars psr j0737-3039 from their birth to coalescence induced by gravitational wave radiation |
magnetic field evolution of neutron star crusts is best described using the formalism of hall magnetohydrodynamics (mhd). for magnetic fields ≥ 1014 g and typical temperatures for most of the lifetime of a magnetar, landau quantization of electrons can significantly alter the crust's thermodynamic and transport properties, with the formation of magnetic domains as one manifestation of this. i will describe how landau quantization modifies hall mhd oscillation modes and ohmic dissipation due to non-zero magnetization. i will then discuss magneto-thermal evolution of neutron star crusts with reference to hall mhd simulations including the combined effects of landau quantization on the magnetization, electrical conductivity and thermal conductivity of the crustal electrons. comparisons to simulations excluding these effects are used to assess their relative importance to the overall magneto-thermal history of magnetars. | hall mhd in magnetar crusts with landau-quantized electrons |
context. density decreases exponentially with height in the gravitationally stratified solar atmosphere, and therefore collisional coupling between the ionized plasma and the neutrals also decreases. reconnection is a process observed at all heights in the solar atmosphere.aims: here, we investigate the role of collisions between ions and neutrals in the reconnection process occurring at various heights in the atmosphere.methods: we performed simulations of magnetic reconnection induced by a localized resistivity in a gravitationally stratified atmosphere, in which we varied the height of the initial reconnection x-point. we compared a magnetohydrodynamic (mhd) model and two two-fluid configurations: one in which the collisional coupling was calculated from local plasma parameters, and another in which the coupling was decreased so that collisional effects would be enhanced. the latter setup has a more representative solar collisionality regime.results: simulations in a stratified atmosphere show similar structures in mhd and two-fluid simulations, with strong coupling. however, when collisional effects are increased to attain representative parameter regimes, we find a nonlinear runaway instability, which separates the plasma-neutral densities across the current sheet (cs). with increased collisional effects, the initial decoupling in velocity heats the neutrals and this sets up a nonlinear feedback loop, according to which neutrals migrate outside the cs, replacing charged particles that accumulate toward the center of the cs.conclusions: the reconnection rate has a maximum value of around 0.1 for both reconnection heights, and is consistent with the locally enhanced resistivity used in all three models. the early-stage plasmoid formation observed near the end of our simulations is influenced by the outflow from the primary reconnection point, rather than by collisions. we synthesized optically thin emission for both mhd and two-fluid models, which can show a very different evolution when the charged-particle density is used instead of the total density. our simulations have relevance for observed plasmoid features associated with chromospheric to low-coronal flare events. movies associated to figs. 3 and 9 are available at https://www.aanda.org. | two-fluid reconnection jets in a gravitationally stratified atmosphere |
we present a three-dimensional, time-dependent mhd simulation of the short-term interaction between a protoplanetary disk and the stellar corona in a t tauri system. the simulation includes the stellar magnetic field, self-consistent coronal heating and stellar wind acceleration, and a disk rotating at sub-keplerian velocity to induce accretion. we find that, initially, as the system relaxes from the assumed initial conditions, the inner part of the disk winds around and moves inward and close to the star as expected. however, the self-consistent coronal heating and stellar wind acceleration build up the original state after some time, significantly pushing the disk out beyond 10r ⋆. after this initial relaxation period, we do not find clear evidence of a strong, steady accretion flow funneled along coronal field lines, but only weak, sporadic accretion. we produce synthetic coronal x-ray line emission light curves, which show flare-like increases that are not correlated with accretion events nor with heating events. these variations in the line emission flux are the result of compression and expansion due to disk-corona pressure variations. vertical disk evaporation evolves above and below the disk. however, the disk-stellar wind boundary stays quite stable, and any disk material that reaches the stellar wind region is advected out by the stellar wind. | three-dimensional, time-dependent mhd simulation of disk-magnetosphere-stellar wind interaction in a t tauri, protoplanetary system |
we investigate the dynamic evolution of the gaseous regions around fs cma post-mergers. owing to the slow rotation of the central b-type star, the dynamics is driven mainly by the magnetic field of the central star. recent observations have allowed us to set realistic initial conditions, such as the magnetic field value ($b_\star \approx 6\times 10^{3}\, \mathrm{g}$), the mass of the central star ($m_\star =6\, \mathrm{m}_\odot$), and the initial disc density $\rho _{d0}\in [10^{-13}\, \mathrm{g\, cm^{-3}},10^{-11}\, \mathrm{g \, cm^{-3}}]$. we use the pluto code to perform 2.5d magnetohydrodynamic simulations of thin and thick disc models. especially relevant for the interpretation of the observed properties of fs cma post-mergers are the results for low-density discs, in which we find a jet emerging from the inner edge of the disc, as well as the formation of the so-called 'hot plasmoid' in the coronal region. jets are probably detected as discrete absorption components in the resonance lines of fs cma stars. moreover, the magnetic field configuration in the low-density plasma region favours the appearance of magnetocentrifugal winds from the disc. the currents towards the star created by the magnetic field may explain accidentally observed material infall. the disc structure is significantly changed owing to the presence of the magnetic field. the magnetic field is also responsible for the formation of a hot corona, as observed in several fs cma stars through the raman lines. our results are valid for all magnetic stars surrounded by a low-density plasma, that is, by some stars showing the b[e] phenomenon. | 2.5d magnetohydrodynamic models of circumstellar discs around fs cma post-mergers - i. non-stationary accretion stage |
we summarize recent attempts to unravel the role of plasma kinetic effects in radiation mediated shocks. such shocks form in all strong stellar explosions and are responsible for the early electromagnetic emission released from these events. a key issue that has been overlooked in all previous works is the nature of the coupling between the charged leptons, that mediate the radiation force, and the ions, which are the dominant carriers of the shock energy. our preliminary investigation indicates that in the case of relativistic shocks, as well as newtonian shocks in multi-ion plasma, this coupling is driven by either, transverse magnetic fields of a sufficiently magnetized upstream medium, or plasma microturbulence if strong enough magnetic fields are absent. we discuss the implications for the shock breakout signal, as well as abundance evolution and kilonova emission in binary neutron star mergers. | anomalous coupling in radiation mediated shocks |
filamentary molecular clouds are regarded as the place where newborn stars form. in particular, a hub region, a place where it appears as if several filaments are colliding, often indicates active star formation. to understand the star formation in filament structures, we investigate the collisions between two filaments using two-dimensional magnetohydrodynamical simulations. as a model of filaments, we assume that the filaments are in magnetohydrostatic equilibrium under a global magnetic field perpendicular to the filament axis. we set two identical filaments with an infinite length and make them collide with a zero-impact parameter (head-on). when the two filaments collide while sharing the same magnetic flux, we found two types of evolution after a merged filament is formed: runaway radial collapse and stable oscillation with a finite amplitude. the condition for the radial collapse is independent of the collision velocity and is given by the total line mass of the two filaments exceeding the magnetically critical line mass for which no magnetohydrostatic solution exists. the radial collapse proceeds in a self-similar manner, resulting in a unique distribution irrespective of the various initial line masses of the filament, as the collapse progresses. when the total line mass is less massive than the magnetically critical line mass, the merged filament oscillates, and the density distribution is well-fitted by a magnetohydrostatic equilibrium solution. the condition necessary for the radial collapse is also applicable to the collision whose direction is perpendicular to the global magnetic field. | simulation of head-on collisions between filamentary molecular clouds threaded by a lateral magnetic field and subsequent evolution |
the spontaneous evolution of magnetic reconnection in generalized situations (with thermodynamic asymmetry regarding the current sheet and magnetic shear) is investigated using a two-dimensional magnetohydrodynamic simulation. we focus on the asymptotic state of temporal evolution, i.e., the self-similarly expanding phase. (1) a long fast-mode shock is generated in front of the shorter plasmoid as in the shear-less thermodynamically asymmetric case; however, the sheared magnetic component weakens the shock. this fast shock may work as a particle acceleration site. (2) the shorter plasmoid-side plasma infiltrates the longer plasmoid across the current sheet. then, the plasmas from both sides of the current sheet coexist on the same magnetic field lines in the longer plasmoid. this may result in efficient plasma mixing. (3) the thermodynamic asymmetry and magnetic shear drastically decrease the reconnection rate in many orders of magnitude. | effects of magnetic shear and thermodynamic asymmetry on spontaneous magnetohydrodynamic reconnection |
magnetic fields play a vital role in numerous astrophysical processes such as star formation and the interstellar medium. in particular, their role in the formation and evolution of galaxies is not well understood. this paper presents high-resolution magnetohydrodynamic (mhd) simulations performed with gizmo to investigate the effect of magnetic fields on primordial galaxy formation. physical processes such as relevant gas physics (e.g., gas cooling and gas chemistry), star formation, stellar and supernova feedback, and chemical enrichment were considered in the simulations. the simulation results suggest that cosmic magnetic fields can be amplified from 1e-13 g to a few microgauss during cosmic structure evolution and galaxy formation. in the ideal mhd setting, in primordial galaxies at z>8, the magnetic energy is less than the thermal and kinetic energy, and therefore, magnetic fields hardly affect the gas dynamics and star formation in these galaxies. specifically, the consideration of micro-physics properties such as metal diffusion, heat conduction, and viscosity in the mhd simulations, could increase the magnetic field strength. notably, metal diffusion reduced gas cooling by decreasing the metallicity and thereby suppresses star formation in the primordial galaxies. as a result, the cosmic re-ionization driven by these primordial galaxies may be delayed. | magnetic fields in primordial galaxies |
we study the impact of an upwind scheme on the numerical convergence of simulations of the hall and ohmic effect in neutron stars crusts. while simulations of these effects have explored a variety of geometries and wide ranges of physical parameters, they are limited to relatively low values of the hall parameter, playing the role of the magnetic reynolds number, which should be not exceed a few hundred for numerical convergence. we study the evolution of the magnetic field in a plane-parallel cartesian geometry. we discretise the induction equation using a finite difference scheme and then integrate it via the euler forward method. two different approaches are used for the integration of the advective terms appearing in the equation: a forward time and central in space (ftcs) and an upwind scheme. we compare them in terms of accuracy and performance. we explore the impact of the upwind method on convergence according to the ratio of planar to vertical field and the hall parameter. in the limit of a low strength planar field the use of an upwind scheme provides a vast improvement leading to the convergence of simulations where the hall parameter is 2 orders of magnitude higher than that of the ftcs. upwind is still better if the planar field is stronger, yet, the difference of the maximum value of the hall parameter reached is within a factor of 10 or a few. moreover, we notice if the schemes diverge their behaviour is very different, with ftcs producing infinite energy, while the upwind scheme only temporarily increasing the overall magnetic field energy. overall, the upwind scheme enhances the efficiency of the simulations allowing the exploration of environments with higher value of electric conductivity getting us closer than before to realistic environmental conditions of magnetars. | application of an upwind integration method to plane parallel hall-mhd |
the observational properties of isolated nss are shaped by their magnetic field and surface temperature. they evolve in a strongly coupled fashion, and modelling them is key in understanding the emission properties of nss. much effort was put in tackling this problem in the past but only recently a suitable 3d numerical framework was developed. we present a set of 3d simulations addressing both the long-term evolution (≈ 104-106 yrs) and short-lived outbursts (≲ 1 yr). not only a 3d approach allows one to test complex field geometries, but it is absolutely key to model magnetar outbursts, which observations associate to the appearance of small, inherently asymmetric hot regions. even though the mechanism that triggers these phenomena is not completely understood, following the evolution of a localised heat injection in the crust serves as a model to study the unfolding of the event. | modelling magnetar behaviour with 3d magnetothermal simulations |
the sunrise chromospheric infrared spectropolarimeter (scip) has been developed for the third flight of the sunrise balloon-borne stratospheric solar observatory. the aim of the scip is to reveal the evolution of three-dimensional magnetic fields in the solar photosphere and chromosphere using spectropolarimetric measurements with a polarimetric precision of 0.03% (1σ). multiple lines in the 770 and 850 nm wavelength bands are simultaneously observed with two 2 k × 2 k cmos cameras at a frame rate of 31.25 hz. stokes profiles are calculated onboard by accumulating the images modulated by a polarization modulation unit, and then compression processes are applied to the two-dimensional maps of the stokes profiles. this onboard data processing effectively reduces the data rate. scip electronics can handle large data formats at high speed. before the implementation into the flight scip electronics, a performance verification of the onboard data processing was performed with synthetic scip data that were produced with a numerical simulation modeling the solar atmospheres. finally, we verified that the high-speed onboard data processing was realized on ground with the flight hardware using images illuminated by natural sunlight or an led light. | high-speed data processing onboard sunrise chromospheric infrared spectropolarimeter for the sunrise iii balloon telescope |
the solar atmosphere is known to contain many different types of wave-like oscillation. waves and other fluctuations (e.g., turbulent eddies) are believed to be responsible for at least some of the energy transport and dissipation that heats the corona and accelerates the solar wind. thus, it is important to understand the behavior of magnetohydrodynamic (mhd) waves as they propagate and evolve in different regions of the sun's atmosphere. in this paper, we investigate how mhd waves can affect the overall plasma state when they reflect and refract at sharp, planar interfaces in density. first, we correct an error in a foundational paper (stein) that affects the calculation of wave energy-flux conservation. second, we apply this model to reflection-driven mhd turbulence in the solar wind, where the presence of density fluctuations can enhance the generation of inward-propagating alfvén waves. this model reproduces the time-averaged elsässer imbalance fraction (i.e., the ratio of inward to outward alfvénic power) from several published numerical simulations. lastly, we model how the complex magnetic field threading the transition region (tr) between the chromosphere and corona helps convert a fraction of upward-propagating alfvén waves into fast-mode and slow-mode mhd waves. these magnetosonic waves dissipate in a narrow region around the tr and produce a sharp peak in the heating rate. this newly found source of heating sometimes exceeds the expected heating rate from alfvénic turbulence by an order of magnitude. it may explain why some earlier models seemed to require an additional ad hoc heat source at this location. | magnetohydrodynamic mode conversion in the solar corona: insights from fresnel-like models of waves at sharp interfaces |
the tayler-spruit dynamo is one of the most promising mechanisms proposed to explain angular momentum transport during stellar evolution. its development in proto-neutron stars spun-up by supernova fallback has also been put forward as a scenario to explain the formation of very magnetized neutron stars called magnetars. using three-dimensional direct numerical simulations, we model the proto-neutron star interior as a stably stratified spherical couette flow with the outer sphere that rotates faster than the inner one. we report the existence of two subcritical dynamo branches driven by the tayler instability. they differ by their equatorial symmetry (dipolar or hemispherical) and the magnetic field scaling, which is in agreement with different theoretical predictions (by fuller and spruit, respectively). the magnetic dipole of the dipolar branch is found to reach intensities compatible with observational constraints on magnetars. | numerical simulations of the tayler-spruit dynamo in proto-magnetars |
the rotation of sunspots around their umbral center has long been considered as an important process in leading to solar eruptions, but the underlying mechanism remains unclear. a prevailing physical picture on how sunspot rotation leads to eruption is that, by twisting the coronal magnetic field lines from their footpoints, the rotation can build up a magnetic flux rope and drive it into some kinds of ideal magnetohydrodynamics (mhd) instabilities which initiate eruptions. here with a data-inspired mhd simulation we studied the rotation of a large sunspot in solar active region noaa 12158 leading to a major eruption, and found that it is distinct from prevailing theories based on ideal instabilities of twisted flux rope. the simulation suggests that, through successive rotation of the sunspot, the coronal magnetic field is sheared with a central current sheet created progressively within the sheared arcade before the eruption, but without forming a flux rope. then the eruption is instantly triggered once fast reconnection sets in at the current sheet, while a highly twisted flux rope is created during the eruption. furthermore, the simulation reveals an intermediate evolution stage between the quasi-static energy-storage phase and the impulsive eruption-acceleration phase. this stage may correspond to the slow-rise phase in observation and it enhances building up of the current sheet. | magnetic reconnection as the key mechanism in sunspot rotation leading to solar eruption |
context. the rotation periods of young low-mass stars after disks have dissipated (≲-pagination10 myr) but before magnetized winds have removed significant angular momentum is an important launch point for gyrochronology and models of stellar rotational evolution; the rotation of these stars also regulates the magnetic activity and the intensity of high-energy emission that affects any close-in planets. a recent analysis of young m dwarf stars suggests a distribution of specific angular momentum (sam) that is mass-independent, but the physical basis of this observation is unclear.aims: we investigate the influence of an accretion disk on the angular momentum (am) evolution of young m dwarfs, whose parameters govern the am distribution after the disk phase, and whether this leads to a mass-independent distribution of sam.methods: we used a combination of protostellar spin and implicit hydrodynamic disk evolution models to model the innermost disk (∼0.01 au), including a self-consistent calculation of the accretion rate onto the star, non-keplerian disk rotation, and the influence of stellar magnetic torques over the entire disk lifetime. we executed and analyzed over 500 long-term simulations of the combined stellar and disk evolution.results: we find that above an initial rate of ṁcrit ∼ 10−8 m⊙ yr−1, accretion "erases" the initial sam of m dwarfs during the disk lifetime, and stellar rotation converges to values of sam that are largely independent of initial conditions. for stellar masses > 0.3 m⊙, we find that observed initial accretion rates ṁinit are comparable to or exceed ṁcrit. furthermore, stellar sam after the disk phase scales with the stellar magnetic field strength as a power law with an exponent of −1.1. for lower stellar masses, ṁinit is predicted to be smaller than ṁcrit and the initial conditions are imprinted in the stellar sam after the disk phase.conclusions: to explain the observed mass-independent distribution of sam, the stellar magnetic field strength has to range between 20 g and 500 g (700 g and 1500 g) for a 0.1 m⊙ (0.6 m⊙) star. these values match observed large-scale magnetic field measurements of young m dwarfs and the positive relation between stellar mass and magnetic field strength agrees with a theoretically motivated scaling relation. the scaling law between stellar sam, mass, and the magnetic field strength is consistent for young stars, where these parameters are constrained by observations. due to the very limited number of available data, we advocate for efforts to obtain more such measurements. our results provide new constraints on the relation between stellar mass and magnetic field strength and they can be used as initial conditions for future stellar spin models, starting after the disk phase. | the post-disk (or primordial) spin distribution of m dwarf stars |
aims: we used solar observations of a plage-enhanced network with the atacama large millimeter/sub-millimeter array (alma) in band 3 and band 6, together with synthetic continuum maps from numerical simulations with bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features.methods: we made use of an algorithm to automatically identify and trace bright features within the field of view (fov) of the alma observations and the simulation. in particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. these quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis.results: we found 193 and 293 features in the bands 3 and 6 observations, respectively. in the degraded simulation, the total number of features were 24 for band 3 and 204 for band 6. in the original simulation, the total number of features were 36 for band 3 and 392 for band 6. based on the simulation, we confirm the magnetic nature of the features. we have obtained average oscillation periods of 30-99 s for the temperature, 37-92 s for size, and 37-78 s for horizontal velocity. there are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1-5.0 km s−1. we find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (mhd) modes. for the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by alma as 453-1838 w m−2 for band 3 and 3640-5485 w m−2 for band 6.conclusions: we have identified mhd waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both alma band 3 and band 6 observations, along with their corresponding synthetic images from simulations. the decrease of wave energy-flux with height (from band 6 to band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere. | the sun at millimeter wavelengths. iv. magnetohydrodynamic waves in small-scale bright features |
the extragalactic gamma-ray sky is dominated by blazars, active galactic nuclei (agn) with a relativistic jet that is closely aligned with the line of sight. galaxies develop an active nucleus if the central supermassive black hole (bh) accretes large amounts of ambient matter and magnetic flux. the inflowing mass accumulates around the plane perpendicular to the accretion flow's angular momentum. the flow is heated through viscous friction and part of the released energy is radiated as blackbody or non-thermal radiation, with luminosities that can dominate the accumulated stellar luminosity of the host galaxy. a fraction of the accretion flow luminosity is reprocessed in a surrounding field of ionised gas clouds. these clouds, revolving around the central bh, emit doppler-broadened atomic emission lines. the region where these broad-line-emitting clouds are located is called broad-line region (blr). about one in ten agn forms an outflow of radiation and relativistic particles, called a relativistic jet. according to the blandford-znajek mechanism, this is facilitated through electromagnetic processes in the magnetosphere of a spinning bh. the latter induces a magnetospheric poloidal current circuit, generating a decelerating torque on the bh and inducing a toroidal magnetic field. consequently, rotational energy of the bh is converted to poynting flux streaming away mainly along the rotational axis and starting the jet. one possibility for particle acceleration near the jet base is realised by magnetospheric vacuum gaps, regions temporarily devoid of plasma, such that an intermittent electric field arises parallel to the magnetic field lines, enabling particle acceleration and contributing to the mass loading of the jets. magnetised structures, containing bunches of relativistic electrons, propagate away from the galactic nucleus along the jets. assuming that these electrons emit synchrotron radiation and that they inverse-compton (ic) up-scatter abundant target photons, which can either be the synchrotron photons themselves or photons from external emitters, the emitted spectrum can be theoretically determined. additionally taking into account that these emission regions move relativistically themselves and that the emission is doppler-boosted and beamed in forward direction, the typical two-hump spectral energy distribution (sed) of blazars is recovered. there are however findings that challenge this well-established model. short-time variability, reaching down to minute scales at very high energy gamma rays, is today known to be a widespread phenomenon of blazars, calling for very compact emission regions. in most models of such optically thick emission regions, the gamma-ray flux is usually pair-absorbed exponentially, without considering the cascade evolving from the pair-produced electrons. from the observed flux, it is often concluded that emission emanates from larger distances where the region is optically thin, especially from outside of the blr. only in few blazars gamma-ray attenuation associated with pair absorption in the blr was clearly reported. with the advent of sophisticated high-energy or very high energy gamma-ray detectors, like the fermi large area telescope or the major atmospheric gamma-ray imaging cherenkov telescopes, besides the extraordinarily fast variability spectral features have been found that cannot be explained by conventional models reproducing the two-hump sed. two such narrow spectral features are discussed in this work. for the nearby blazar markarian 501, hints to a sharp peak around 3 tev have been reported from a multi-wavelength campaign carried out in july 2014, while for 3c 279 a spectral dip was found in 2018 data, that can hardly be described with conventional fitting functions. in this work it is examined whether these spectral peculiarities of blazar jet emission can be explained, if the full radiation reprocessing through an ic pair cascade is accounted for. such a cascade is the multiple concatenation of ic scattering events and pair production events. in the cascades generally considered in this work, relativistic electrons and high-energy photons are injected into a fixed soft target photon field. a mathematical description for linear ic pair cascades with escape terms is delivered on the basis of preliminary works. the steady-state kinetic equations for the electrons and for the photons are determined, whereby it is paid attention to an explicit formulation and to motivating the correct integration borders of all integrals from kinematic constraints. in determining the potentially observable gamma-ray flux, both the attenuated injected flux and the flux evolving as an effect of ic up-scattering, pair absorption and escape are incorporated, giving the emerging spectra very distinct imprints. much effort is dedicated to the numerical solution of the electrons' kinetic equation via iterative schemes. it is explained why pointwise iteration from higher to lower lorentz factors is more efficient than iterating the whole set of sampling points. the algorithm is parallelised at two positions. first, several workers can perform pointwise iterations simultaneously. second, the most demanding integral is cut into a number of part integrals which can be determined by multiple workers. through these measures, the python code can be readily applied to simulate steady-state ic pair cascades with escape. in the case of markarian 501 the developed framework is as follows. the agn hosts an advection-dominated accretion flow with a normalised accretion rate of several 10^{‑4} and an electron temperature near 10^{10} k. on the one hand, the accretion flow illuminates the few ambient gas clouds with approximate radius 10^{11} m, which reprocess a fraction 0.01 of the luminosity into hydrogen and helium emission lines. on the other hand, the gamma rays from the accretion flow create electrons and positrons in a sporadically active vacuum gap in the bh magnetosphere. in the active gap, a power of roughly 0.001 of the blandford-znajek power is extracted from the rotating bh through a gap potential drop of several 10^{18} v, generating ultra-relativistic electrons, which subsequently are multiplied by a factor of about 10^6 through interaction with the accretion flow photons. this electron beam propagates away from the central engine and encounters the photon field of one passing ionised cloud. the resulting ic pair cascade is simulated and the evolving gamma-ray spectrum is determined. just above the absorption troughs due to the hydrogen lines, the spectrum exhibits a narrow bump around 3 tev. when the cascaded emission is added to the emission generated at larger distances, the observed multi-wavelength sed including the sharp peak at 3 tev is reproduced, underlining that radiation processes beyond conventional models are motivated by distinct spectral features. the dip in the spectrum of 3c 279 is addressed by a similar cascade model. three types of injection are considered, varying in the ratio of the photon density to the electron density and varying in the spectral shape. the ic pair cascade is assumed to happen either in the dense blr photon field with a luminosity of several 10^{37} w and a radial size of few 10^{14} m or in the diluted photon field outside of the blr. the latter scenario is however rejected as the spectral slope around several 100 mev and the dip at few 10 gev cannot be reconciled within this model. the radiation cascaded in the blr can explain the observational data, irrespective of the assumed injected rate. it is therefore concluded that for this period of gamma-ray emission, the radiation production happens at the edge of the blr of 3c 279. both investigations show that ic pair cascades can account for fine structure seen in blazar seds. it is insufficient to restrict the radiation transport to pure exponential absorption of an injection term. pair production and ic up-scattering by all generations of photons and electrons in the optically thick regime critically shape the emerging spectra. as the advent of future improved detectors will provide more high-precision spectra, further observations of narrow spectral features can be expected. it seems therefore recommendable to incorporate cascading into conventional radiation production models or to extend the model developed in this work by synchrotron radiation. | spectral imprints from electromagnetic cascades in blazar jets |
stars spend most of their lives in rather quiescent phases dominated by slow fluid flow, be it low mach number convection or wave-like motions. yet accurately modeling these phases is of great importance for understanding mixing in stars, the resulting impact on nucleosynthesis, and the rich data coming from asteroseismology. this influences the ultimate fate of the star and, in turn, galactic chemical evolution. it turns out that slow flows pose a tough computational challenge for several reasons. many of the numerical solvers show excessively dissipative behavior at low mach numbers, which can completely obfuscate the physical solution. also the very long time scales involved need special treatment, either at the level of the equations or in the time-stepping scheme. finally, the different physical effects, such as fluid dynamics, gravity, magnetic fields, and nuclear burning, are in a delicate balance, which requires careful coupling. i will talk about different approaches to solve these challenges and how to make these simulations work efficiently on large supercomputers. in particular i will talk about the merits of implicit time-stepping, well-balancing of different physics terms, and flux functions for all mach numbers. | the challenge of simulating slow flows in stellar astrophysics |
how molecular clouds fragment and create the dense structures which go on to eventually form stars is an open question. this thesis numerically investigates various aspects of fragmentation and structure formation in young molecular clouds based on the silcc-zoom and silcc deep-zoom simulations. the silcc-zoom simulations follow the self-consistent formation of molecular clouds in a few hundred parsec sized region of a stratified galactic disc, which include (self-) gravity, magnetic fields, supernova driven turbulence, as well as a non-equilibrium chem- ical network and treatment of the interstellar radiation field, with resolutions of ∼ 0.1 parsec. the silcc deep-zoom simulations are an extension of the cloud scale silcc-zoom simulations and allow us to resolve structures with a maximum resolution of 0.0078 parsec ( ∼ 1600 au). we identify 3d volumes inside the simulated clouds as structures using dendrograms and analyze their behaviour. by considering the energetic balance of cloud scale sub-structures, we find that our molecular clouds are dominated by the interplay of turbulence and self-gravity - with self- gravity becoming dynamically dominant only over time. this supports the gravo-turbulent scenario of structure formation. by tracing the morphology of cloud scale structures, we evaluate our clouds to be sheet-like on larger scales, likely tracing the shells of bubbles driven by supernovae. we estimate the effect of magnetic fields in molecular clouds and their atomic envelopes and find that magnetic fields alter the nature of fragmentation at low densities, slow down the formation of denser structures, but do not seem to be dynamically important in the further evolution of these potentially star forming sub-structures. we extend the study of energetics and morphology to sub-pc scale structures using the novel silcc deep-zoom simulations. we find different methods of forming filaments - fragmentation of mostly self-gravitating structures, as well as shock compression. moreover, we find that gravitationally bound, spheroidal cores emerge at ∼ 0.1 parsec scales and are embedded inside gravitationally dominated filaments. | morphology, fragmentation, and dynamic balance: an investigation into early stages of structure formation in molecular clouds |
the recent discovery of the moderate differential rotation between the core and the envelope of intermediate-mass (im) main-sequence and evolved stars, and the population of im red giants presenting a surprisingly low-amplitude of their mixed modes (i.e. modes that behave as acoustic modes in their external envelope and as gravity modes in their core) could both be the signature of a strong magnetic field trapped inside the radiative regions of im stars. indeed, stars more massive than 1.1 solar mass are known to develop a convective core during their main sequence. the field generated by the dynamo triggered by this convection could be the progenitor of a strong fossil magnetic field trapped inside the core of the star for the rest of its evolution. in this context, the mixed modes observed thanks to space-based asteroseismology can constitute an excellent probe of the deepest layers in im evolved stars: such magnetic fields may impact their propagation inside the core of these stars, and these perturbations should be visible in asteroseismic data. to unravel which constraints can be obtained from these observations, we theoretically investigate the effects of a plausible mixed magnetic field with various amplitudes on the mixed-mode frequencies of red giants. applying a perturbative method, we estimate the magnetic splitting of the frequencies of simulated mixed dipolar modes that depends on the magnetic field strength and its configuration. a complete asymptotic analysis is derived, showing the potential of asteroseismology to probe the magnetism at each depth as this is done for stellar rotation. the effects of the mass and the metallicity of the stars are also explored. finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action to redistribute angular momentum in stellar interiors. | probing fossil magnetic field effects in the core of evolved low-mass stars using mixed-mode frequencies |
a radio telescope on the far side of the moon can have a revolutionary impact in the field of cosmology. it can measure signals at frequencies below 30 mhz, which represent some of the earliest signals in the cosmological history of the universe, but are blocked from reaching terrestrial radio telescopes by the earth's ionosphere. measuring radio signals in the 6-30 mhz band allows us to track the evolution of the neutral intergalactic medium (igm) before and during the formation of the first stars by measuring the signal associated with the highly red-shifted hyperfine transition of neutral hydrogen. in addition, being on the far side of the moon, it is shielded from terrestrial sources of radio frequency interference. we propose to build a ~1 km aperture radio telescope on the far side of the moon, by robotically suspending a wire mesh into an appropriately-sized crater. the concept of operations requires two landers, one of which lands near the center of the crater, carrying the wire mesh. the other lander lands on the rim, carrying duaxel robots. these are four-wheeled rovers with an ability to split and rappel their front wheels and axles down crater walls while remaining tethered to the rest of the system that serves as an anchor and provides power and communication to the rappelling two-wheeled rover. upon reaching the lander at the center of the crater, it helps deploy the mesh by carrying its ends up the crater walls to different points on the rim. the duaxel rovers also help in the deployment of guy-wires for hoisting the receiver. suspending a wire mesh by anchoring its ends will lead to a catenary shape, which is not ideal for focusing the signal at the receiver. this can be compensated for by using wires with variable linear mass density. the density of the wire mesh, thickness and material of the wire, and tolerance for non-parabolic shape of the telescope are all parameters that dictate the rf performance of the telescope and also dictate the complexity and cost of the robotic operations that need to be carried out. we describe the results obtained by simulating the electromagnetic performance of different antenna configurations generated by varying the aforementioned parameters. these simulations help us optimize the engineering design of the telescope for meeting our science objectives. | ultra-long wavelength radio astronomy using the lunar crater radio telescope (lcrt) on the farside of the moon |
hex-p is a probe-class mission concept that will combine high spatial resolution x-ray imaging (<10 arcsec fwhm) and broad spectral coverage (0.1-150 kev) with an effective area far superior to current facilities (including xmm-newton and nustar) to enable revolutionary new insights into a variety of important astrophysical problems. hex-p observations will address important open questions in magnetar and high-b pulsar physics such as (1) the trigger of the 'transient' outburst epochs in high-b neutron stars and the dissipative mechanism of the ensuing surface thermal and magnetospheric non-thermal energy enhancement, thereby constraining untwisting magnetic-field loops and thermal crustal relaxation models; (2) the mechanism behind magnetar short bursts and their connection to fast radio bursts; (3) establishing the broad-band sed of persistently luminous magnetars and its variability pattern with rotational phase, potentially testing fundamental predictions of quantum electrodynamics. finally, hex-p will illuminate our understanding of the evolutionary pathways of young nss, with the goal of constraining the populations of central compact objects (ccos), magnetars, gamma-ray pulsars, and normal radio pulsars. we will present simulations of hex-p observations of the diverse types of young isolated nss, and demonstrate how hex-p's unique broadband spectral and timing capabilities will advance our understanding of these topical sources. more information on hex-p, including the full team list, is available at http://hexp.org. | the high energy x-ray probe (hex-p): broadband x-ray observations of magnetars and other young isolated neutron stars |
we investigate, using a multi-fluid approach, the main properties of standing ion-acoustic modes driven by non-linear standing alfvén waves. the standing character of the alfvénic pump is due to the superposition of two identical circularly polarised counter-propagating waves. we consider parallel propagation along the constant magnetic field and we find that left- and right-handed modes generate via ponderomotive forces the second harmonic of standing ion-acoustic waves. we demonstrate that parametric instabilities are not relevant in the present problem and the secondary ion-acoustic waves attenuate by landau damping in the absence of any other dissipative process. kinetic effects are included in our model where ions are considered as particles and electrons as a massless fluid, and hybrid simulations are used to complement the theoretical results. analytical expressions are obtained for the time evolution of the different physical variables in the absence of landau damping. from the hybrid simulations we find that the attenuation of the generated ion-acoustic waves follows the theoretical predictions even under the presence of an alfvénic pump. due to the non-linear induced ion-acoustic waves the system develops density cavities and an electric field parallel to the magnetic field. theoretical expressions for this density and electric field fluctuations are derived. the implications of these results in the context of standing slow mode oscillations in coronal loops is discussed. movies are available at https://www.aanda.org | excitation of ion-acoustic waves by non-linear finite-amplitude standing alfvén waves |
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