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we propose a 100ks observation of the unique outbursting seyfert galaxyic 3599, to be triggered if it flares again. two giant-amplitude x-rayoutbursts (factor > 100) of ic 3599 in 1990 and 2010 were accompaniedby dramatic emission-line response never seen before in any agn, buthigh-resolution x-ray spectra where missed. among the two outburst models-- a repeat stripping of the same star in orbit of a tidal disruptionevent or an accretion-disk instability. we propose to obtain the firsthigh-resolution x-ray spectrum of ic 3599 in one of these rare outburststates in order to understand accretion and wind physics under extremeconditions. | the nature of the x-ray outbursts in ic 3599 |
wz sge is the prototype of highly evolved, low-accretion rate dwarf novae (dne). during the decline from eruptions, its light curve displays a 'dip' followed by ≃10 'echo outbursts'. the standard disc instability model does not account for this behaviour, which is also seen in other low-accretion rate dne. one recent interpretation for these rapid brightness changes is that they represent transitions into and out of a magnetic propeller regime. here, we test this scenario with time-resolved, ultraviolet spectroscopy taken with the hubble space telescope just before, during and after the dip in wz sge's 2001 eruption. we find no distinctive or unique signatures that could be attributed to a propeller in either the time-averaged uv spectrum or the variability spectrum. thus the data do not support the magnetic propeller scenario. instead of resolving the mystery of wz sge's outburst light curve, our study has actually added another: the origin of the narrow absorption features seen in all outburst phases. we show explicitly that these features are likely formed in a high-density 'veiling curtain' with a characteristic temperature $\rm t \simeq 17,000~\mathrm{k}$. however, the nature and origin of this veil are unclear. given that wz sge-type dne are the most intrinsically common class of accreting white dwarfs, resolving these questions should be a high priority. | plateaus, dips, and rebrightenings during the outbursts of wz sge: no magnetic propeller, but a veiling curtain |
planets form from gas and dust discs in orbit around young stars. the timescale for planet formation is constrained by the lifetime of these discs. the properties of the formed planetary systems depend thus on the evolution and final dispersal of the discs, which is the main topic of this thesis. observations reveal the existence of a class of discs called "transitional", which lack dust in their inner regions. they are thought to be the last stage before the complete disc dispersal, and hence they may provide the key to understanding the mechanisms behind disc evolution. x-ray photoevaporation and planet formation have been studied as possible physical mechanisms responsible for the final dispersal of discs. however up to now, these two phenomena have been studied separately, neglecting any possible feedback or interaction. in this thesis we have investigated what is the interplay between these two processes. we show that the presence of a giant planet in a photo-evaporating disc can significantly shorten its lifetime, by cutting the inner regions from the mass reservoir in the exterior of the disc. this mechanism produces transition discs that for a given mass accretion rate have larger holes than in models considering only x-ray photo-evaporation, constituting a possible route to the formation of accreting transition discs with large holes. these discs are found in observations and still constitute a puzzle for the theory. inclusion of the phenomenon called "thermal sweeping", a violent instability that can destroy a whole disc in as little as 10 4 years, shows that the outer disc left can be very short-lived (depending on the x-ray luminosity of the star), possibly explaining why very few non accreting transition discs are observed. however the mechanism does not seem to be efficient enough to reconcile with observations. in this thesis we also show that x-ray photo-evaporation naturally explains the observed correlation between stellar masses and accretion rates and is therefore the ideal candidate for driving disc evolution. another process that can influence discs is a close encounter with another star. in this thesis we develop a model to study the effect of stellar dynamics in the natal stellar cluster on the discs, following for the first time at the same time the stellar dynamics together with the evolution of the discs. we find that, although close encounters with stars are unlikely to change significantly the mass of a disc, they can change substantially its size, hence imposing an upper limit on the observed disc radii. finally, we investigated in this thesis whether discs can be reformed after their dispersal. if a star happens to be in a region that is currently forming stars, it can accrete material from the interstellar medium. this mechanism may result in the production of "second generation" discs such that in a given star forming region a few percent of stars may still possess a disc, in tentative agreement with observations of so called "old accretors", which are difficult to explain within the current paradigm of disc evolution and dispersal. | proto-planetary disc evolution and dispersal |
plasma jets can be found in astrophysical systems (accretion disks, polars or young stellar objects), but they are also useful as a platform to study plasma properties and transport effects. on a experiment at the pals facility, we have studied the formation and propagation of rear-driven, collisional plasma jets from different foil thicknesses and materials when subject to an intense external magnetic field. magnetic fields were generated using a pair of helmholtz coils that provide 5-10 t in the direction perpendicular to the jet propagation. the diagnostics used were the streaked optical self-emission as a measurement of jet velocity, and 4-frame interferometry as a measurement of the jet density. with the right scaling factors, this data can help model the accretion of matter into magnetized astrophysical systems, such as the surface of young stellar objects, as well as the role that instabilities play in this process. this work was supported by the helmholtz association under grant no. vh-ng-1338. | modeling of young stellar objects through the study of magnetized rear-driven plasma jets from thin foil targets |
i present new narrow-band h-alpha imaging for 24 nearby edge-on galaxies in the chang-es survey. i use the images in conjunction with wise 22 micron imaging of the sample to estimate improved star formation rates (sfrs) using the updated recipe from vargas et al. (2018). i explore correlations between the updated star formation properties and radio continuum scale heights, scale lengths, and diameters, measured in krause et al. (2018). i discuss a newly discovered correlation between sfr and radio scale height that did not exist using mid-ir only sfr calibrations. this implies that a mid-ir extinction correction should be applied to sfr calibrations when used in edge-on galaxies, due to attenuation by dust. the updated sfr values also show newly discovered correlations with radio scale length and radio diameter, implying that the previously-measured relationship between radio scale height and radio diameter originates from star formation within the disk. we also identify a region of star formation located at extreme distance from the disk of ngc 4157, possibly ionized by a single o5.5 v star. this region is spatially coincident with an xuv disk feature, as traced by galex nuv imaging. we theorize that the star formation feature arose due to gravitational instability within gas from an accretion event. new h-alpha images from this work can be found at the chang-es data release web site, https://www.queensu.ca/changes. | h-alpha imaging of nearby edge-on galaxies, new sfrs, and an extreme star formation region (data release) |
cannizzo & nelemans (2015) applied the accretion disk limit cycle model to am cvn stars, using the width of the observed instability strip to derive three relations - (i) the mass transfer rate versus orbital period p_orbital, (ii) the recurrence time for outbursts versus p_orbital, and (iii) the outburst duration t_duration versus p_orbital. the first two relations were consistent with observations; the third was not. the inconsistency lay in the fact that the predicted variation of outburst duration on orbital period was much flatter d log(t_duration)/d log(p_orbital) ~ 0.4 than the observationally determined value ~4.5 from levitan et al. (2015). we show that the previously determined steep relation was strongly influenced by upper limit values at high p_orbital which were included in the fit, and that if one excludes the upper limit values and includes t_duration values for three systems at high p_orbital which were not considered previously (because they are in systems with only one historical outburst), then d log (t_duration)/ d log (p_orbital) is flat. | the outburst duration versus orbital period relation for am cvn stars and the 2018 may/june outburst of sdss j141118.31+481257.6 |
binary black holes in which both spins are aligned with the orbital angular momentum do not precess. however, the up-down configuration, in which the spin of the heavier (lighter) black hole is aligned (anti-aligned) with the orbital angular momentum, is unstable to spin precession at small orbital separations. we first cast the spin precession problem in terms of a simple harmonic oscillator and provide a cleaner derivation of the instability onset. surprisingly, we find that following the instability, up-down binaries do not disperse in the available parameter space but evolve towards precise endpoints. namely, they approach merger with the two spins co-aligned with each other and equally misaligned with the orbital angular momentum. merging up-down binaries relevant to ligo/virgo and lisa may be detected in these endpoint configurations if the instability onset occurs prior to the sensitivity threshold of the detector. we finally apply these findings to a simple astrophysical population of binary black holes where a formation mechanism aligns the spins without preference for co- or counter-alignment, as might be the case for stellar-mass black holes embedded in the accretion disk of a supermassive black hole. see arxiv:2003.02281. | unstable spin precession in binary black holes |
v630 cas is one of several long-period cataclysmic variables withrare, long-duration, dwarf nova outbursts. the characteristics of theseoutbursts are likely shaped by the large physical size of the accretiondisks. moreover, they often are luminous (> 10e33 ergs/s) x-ray sourcesin quiescence, severely challenging the disk instability model. herewe propose an xmm-newton observation of v630 cas in quiescence toconstrain the white dwarf mass and the accretion rate, thereby testingthe hypothesis that a large accretion disk can remain in quiescencewhile maintaining a high accretion rate. we also propose to obtain fastuv photometry with the om, since uv flickering has emerged as a keydiagnostic of accretion for the related class of symbiotic stars. | probing accretion disks on intermediate size scales: the case of v630 cas |
the young star elias 2-27 has recently been observed with alma to posses a massive circumstellar disc with two prominent large-scale spiral arms out to 250au. these are the frst observations of extended spirals in the disc midplane. we perform three-dimensional smoothed particle hydrodynamics simulations, radiative transfer modelling, synthetic alma imaging and an unsharped masking technique to explore three possibilities for the origin of the observed structures - an undetected companion either internal or external to the spirals, and a self-gravitating disc. we find that a gravitationally unstable disc and a disc with an external companion can produce morphology that is consistent with the observations. in addition, for the latter, we find that the companion could be a relatively massive planetary mass companion (< 10 - 13 mjup) and located at large radial distances (between 300-700 au). such a companion could not have formed by core accretion so quickly at such large distances. we therefore suggest that elias 2-27 may be one of the frst detections of a disc undergoing gravitational instabilities, or a disc that has recently undergone fragmentation to produce a massive companion. | the spiralling signatures of planet formation |
dwarf nova outbursts are nonlinear phenomena, and a time-dependent disk model is necessary to account for observations in detail. however, it is also necessary to elaborate a simpler steady-state fit to interpret observations. to know in what condition the outburst is initiated, understanding of the dwarf nova outburst is important. the parameterized, steady-state fitting formulae are suggested by smak (acta astron. 52, 429 (2002); ibid 60, 83 (2010)) for the critical disk temperature and mass accretion rate above which the disk becomes thermally unstable. the fits give a single-valued temperature and accretion rate and are radius-independent whereas the observations show that the outbursts are radius-dependent phenomena of the ionizaton propagating in the disk. the fits have been tested to account for the observed outbursts only for systems with orbital periods shorter than a half day. therefore, we examine the fits for orbital period as long as 2 days and compare the fits to the time-dependent model of a long-period dwarf nova gk per. the fits are not much different from the time-dependent result for the critical temperature. however, the fits for the critical mass accretion rate above which the disk enters the hot state overestimate the time-dependent model for a long-period system like gk per. the critical mass accretion rate in the intermediate state is consistent with that from the time-dependent disk model. however, the fit value should be treated as a maximum possible value below which the disk maintains the intermediate state, which is consistent with an interpretation for the observations of the z cam stars. | critical temperature and accretion rate of outbursts in long-period dwarf novae |
young protostars that undergo episodic accretion can provide insight into the effects of stellar evolution and impact on their circumstellar environments. l1251 vla 6 is a four component protostar system with one of those being a fading outbursting protostar. here we examine structure in the disk around l1251 vla 6 at a frequency of 33 ghz with the very long array (vla). given the rarity of ysos undergoing this type of accretion, l1251 vla 6 can provide insight into the fading post-outburst process. to constrain the disk structure, we adopt a modeling approach specifically taking the protoplanetary disk density profile equations laid out in andrews et al. (2009) and then follows the same technique as in white et al. (2020) that uses an mcmc approach to constrain the most probable values. this model then combined with a parametric ray-tracing code to generate synthetic model images of an axisymmetric disk with an mcmc fitting algorithm, allowing us to characterize the radial distribution of dust in the system. the results of our mcmc fit shows that the most probable values for the mass and radius are more consistent with class i. we find that the mass of the disk to be 0.070, consistent with calculated values. additionally, we investigate the reasoning for the accretion outburst, which we determine is not caused by gravitational instability. | observations of the fading outburst system l1251 vla 6 |
in order to improve the physical interpretation about innermost dusty regions in protoplanetary discs around brown dwarf (bd), and even very low mass star (vlms), we present a grid of models taking into account two different sets: (i) the set called standard model, that simulates an axisymmetric dusty disc with an inner curved wall. (ii) the perturbed one called non-standard where the axisymmetry of the inner edge has been broken. we have achieved a fitting for the disc structure able to explain the spectral energy distribution (sed). as the main condition, we assume that the changes of the inner wall geometry in the tongue-like shape depend on the rayleigh-taylor instability (r-tins) generated in the inner disc edge. for each object, we parametrize the shape of the inner wall to find a time-dependent model that enables us to explain the photometric near-infrared variability and connect the changes on the inner disc structure with the amplitude of such variability. we re-analysed photometric measurements from near to mid-infrared wavelengths of a sample of 6 bds and one vlms in different cloud associations which were previously studied by other authors. we also show that the flux change calculated between the non-standard and the standard configurations models the observed variability in lrll 1679. the magnitude changes due to these fluctuations slightly depend on the wavelength and they can present differences of up to 0.9 mag. we suggest that if the r-tins persist enough time, the features in the protoplanetary inner disc, e.g. inner holes or gaps evolve. | about the modelling of the sed for the inner boundary of protoplanetary discs at the lower stellar mass regime |
a young star inevitably accretes through a surrounding disk due to the conservation of angular momentum of the parent molecular cloud. such a protoplanetary disk typically forms a magnetically dead zone at the distance of a few au from the central protostar, where the turbulence resulting from magnetorotational instability (mri) is suppressed. the dead zone results in a bottleneck in the angular momentum transport and as a consequence, a large amount of mass may accumulate in the inner disk. accretion through such a layered disk is not steady but intrinsically unstable and can exhibit powerful outbursts known as fuor and exor events. in this talk, i will present the results of the first investigation into the effects of low metallicity environment on the structure of the dead zone as well as outbursting behavior of the protoplanetary disk. a metal poor disk accumulates much more mass in the innermost regions, as compared to its solar metallicity counterpart. the duration of the burst phase is also reduced significantly at low metallicities and is confined mostly to the early, embedded stages. in conclusion, metallicity of a protoplanetary disk can have profound effects on both its structure and evolution in terms of episodic accretion. | effects of metal poor environments on the episodic accretion in protoplanetary disks |
the properties of protostellar (class 0/i) disks play an important role in planet formation by setting the environment for dust grain growth and the initial conditions for subsequent protoplanetary (class ii) disk evolution. however, obtaining these properties, especially the disk mass, has been challenging because translating observations of dust thermal emission to disk mass are subject to large uncertainties in dust optical depth and dust temperature. we present new estimates on the mass of typical protostellar disks through a synergy between theory and observation. using a radiative non-ideal mhd simulation that self-consistently traces the formation of a protostellar disk together with a set of analytic arguments, we argue that a typical protostellar disk should be gravitationally self-regulated, where a balance between infall from the envelope and accretion driven by gravito-viscous angular momentum transport keeps the disk at a marginally gravitationally unstable state with toomre q parameter of order unity. following this physical picture, we formulate a simple model of gravitationally self-regulated disk and use it to fit multi-wavelength dust continuum observations in the vandam orion survey. we find that the majority of observed disks can be fit well with this model. moreover, the data are better fit by disk models assuming order-unity q than those assuming larger q values, suggesting that gravitational instability might be common among protostellar disks. we use this model to produce new estimates of disk properties, and find that typical protostellar disks are significantly more massive than previously expected, with typical disk-to-star mass ratio ~ 1. such high disk mass may have important implications for planet formation. | the majority of protostellar disks might be way more massive than we thought: new insights from theory and observation |
in accretion disks, magneto-rotational instability (mri; balbus & hawley, 1991) makes the disk gas in the magnetic turbulent state and drives efficient mass accretion into a central star. mri drives turbulence through the evolution of the parasitic instability (pi; goodman & xu, 1994), which is related to both kelvin-helmholtz (k-h) instability and magnetic reconnection. the wave number vector of pi is strongly affected by both magnetic diffusivity and fluid viscosity (pessah, 2010). this fact makes mhd simulation of mri difficult, because we need to employ the numerical diffusivity for treating discontinuities in compressible mhd simulation schemes. therefore, it is necessary to use an mhd scheme that has both high-order accuracy so as to resolve mri driven turbulence and small numerical diffusivity enough to treat discontinuities. we have originally developed an mhd code by employing the scheme proposed by kawai (2013). this scheme focuses on resolving turbulence accurately by using a high-order compact difference scheme (lele, 1992), and meanwhile, the scheme treats discontinuities by using the localized artificial diffusivity method (kawai, 2013). our code also employs the pipeline algorithm (matsuura & kato, 2007) for mpi parallelization without diminishing the accuracy of the compact difference scheme. we carry out a 3-dimensional ideal mhd simulation with a net vertical magnetic field in the local shearing box disk model. we use 256x256x128 grids. simulation results show that the spatially averaged turbulent stress induced by mri linearly grows until around 2.8 orbital periods, and decreases after the saturation. we confirm the strong enhancement of the k-h mode pi at a timing just before the saturation, identified by the enhancement of its anisotropic wavenumber spectra in the 2-dimensional wavenumber space. the wave number of the maximum growth of pi reproduced in the simulation result is larger than the linear analysis. this discrepancy is explained by the simulation result that a shear flow created by mri locally becomes thinner and faster due to interactions between antiparallel vortices induced by k-h mode pi, and this structure induces small scale waves which break the shear flow itself. we report the results of the simulation, and discuss how the saturation amplitude of mri is determined. | mhd simulation of transition process from the magneto-rotational instability to magnetic turbulence by using a high-order mhd simulation scheme |
we investigate dynamics of slender magnetic flux tubes (mft) in the accretion disks of young stars. simulations show that mft rise from the disk and can accelerate to 20-30 km/s causing periodic outflows. magnetic field of the disk counteracts the buoyancy, and the mft oscillate near the disk's surface with periods of 10-100 days. we demonstrate that rising and oscillating mft can cause the ir-variability of the accretion disks of young stars. | rising magnetic flux tubes as a source of ir-variability of the accretion disks of young stars |
the large-scale magnetic field in the accretion disks of young stars is investigated. main features of our magnetohydrodynamical (mhd) model of the accretion disks and typical simulation results are presented. we discuss the role of mhd effects, ionization structure, magnetic field geometry and strength of the accretion disks. | large-scale magnetic field of the accretion disks of t tauri stars |
calm, "secular" accretion and evolutionary processes, once thought to be relegated to the sidelines of galaxy evolution, are now understood to play a significant role in the buildup of stellar mass in galaxies. most galaxies are formed and evolve via a mix of secular-driven evolution and more violent processes like strong disk instabilities and galaxy mergers; this makes isolating the effects of secular evolution in galaxies very difficult. massive pure disk galaxies, lacking the classical or "pseudo" bulge components that arise naturally from mergers and disk instabilities (respectively), are a unique opportunity to study galaxy evolution in the absence of violent processes. previous studies have disagreed on whether the black hole-galaxy mass correlation is driven by galaxy-galaxy interactions or something more fundamental. here we present new evidence using a statistically significant sample of agn hosted in bulgeless disk galaxies at z < 0.2 to constrain black hole-galaxy co-evolution in the absence of mergers. | the merger-free co-evolution of galaxies and supermassive black holes |
accretion disks are important at different {scales} in the physics of the universe. for instance, binaries systems called {redback} (close binaries systems composed of a normal giant star and a neutron star) have shown transitions from an x-ray emission state (the neutron stars is accreting material) to {a} radio pulsar state ({where a} milisecond pulsar is observed). this transitions would be connected with instabilities in the accretion disk.in this paper, we show our first study of local instabilities in the disk due to a magnetic field and its influence in the viscosity. we analyze the increment of magnetic field by dynamo process, taking {into} account the {characteristic} timescales of parker instability, magnetic reconnection, and magneto-rotational instability. | estudio de la viscosidad en un disco de acreción delgado |
the evolution of the white dwarf remnant of the merger of two white dwarfs is still an open problem, and even more in the case when the mass of the remnant is larger than the chandrasekhar limiting mass, namely when a metastable super-chandrasekhar white dwarf is formed. angular momentum loss might bring the white dwarf to conditions for a thermonuclear explosion or to gravitational and/or rotational instabilities. dipole magnetic braking is one of the mechanisms that can drive such loss of angular momentum providing the white dwarf is highly magnetized. however, the timescale on which this process occurs is still the matter of an active debate, as it depends on many factors, like the strength of the magnetic field, its angle of inclination with respect to the rotation axis, and the structure properties of the white dwarf. in addition, the coalescence leaves a surrounding keplerian disk that can be accreted onto the newly formed white dwarf. here we compute the post-merger evolution of a super-chandrasekhar magnetized white dwarf taking into account all the relevant physical processes. these include magnetic torques acting on the star, accretion from the keplerian disk, and the threading of the magnetic field lines through the disk. | induced compression by angular momentum losses in fast-rotating, magnetized super-chandraskehar white dwarfs |
planetary mass companions to solar mass stars may form through core accretion, gravitational instabilities in the disk, or protostellar core fragmentation. recent searches have uncovered about 15 planetary mass companions to young sun-like stars in orbits of 50-300 au from the central star. these objects pose significant challenges for formation theories. one particular object, fw tau b, may be caught in the process of formation. fw tau b has an apparent brightness and colors consistent with an early l-dwarf, bright emission lines consistent with strong accretion, heavy k-band veiling due to warm dust, and a spatially resolved jet. however, the object mass is not known because the accretion/disk signatures veil the photosphere and have prevented detection of photospheric features and along with a precise measurement of spectral type or temperature. we propose to use stis g750l to obtain a deep red-optical spectrum of this very faint and enigmatic object to measure its spectral type and thereby infer its mass. if the photosphere is visible, an m-dwarf spt would mean that the object is a brown dwarf obscured by an edge-on disk. an l-dwarf spt would indicate a planetary mass object with strong accretion. if entirely veiled, this object would have a remarkably high accretion rate, consistent with it being caught either in its main formation stage or in outburst. a wealth of data, including hst optical photometry, keck-ao near-ir spectra, and alma observations, all suggest that fw tau b is a very low mass companion at an important stage of evolution. these stis data are crucial to finally tie these observations together with a spt and refined mass estimate of this object. | the very low mass object fw tau b: an edge-on brown dwarf disk or a planet caught in formation? |
the magnetorotational instability (mri) is a crucial mechanism of angular momentum transport in a variety of astrophysical scenarios, as e-e+ plasmas accretion disks nearness neutron stars and black holes. the mri has been widely studied using mhd models and simulations, in order to understand the behavior of astrophysical fluids in a state of differential rotation. when the timescale for electron and ion collisions is longer than the inflow time in the disk, the plasma is macroscopically collisionless and mhd breaks down. this is the case of the limit of weak magnetic field, i.e., as the ratio of the ion cyclotron frequency to orbital frequency becomes small. leveraging on the recent addition of the shearing co-rotating frames equations of motion and maxwell's equations modules in our pic code osiris 3.0, we intend to present our recent results of the analysis of mri in electron-positron plasma in the limit of weak magnetic field. we will recall the theoretical 1d linear model of krolik et zweibel that describes the behavior of mri in the limit of weak magnetic field and use it to support our results. moving to 2d simulations, the analysis of mri via pic code permits to investigate also how mri will act in comparison with other kinetic instabilities, like mirror instability. | pic simulations of the magnetorotational instability in electron-positron plasmas |
the post-merger evolution of black hole-neutron star and neutron star-neutron star systems is driven by magnetohydrodynamic turbulence. such multiscale problems are very costly to simulate. one approach is to use artificially large seed magnetic fields to resolve the magnetorotational instability. another is to add some model of subgrid-scale effects, with subgrid angular momentum transport often being modeled as a shear viscosity. we simulate the disk from a black hole-neutron star model in both ways and compare results. we present a new implementation of the relativistic navier-stokes equations in the spectral einstein code, with accompanying star and accretion torus tests. for the post-merger system, we analyze the combination of shocks and turbulent/viscous dissipation acting to heat the disk in the early post-merger phase. | black hole-neutron star post-merger evolution using viscous relativistic hydrodynamics |
the tidal disruption of a star by a massive black is expected to yield a luminous flare of thermal emission. optical transient surveys have collected about two dozen similar-looking nuclear transients that are consider examples of these stellar tidal disruption flares (tdfs). however, explaining the observed properties of these events within the tidal disruption paradigm is challenging. for example, there is no consensus on the origin of the optical emission. this theoretical ambiguity leaves open the possibility that the flares we call tdfs are instead due to a completely different process, such a nuclear supernovae or accretion disk instabilities. fortunately, the number of discovered tdfs recently became large enough to test a fundamental prediction of the stellar tidal disruption paradigm. at high black hole mass (greater than 108 m⊙), the star will be swallowed whole before being disrupted. using a recently compiled catalog of candidate tdfs with black hole mass measurements, plus a careful treatment of selection effects in this flux-limited sample, we robustly detect a suppression of flares from high-mass black holes. this dearth of observed tdfs from the upper end of the black hole mass distribution is naturally explained by suppression due the event horizon and implies a moderate mean spin of these black holes (a > 0.5). conversely, if we start by assuming that current tdf candidates are indeed due to stellar tidal disruptions, our sample can be used to constrain the existence of naked singularities. | stellar tidal disruption flares provide evidence for a black hole event horizon |
in recent years, the first accretion bursts from massive ysos have been detected. this confirmed that episodic accretion, possibly driven by disk instabilities, is a feature of high-mass star formation as well. these bursts were accompanied by flares of class ii methanol masers. thus, maser monitoring will improve prospects to discover myso bursts which are hard to find otherwise. this is the objective of the maser monitoring organization (m2o), founded in 2017. the first m2o alert candidate, g358.93-0.03, spotted in early 2019, lacked clear excess emission at nir/mir and (sub)mm wavelengths. thus, sofia fifi-ls observations were requested to verify the burst by detecting its fir emission. the sofia data clearly proved the presence of an accretion burst. from two epochs, and by incorporating alma data, major burst properties could be derived by radiative-transfer modeling (rtm) of the pre-burst, bust, and post-burst seds. the analysis also revealed that accretion bursts cause thermal afterglows of dust continuum emission which fade away with a wavelength-dependent time-scale. the use of time-dependent rtm opens up the possibility to better understand the physical and chemical processes induced by the bursts in the protostellar environment. this holds particularly for the dust-maser symbiosis. | ir observations of a flaring maser source - revealing the unsteady growth of massive stars |
close binary systems are formed by a varied family of objects, in particular, the named redback systems, i.e. the donor star transfers material to the neutron star, putting it in an accretion disc surrounding this star. later, this material falls on the neutron star. in the last years it was observed that some members of the redback family experienced transition from the state of low mass x-ray binary system to the pulsar state, and in the opposite way. the time scales associated with these transitions suggest that they are related to instabilities in the accretion disc. that fact motivates us to model the accretion disc around the neutron star in this kind of systems. we present our first results, associated with instabilities in the disc by irradiation of the neutron star. | inestabilidad radiativa en un disco de acreción en sistemas binarios interactuantes |
the magnetorotational instability (mri) is a crucial mechanism of angular momentum transport in a variety of astrophysical scenarios, as accretion disks nearness neutron stars and black holes. the mri has been widely studied using mhd models and simulations, in order to understand the behaviour of astrophysical fluids in a state of differential rotation. when the timescale for electron and ion collisions is longer than the inflow time in the disk, the plasma is macroscopically collisionless and mhd breaks down. this is the case of the limit of weak magnetic field, i.e., as the ratio of the ion cyclotron frequency to orbital frequency becomes small. leveraging on the recent addition of the shearing co-rotating frames equations of motion and maxwell's equations modules in our pic code osiris 3.0, we intend to present our recent results of the analysis of mri in collisionless plasma. increasing the scale of our simulations, we will show the first ab-initio pic simulations of a 2d turbulence induced consistently during the saturation regime of the mri. we will demonstrate the existence of a minimum scale λkink that determine the comparison of a drift-kink instability in the plasma. this instability will activate the turbulence during the saturation regime of the mri. | 2d mri-induced turbulence in high β pic simulation |
using 0d, 1d, and 3d models of galaxies, i explore different problems in galaxy evolution most suited to each technique. in the simplest case, a galaxy is described by a few numbers integrated via coupled ordinary differential equations. by allowing the galaxies to respond to a stochastic accretion rate, i show a natural way of generating the finite scatter observed in several galaxy scaling relations: the correlation between a galaxy's stellar mass and its star formation rate or metallicity. by comparing this simple model to observations, we constrain the process by which gas accretes onto galaxies, which must occur, but is essentially impossible to observe directly. adding an additional dimension to the models, we explore the structure of galactic disks as a function of radius. we find that turbulence driven by gravitational instability in the disks and the resulting migration of gas can explain a wide variety of phenomena, including the age-velocity dispersion correlation of stars in the solar neighborhood, the central quenching star formation in disk galaxies, rings of star formation, and the observed radial profile of gas column densities. finally, we run a set of fully three-dimensional galaxy simulations to try to understand what physics is responsible for basic properties of galaxies, including the rate at which they form stars, and the rate at which they eject mass in large-scale winds. we find that supernovae are capable of driving moderate metal-enhanced winds, but surprisingly they have very little effect on the star formation rates of dwarf galaxies. instead, ordinary photoelectric heating dominates the star formation law in low-mass galaxies. | numerical experiments in galactic disks: gravitational instability, stochastic accretion, and galactic winds |
the magnetorotational instability (mri) is the most promising mechanism in driving angular momentum transport in the radial direction in accretion discs associated to t tauri stars. however, the fact that this instability requires a minimum ionization fraction, makes it ineffective in the inner regions (i.e. at the midplane of the regions close to the central object) of these discs. in this work, we consider damping of alfvén waves as a possible source of extra heating in the disc and analyse its effects for the ocurrence of the mri in these systems. in particular, we focus on how the kelvin-helmholtz instability (khi), associated with the presence of surface alfvén waves, can develop, enhancing the energy dissipation due to a cascading of the wave energy. we take the keplerian shear to be the main responsible for the onset of khi and study how the development of such instability can influence the wave dissipation. our results confirm that the triggering of this instability can greatly amplify the amount of energy released, as previously stated in the literature, and thus make mri effective in a larger region of the disc. finally, we argue that this mechanism, when applied to t tauri discs, can couple both resonant absorption and turbulent damping of alfvén waves. | heating of protostellar accretion discs associated with plasma inhomogeneities |
redback millisecond pulsars (hereafter redbacks) are a sub-population of eclipsing millisecond pulsars in close binaries. the formation processes of these systems are not clear. the three pulsars showing transitions between rotation- and accretion-powered states belong to both redbacks and transient low-mass x-ray binaries (lmxbs), suggesting a possible evolutionary link between the them. through binary evolution calculations, we show that the accretion disks in almost all lmxbs are subject to the thermal-viscous instability during certain evolutionary stages, and the parameter space for the disk instability covers the distribution of known redbacks in the orbital period - companion mass plane. we accordingly suggest that the abrupt reduction of the mass accretion rate during quiescence of transient lmxbs provides a plausible way to switch on the pulsar activity, leading to the formation of redbacks, if the neutron star has been spun up to be an energetic millisecond pulsar. we investigate the evolution of redbacks, taking into account the evaporation feedback, and discuss its possible influence on the formation of black widow millisecond pulsars. | evolution of transient low-mass x-ray binaries to redback millisecond pulsars |
in march 2018, the transient asassn-18el occurred in the nucleus of seyfert 2 galaxy 1es1927+654. monitoring the source over several months our team discovered the emergence of broad balmer emission lines, suggesting a transition from type 2 to a type 1 agn on timescales consistent with the light travel time between the central black hole and the broad line region. archival x-ray and optical observations of 1es1927+654 in 2011 already showed an interesting agn, which, while optically classified as a type-2, showed an x-ray spectrum typical of an unobscured type 1 agn. after the recent optical outburst, the x-ray spectrum changed dramatically, with the hard x-ray corona decreasing by >2 orders of magnitude, and instead now resembles a 10^6 k thermal spectrum. in this talk, i will give an overview of our ongoing multi-wavelength follow-up with nicer, hst, xmm-newton, nustar, keck, swift and las cumbres observatory of this unprecedented changing-look agn. i discuss potential reasons for the onset of this accretion activity, including the tantalizing possibility that we observed the tidal disruption of a star that caused an instability in the pre-existing agn disc and corona. | the ongoing transformation of seyfert galaxy 1es 1927+654 |
investigation of instability in fluid galactic and accretion discs, in contrast to stellar discs, has not yet been formalised in standard methods. recently, it has been proposed to use the finite element method for this purpose, which makes it possible to reduce the problem of finding unstable modes to a linear algebraic eigenvalue problem. we show that this method can be successfully applied to the exactly solvable hunter disc model with pressure, and analyse the method errors. | investigation of instability by the finite element method on the example of one exactly solvable model of the fluid disc |
to study the effects of a gas component on the formation and evolution of a stellar bar, we run fully self-consistent three-dimensional simulations of isolated barred galaxies similar to the milky way. our models consider feedbacks from star formation and accretion to a black hole, but neglect the effects of magnetic fields. we vary the gas fraction in the disk as well as the toomre stability parameter q in the stellar disk. in models with q=1.2, the presence of gas tends to delay the bar formation and make a bar weaker since the gas disk does not actively participate in a bar-forming large-scale gravitational instability. in models with q=1.0, on the other hand, the gas rapidly turns into stars with low velocity dispersions, which cools down the stellar disk and thus promotes the bar formation. while gas-free disks are subject to buckling instability, disks with the gas fraction more than 5% are found to thicken secularly without undergoing the buckling instability. a stellar bar that forms is efficient in redistributing the gas in the bar regions and produces a star-forming nuclear ring. the ring is very small when it first forms and grows in size over time. the bars and nuclear rings formed in our models have properties similar to the central molecular zone in the milky way. | effects of gas on the formation and evolution of a bar in milky-way sized galaxies |
the majority of detected exoplanets are close-in super earths (planets of a few earth masses) orbiting their host star roughly inside 0.5 au. additionally nearly all systems of super earths feature multiple planets within the same system, where the period ratios between adjacent planets are mostly outside of resonance. however, their formation is still mysterious. we present here new simulations that have the potential to explain the formation pathway of super earths. we follow the growth, migration, and composition of planetary embryos of just a tiny fraction of the earth mass to full grown planetary systems, where solid accretion is enhanced through pebble accretion. our main findings include: a) a difference of about a factor two in pebble flux separates true terrestrial planet analogues, which are finally assembled after the gas disc dissipates, with super earths formed completely during the gas-disc phase b) chains of migrating super earths pile up in resonances, which break during late instabilities, where the resulting planetary systems match very well with the kepler observations co-authors: a. izidoro, a. johansen, a. morbidelli, s. raymond, s. jacobson, m. lambrechts | origin of super-earths planets: influence of pebble accretion, migration and instabilities |
protoplanetary discs (ppds) have been widely observed around young stars and are the birth cradle of planets. they are cold, dense and magnetised objects among which stand the transition discs (tds) characterised by a dust cavity in the inner regions that extends from a few au to a few hundreds au and whose formation remains yet unexplained. not only are such cavities seen in the dust profiles, but they are also detected in the gas profiles. a striking observation that challenges intuition states that despite their diminished surface density profile, tds accrete at a rate similar to the accretion rates of full ppds ($\dot{m}_ppd\sim 10^{-7} m_\odot \mathrm{yr}^{-1}$), suggesting a fast inward motion of matter in their cavity. a possible explanation for these high accretion rates is the presence of magnetised winds that would allow matter to fall onto the star at high radial velocity. the aim of this work is to tackle this observational discrepancy using magnetohydrodynamic (mhd) winds to account for accretion in tds with the help of global numerical simulations. i will present the results of $2.5 d$ and $3 d$ simulations modelling tds with non-ideal magnetic winds. i will in particular focus on mass accretion through the cavity and on the $3 d$ stability of this cavity against hydro and magnetohydrodynamic instabilities. | transition discs and magnetohydrodynamic winds: global numerical simulations |
clone from phase 1: this 8-orbit mid-cycle proposal requests hst/stis coronagraphic imaging to confirm a candidate wide-separation protoplanet seen around ab aur. near infrared data sets obtained over multiple epochs but only fully analyzed in may 2020 suggest a protoplanet whose colors/spectrum are distinguishable from scattered starlight and whose astrometry hints at orbital motion. optical imaging roughly contemporaneous with recently scheduled ground-based extreme ao follow-up observations in fall 2020/winter 2021 are required. only optical imaging can conclusively rule out the alternate hypothesis: that this signal is a static disk feature. stis is the only suitable optical high-contrast imaging instrument: its ability to yield unbiased detections of disks and point sources is unmatched by any ground or space-based facility. the analysis will decisively point to one of 3 interpretations: 1) thermal emission from a protoplanet formed by disk instability, 2) an orbiting disk region heated and puffed up by an unseen planet, or 3) a non-rotating disk feature whose pathological colors and near-ir variability can be mistaken for an orbiting protoplanet. our stis imaging multi-roll reference star differential imaging will easily achieve the required contrast to distinguish these scenarios. if interpretations 1) or 2) are supported, this candidate would be hst's first bona fide exoplanet direct imaging discovery, given the recent controversy over fomalhaut b. moreover, if 1) is correct, the program rewrites the field's understanding of planet formation, decisively demonstrating the existence of multiple mechanisms for forming jovian planets: core accretion and disk instability. | confirming a wide-separation directly-imaged infant planet around a young, dusty star |
complex a is a high-velocity cloud (hvc) that is traversing through the galactic halo toward the milky way's disk. we used green bank telescope observations to construct a spectroscopically resolved hi 21 cm map of this entire complex at roughly log(nhi/cm-2) = 17.9 (1-sigma) sensitivity and 25 pc spatial resolution. we find that that complex a will reach the galactic plane in roughly 70 myr if it can survive its journey. we have identified numerous signatures of gas disruption, including an elongated and multi-core structure that are characteristic of fragmentation due to either thermodynamic instabilities or shock-cascade processes. we also find rayleigh-taylor fingers on the low-latitude edge of this hvc; many have been pushed backward by ram pressure stripping. on the high-latitude side of the complex, kelvin-helmholtz instabilities have generated two large wings that extend tangentially off complex a; these wings are forming rayleigh-taylor globules at their tips. these observations provide new insights on the survivability of low-metallicity gas streams that are accreting onto l-star galaxies. | exploring hydrodynamic instabilities along the infalling high-velocity cloud complex a |
neutron star high mass x-ray binaries that exhibit superorbital variability offer an opportunity to study the geometry and stability of warped accretion disks. the high mass x-ray binary smc x-1 is an ideal system in which to investigate these questions because the supeorbital period has epochs of instability known as excursions likely caused by disk instability. using the high resolution spectral and timing capabilities of the neutron star interior composition explorer (nicer) we examined the high state of four different superorbital cycles of smc x-1 to search for short term changes in spectral shape and any connection to the unstable accretion disk geometry. we performed phase averaged and phase resolved spectroscopy, as well as principal component analysis to closely compare the spectral characteristics and any cycle-to-cycle variations. while soft, disk-related spectral components showed variations with time, the accretion column related parameters (i.e. photon index) remained mostly constant, indicating that the disk instability does not significantly change smc x-1's accretion process. | constraining the evolution of the unstable accretion disk in smc x-1 with nicer |
in recent years, the first accretion bursts from massive ysos (mysos) have been detected. this confirmed that episodic accretion, possibly driven by disk instabilities, is a feature of high-mass star formation as well. since mysos are usually deeply embedded, such events are hard to find at near-ir wavelengths. moreover, the burst-induced flux rise in the (sub)mm is much shallower than in the far-ir, where the peak of the spectral energy distribution is located. thus, sofia observations provide the best prospects to reveal the burst by detecting its fir emission which, at the same time, provides crucial information on the luminosity increase. our early results from the first time-dependent radiative transfer modeling of dust continuum emission, applied to this science case, indicate that the burst leads to a thermal afterglow which fades away with a wavelength-dependent time-scale. this implies requirements on the selection and scheduling of sofia instruments to follow the evolution of the spectral energy distribution over time. moreover, since the targets are usually bright, in particular during the burst, sensitivity is no issue. thus, snapshot measurements would suffice. establishing a sample of myso bursts is mandatory to assess the credibility of corresponding simulations. moreover, this will lead to better understanding of the burst-induced physical and chemical changes in the protostellar environment. | watching massive stars grow with sofia |
v630 cas and v1017 sgr are long-period cataclysmic variables withrare, long-duration, dwarf nova outbursts. the characteristics of theseoutbursts are likely shaped by the large physical size of the accretiondisks. they also appear to be luminous (> 10e33 ergs/s) x-ray sourcesin quiescence, severely challenging the disk instability model. here wepropose xmm-newton observations of these objects to obtain x-ray spectraof sufficient quality to constrain the white dwarf mass and the accretionrate. we also propose to obtain fast uv photometry with the om, since uvflickering has emerged as a key diagnostic of accretion for the relatedclass of symbiotic stars. | v630 cas and v1017 sgr: probing accretion disks on intermediate scales |
stratified vortices can be found from small to large scales in geophysical and astrophysical flows. on the one hand, tornadoes and hurricanes can lead to devastation and even a large number of casualties. on the other hand, vortices can distribute heat and momentum in the atmosphere which is important for a habitable environment on earth. in the astrophysical context, accretion disks (from which solar systems are formed) can be seen as stratified vortices. in such systems, understanding the mechanisms that can result in an outward transport of angular momentum is a central problem. for a planet or star to be formed in a disk, angular momentum has to be carried away from its center to allow matter aggregation by gravity; otherwise, its rotation speed would be far too large, avoiding this matter aggregation (and the consequent star formation) to happen. in such gas systems, turbulence is the most likely mechanism to achieve such a large angular momentum transport. however, it was shown that the flow profile of accretion disks is stable with respect to purely shear instabilities, and the question arises about how the turbulence can be generated. among other candidates, the strato-rotational instability (sri) has attracted attention in recent years. the sri is a purely hydrodynamic instability that can be modeled by a classical taylor-couette (tc) system with stable density stratification due to axial salinity or temperature gradients. in this thesis, a combined experimental and high-performance computing study of new specific behaviors of the strato-rotational instability (sri) is performed. the density stratification causes a change in the marginal instability transition when compared to classical non-stratified tc systems, making the flow unstable in regions where - without stratification - it would be stable. this characteristic makes the sri a relevant phenomenon in planetary and astrophysical applications, particularly in accretion disk theory. despite many advances in the understanding of strato-rotational flows, the confrontation of experimental data with non-linear numerical simulations remains relevant, since it involves linear aspects and non-linear interactions of sri modes which still need to be better understood. these comparisons also reveal new non-linear phenomena and patterns not yet observed in the sri, that can contribute to our understanding of geophysical flows. | high-performance computing and laboratory experiments on strato-rotational instabilities |
twenty-five years ago, pringle suggested a boundary-layer origin for jets from ysos. the jets were driven by a toroidal magnetic field generated by strong shear in the accretion boundary layer. such a mechanism is clearly non-magnetocentrifugal in nature.nearly fifteen years ago, we suggested a cartoon of the jet-launching mechanism in protostars in which shear, acting upon mhd turbulence generated by the magnetorotational instability (mri), generated a tangled, toroidal magnetic field capable of driving a jet. this picture, which is also manifestly non-magnetocentrifugal in nature, relied upon a novel model for mri-driven mhd turbulence based on a viscoelastic, rather than a viscous, prescription for the turbulent stress. our hypothesis has some clear similarities to pringle's mechanism, but it relied upon a large envelope surrounding the central star.an accretion boundary layer has long been recognized as a promising source for protostellar jets in good part because in a standard thin disk, matter loses circa half of all its accretion energy in this layer, but it is problematic to drive a well-collimated outflow from a boundary layer in a thin disk. in this presentation, we argue paradoxically that the "boundary layer" can drive jets when a true boundary layer, like the thin disk, does not exist. this changes the inner boundary condition for viscous angular momentum flux in the disk.the standard argument for a thin boundary layer is, we argue, circular. in high accretion-rate systems, or when the gas cannot cool efficiently, there is no reason to suspect the turbulent viscosity in this boundary layer to be small, and therefore neither is the boundary layer. when the boundary layer becomes larger than the central accretor itself, it is arguably no longer a boundary layer, but rather an envelope. it is still, however, a substantial source of power and toroidal mri-driven magnetic fields.it is, again, only in relatively hot or high-accretion rate systems in which the boundary layer would be expected to inflate and so disappear. not coincidentally, it is in such systems, such as class 0 and class i protostars, in which we have the strongest evidence for powerful, well-collimated jet outflows. | boundary-layer origin for jets, and non-existence of the boundary layer in young jet-producing protostars |
we report the analysis of infrared jhk_s high speed photometry of the dwarf nova v2051 oph in quiescence. we model the ellipsoidal variations in the light curve to measure the fluxes of the mass donor star. its colors are consistent with an m8 ± 1 spectral type with an equivalent blackbody temperature of t_{bb}= (2700± 300) k, in agreement with spectroscopic measurements and with theoretical expectation for donor stars at the same orbital period. we use the mass donor star fluxes and the barnes & evans relation to find a photometric parallax distance of (102 ± 16) pc to the binary. at this distance the outbursts of v2051 oph occur at disc temperatures everywhere lower than the minimum/critical temperature predicted by the disc instability model, underscoring previous suggestions that they are powered by mass transfer bursts. we subtract the contribution of the mass donor star and apply eclipse mapping techniques to the remaining light curve in order to investigate the structure and emission of its accretion disc. the infrared accretion disc is bright and 'blue' in the inner regions and becomes progressively fainter and redder with increasing radii, indicating that the disc temperature decreases with radius. bulges in the eclipse shape, more prominent in the h and k_s bands, lead to asymmetric arcs in the eclipse maps reminiscent of the spiral arms found in disc maps of outbursting dwarf novae. the arcs show an azimuthal extent of ∼90^o, extend from the intermediate to the outer disc regions (0.3-0.4 r_{l1}, where r_{l1} is the distance from disc center to the inner lagrangian point), and account for ≃ 30 per cent of the total flux in the h and k_s bands. | the mass donor star and the accretion disc of the dwarf nova v2051 ophiuchi in the infrared |
galaxies need to accrete gas to form stars. stars form near the center of the galactic halo within the galactic disc. the most proficient star forming galaxies, starburst galaxies, are those that involve filaments that withstand the shock that forms at the edge of the galactic halo and transport matter deep into the galactic disc. the cold, dense matter within the filament moves within the hot gaseous background, indicating that the filament boundary is likely kelvin-helmholtz (kh) unstable. if the kh instability is allowed enough time to evolve, it will potentially disrupt the filaments before they can penetrate deeply within the galaxy. galactic scale simulations capable of modeling the filament dynamics lack the spatial resolution to capture these hydrodynamics. therefore, we have conducted a scaled, high-energy-density laboratory experiment on the omega-ep laser that emulates and studies the cosmological process of a cold stream penetrating a shocked region. we use a radiography diagnostic to observe the kh instability on the filament boundary and help tune hydrodynamic simulations performed using crash. from the data and tuned simulations, we determine whether the kh instability time-scale is relevant and to what extent it can inhibit mass delivery to the galactic disc. this work is funded by the u.s. department of energy nnsa center of excellence under cooperative agreement number de-na0003869. | experiments to study kh evolution of filaments feeding starburst galaxies on omega-ep |
exoplanet surveys have discovered that a large fraction of planetary systems (perhaps, a third around sun-like stars) possess super-earth planets on orbits tighter than earth's. these super-earths with masses between that of earth and neptune are not found in the solar system, however it has been proposed that they formed in a similar way to our terrestrial planets. instead, we find that these two inner planetary system architectures correspond to two clearly defined planet formation pathways regulated by the pebble mass-flux. radio observations of protoplanetary disks reveal large reservoirs of "pebbles" (approximately cm-sized objects), which spiral inwards towards the central star through the disk due to drag with the nebular gas. protoplanets embedded in the disk accrete a portion of these pebbles as they drift by. pebble accretion can be the most efficient growth process of solid material. we modeled the growth of a system of rocky protoplanets embedded in disks of varying pebble mass-fluxes, and we find that a change of less than a factor of two in the pebble mass-flux significantly alters the architecture of the final planetary system. when the pebble mass-flux is low, protoplanets grow slowly and remain small. their low-mass strongly limits their migration and so they are located near where they originally grew in the disk. when the gas disappears, they naturally become dynamically unstable, collide with one another, and a smaller number of larger planets remain, i.e. the terrestrial planet formation process. we find the largest final planets are at most a few earth masses, which grow from embryos that are at most a third of an earth mass when the gas disappears and pebble accretion ceases. if the pebble mass-flux is higher, protoplanets grow faster and become more massive while the protoplanetary disk still possesses nebular gas. these massive protoplanets interact with the gas disk and migrate inward leading to stronger planet-planet interactions, mutual merging, and even faster growth. as the nebular gas dissipates, it leaves behind a system of close-in super-earths, which eventually undergo dynamical instabilities responsible for further growth and the removal of orbital resonances. | formation of planetary systems from pebble accretion and migration i: the growth dichotomy between close-in rocky super-earth systems and terrestrial planets |
laboratory astrophysics reproduces on laboratory scales processes that in reality are tremendously larger. scaling can be formalised rigorously [1] but the best tool to bridge the scale gap is numerical simulation. it can consider the real astrophysical processes and the actual laboratory using very different scales. simulations can bridge this tremendous gap and allow a rigorous validation of the applicability of laboratory results to astrophysical systems.laboratory astrophysics than calls into action a triangle among plasma astrophysics, laboratory experiments and high performance computing. however, not all methods of plasma simulation are capable of handling the task. mhd methods are scale-less and therefore capable of scaling from laboratory to the cosmos. but their limitation is that the actual intrinsic scales are lost. kinetic methods recover those intrinsic scales but are less easily scaled. we focus on the evolution of flux ropes. flux ropes are typical of the solar corona and interplanetary space, are observed in astrophysical jets around accretion disks (ranging in scale from forming stars to extragalactic jets produced by supermassive black holes) and are reproduced in experiments [2,3].we report our modelling efforts in the modelling of flux ropes over all these scales using methods ranging from mhd [2,4] to full kinetic [5].[1] ryutov, d. d., et al. magnetohydrodynamic scaling: from astrophysics to the laboratory. pop, 8.5 (2001): 1804.[2] intrator, t. p., sun, x., lapenta, g., dorf, l., & furno, i. (2009). experimental onset threshold and magnetic pressure pile-up for 3d reconnection. nature physics, 5(7), 521.[3] gekelman, w., e. lawrence, and b. van compernolle. three-dimensional reconnection involving magnetic flux ropes. apj 753.2 (2012): 131.[4] lapenta, giovanni, et al. kink instability of flux ropes anchored at one end and free at the other. jgr, 111.a12 (2006).[5] restante, a. l., markidis, s., lapenta, g., & intrator, t. (2013). geometrical investigation of the kinetic evolution of the magnetic field in a periodic flux rope. pop, 20(8), 082501. | high performance computing (hpc) simulations of laboratory experiments probing astrophysical processes |
high-contrast imaging surveys are discovering a growing number of brown dwarf companions and giant planets orbiting stars at wide separations between 10-100 au, but the formation of these objects is poorly understood because multiple routes (disk instability, core accretion plus dynamical scattering, and cloud fragmentation) may contribute to this population. i will describe recent observations of 2m0441+2301 aabbab, a unique young (1-3 myr) hierarchical quadruple system comprising a low-mass star, two brown dwarfs, and a planetary-mass companion in taurus. our near-infrared imaging and spectroscopy with keck/nirc2 and osiris confirm the young age, late spectral type (~l1), and low temperature (~1800 k) of the faintest component, 2m0441+2301 bb. with individual masses of ~200 mjup, 35 mjup, 19 mjup, and 9.8 mjup, 2m0441+2301 aabbab is the lowest-mass quadruple system known. its hierarchical orbital architecture and mass ratios imply that it formed from the collapse and fragmentation of a molecular cloud core, demonstrating that planetary-mass companions can originate from a stellar-like pathway analogous to higher-mass quadruple star systems. more generally, cloud fragmentation may be an important formation pathway for the massive exoplanets that are now regularly being imaged on wide orbits. | near-infrared spectroscopy of a quadruple system spanning the stellar to planetary mass regimes |
we analyze two sectors of transiting exoplanet sky survey (tess) photometry of the nova-like cataclysmic variable star v533 her. we detect a periodicity consistent with the binary orbital period and estimate a revised value of 3.53709(2) hr. we also detect a strong signal near a period of 3.8 hr that we associate with positive superhumps. the superhump frequency varies over the tess observations with the fractional difference between the superhump and orbital periods, ϵ, ranging between 0.055 ≤ ϵ ≤ 0.080. the superhump amplitude is correlated with its frequency such that the amplitude increases as ϵ decreases. positive superhumps result from an instability that generates an eccentric accretion disk and ϵ is a measure of the disk precession rate in the binary rest frame. the observed correlation implies that as the disk precession rate slows, the disk eccentricity increases. | characteristics of the permanent superhumps in v533 herculis |
compact binaries consist of a "normal" star accreting material onto the surface of a white dwarf, neutron star, or black hole. the white dwarf case constitutes cataclysmic variables (cvs), which are often distinguishable from the others by their characteristic optical light curve variability. cvs make up the vast majority of compact binaries since more stars end their lives as white dwarfs than the more massive alternatives. compact binaries with a neutron star or black hole primary are collectively known as x-ray binaries (xrbs). identification of xrbs, particularly those at low accretion levels, is more difficult, typically requiring multi-wavelength follow-up observations. in all cases, instabilities in the accretion disk can lead to changes in brightness. the zwicky transient facility (ztf) provides a photometric variability catalog of the northern sky to a depth of 20.5 mag. this provides considerable overlap with the rosat all-sky survey source catalog, which identified x-ray sources in the soft 0.1‑2.4 kev energy band. we perform a cross match between these two catalogs and investigate well-observed ztf counterparts that are unclassified in the simbad astronomical database. we implement a step function model-fitting algorithm to identify light curves that exhibit a consistent sustained increase or decrease in magnitude, which is a possible signature of an accretion state-change. we present several candidate compact binaries, with analysis of follow-up spectra and archival multi-wavelength observations. | outbursting compact binary candidates from the zwicky transient facility |
space telescopes have provided unprecedented depictions of the manifold variability behaviors typical of young stellar objects (ysos). however, the lack of coordinated, multiband data often limited our understanding of the observed flux patterns. we conducted a comprehensive variability survey of 278 b-to-k ysos in the 1-2 myr-old cluster ngc 6530. our sample was monitored with kepler/k2, and simultaneous u,g,r,i,halpha time series were acquired with vst/omegacam. we detected substantially lower variability on b/a stars than on g/k stars for any light curve morphology, and a dearth of some distinctive disk-driven behaviors among stars earlier than g. we could also pinpoint distinct contributions to the leading day-to-week variability timescales, from intense accretion triggered by inner disk instabilities, to variable accretion efficiency in the outer magnetosphere. on the heels of that project, we are undertaking a new survey of the taurus star forming region, coupling high-precision photometry from tess with simultaneous u,g,r,i data from the las cumbres observatory. this program will provide unique constraints on the physical drivers of yso variability, and on the mid- to long-term stability of inner disks around young stars. | multicolor variability of young stars with disks: insights from coordinated space and ground observations |
bright hard states are observed during the outbursts of stellar mass black hole candidates. this state appears when the luminosity exceeds 0.1% of the eddington luminosity in the hard x-ray dominant state. when the luminosity increase further, the black hole candidate transit to the soft x-ray dominant state. such a transition is called hard-to-soft transitions. similar state transitions are observed in agns. in some seyfert galaxies, soft x-ray excess component appears when the luminosity exceeds 0.1% of the eddington luminosity. furthermore, rapid time variabilities are observed during the hard-to-soft state transition. since the luminosity of the bright hard state exceeds the upper limit for radiatively inefficient accretion flows (riafs), radiative cooling should be taken into account to simulate the accretion flow in this state. we carried out 3d radiation magnetohydrodynamic simulations of the hard-to-soft state transition. the initial disk density is determined, such that the mass accretion rate is around 10% of the eddington accretion rate. we found that radiation pressure dominant, relatively cool (t $<$$ 10^{8}$ k) region appears outside the optically thin, hot accretion flow near the black hole. we also found that the radiation pressure dominant region oscillates quasi-periodically. the possible mechanism of the oscillation is the radial pulsational instability (e.g., kato 1978; blumenthal et al. 1984). in this talk, we discuss the possibility that the non-axisymmetric radial pulsation in the radiation pressure dominant region is the origin of low frequency qpos in black hole candidates and rapid time variabilities in changing look agns. | radiation magnetohydrodynamic simulations of black hole accretion flows in bright hard state |
dense cores are the places where stars are formed within the supersonic molecular clouds. these dense regions (n ∼ 105 cc) are cold (t ∼10 k) and display subsonic levels of turbulence (mach ∼ 0.5), and represent the initial conditions for both star and disk formation. however, the influence of the parental core properties on the disk formation process is still not well constrained, and it is therefore crucial to study dense cores with interferometers to better understand the dense core and disk connection. we present noema observations of a class 0 object, which has been suggested to present a disk under gravitational instability (gi) (asymmetrical features in alma high resolution dust continuum emission). our new data reveal a previously unseen large scale (∼10,000 au, or 33′′) streamer of fresh gas from the surrounding dense core down to the disk scales. this streamer is almost perpendicular to the outflow, and it contains material with subsonic levels of turbulence, and therefore unperturbed by the outflow. based on the total mass in the streamer and the free-fall timescale, we estimate infall rates to the disk scales, which clearly show that accretion via streamer can have an important role in the disk formation. moreover, these results show that previously observed disk asymmetries could also be driven by large scale asymmetric flows instead of gi. this result shows the power and importance of studying dense cores with interferometers to provide a complete and proper picture of star and disk formation. | feeding a protostar with 10 000 au scale streamers |
black hole x-ray binaries (bhxbs) provide the easiest means to study stellar-mass black holes. outbursts of bhxbs typically last months-years and are quite often characterized by a fast-rise, exponential decay light curve profile. however there are many exceptions, with some sources rising slowly, some having multiple peaks, and some displaying flares, dips, plateaus and re-brightenings. while re-flares during outburst decays are fairly common, re-brightenings after the end of the outbursts, when the source has reached quiescence, have been reported in far fewer bhxb sources. the origin of re-flares and mini-outbursts are a matter of debate, but can be a powerful tool to either strengthen or challenge the disk-instability model (which predicts that outbursts are regulated by thermal-viscous instabilities in the accretion disk), and help us understand the outburst cycles in bhxbs. we present long-term optical monitoring of the black hole candidate x-ray binary swift j1910.2-0546 with the faulkes telescopes, and report two periods of re-brightening activities previously undocumented in the literature. we find that following the bright 2012 outburst, the source has displayed a series of at least seven quasi-periodic, high amplitude optical re-flares in 2013, with a recurrence time increasing from ∼42 days to ∼49 days. this was followed by a mini-outburst with two peaks in 2014. the flaring behavior during both the re-brightening periods show a bluer when brighter behavior, having optical colors consistent with a blackbody heating and cooling between 4500 and 9500 k, the temperature range in which hydrogen ionizes. we compare the flaring behavior of the source with re-brightening events observed in other bhxbs within one year of an outburst. we discuss the different scenarios which could cause such extreme flaring, and propose that the highly unusual repeated flaring in swift j1910.2-0546 can arise from a sequence of heating and cooling front reflections in the accretion disk following the disk instability model, probably due to the presence of a hot inner disk at the end of the main outburst. | seven re-flares and a mini-outburst: high amplitude optical variations in swift j1910.2-0546 |
the vast majority of the known galactic population of stellar-mass black holes and a significant fraction of the neutron stars are found in low mass x-ray binaries (lmxbs). these stellar systems have sub-solar companion stars that transfer material to the compact object via roche lobe overflow. among lmxbs, the population of very faint x-ray binaries (systems displaying accretion luminosities in the range of lx~1e34-36 ergs/s) have opened new, exciting windows for accretion studies, but their existence also raises some intriguing questions. in particular, persistent systems accreting at very low accretion rates challenges the standard and well tested transient/persistent paradigm for x-ray binaries explained by the disc instability model. a possible solution requires they having very short orbital periods, even within the ultra-compact regime. these ultra-compact x-ray binaries, comprised by systems with orbital periods shorter than 80 min, are of great interest. these short periods imply small roche lobes, in which only degenerated (hydrogen poor) donor stars can fit. thus, they are unique laboratories to study accretion processes in hydrogen deficient environments as well as some of the fundamental stages of binary evolution. last but not least, they will be primary sources for gravitational waves studies at low-frequencies by the forthcoming lisa mission. however, the discovery and study of these tantalising (but faint) systems are usually hampered by the limitations of current instrumentation, being only possible by pushing the largest optical telescopes to their limits. i will review the state-of-the-art of the field and present the latest results on the hunt for new members for the scarce family of ultra-compact x-ray binaries. these are based on the investigation of the chemical composition of the donor star by using deep x-ray spectra as well as optical spectroscopy carried out with the 10.4 m gtc. | the hunt for ultracompact x-ray binaries |
in this contribution we present an initial analysis of the properties of a set of about 30 planets and brown dwarfs associated with forming stars of the t tauri type, with ages 10 10 years, belonging to different star-forming regions. the 18 of the sample (6 out of 33) are hot-jupiter-like planets. the remaining 82, detected by direct imaging, corresponds to giant and distant objects (a several hundred au), which represent a challenge for the models in vogue of planetary formation (core accretion and gravitational instability). it is believed that these planets would not have been formed in situ, but would rather have been gravitationally scattered from the internal disk towards the external disk. they have colors of spectral types m8-l5 and, for about half of the sample, errors in the masses do not rule out that they could be brown dwarfs. | propiedades de planetas y enanas marrones en estrellas jóvenes |
planets form in the gaseous and dusty disks orbiting young stars. these protoplanetary disks are dispersed in a few million years, being accreted onto the central star or evaporated into the interstellar medium. to explain the observed accretion rates, it is commonly assumed that matter is transported through the disk by turbulence, although the mechanism sustaining turbulence is uncertain. on the other side, irradiation by the central star could heat up the disk surface and trigger a photoevaporative wind, but thermal effects cannot account for the observed acceleration and collimation of the wind into a narrow jet perpendicular to the disk plane. both issues can be solved if the disk is sensitive to magnetic fields. weak fields lead to the magnetorotational instability, whose outcome is a state of sustained turbulence. strong fields can slow down the disk, causing it to accrete while launching a collimated wind. however, the coupling between the disk and the neutral gas is done via electric charges, each of which is outnumbered by several billion neutral molecules. the imperfect coupling between the magnetic field and the neutral gas is described in terms of "non-ideal" effects, introducing new dynamical behaviors. this thesis is devoted to the transport processes happening inside weakly ionized and weakly magnetized accretion disks; the role of microphysical effects on the large-scale dynamics of the disk is of primary importance. as a first step, i exclude the wind and examine the impact of non-ideal effects on the turbulent properties near the disk midplane. i show that the flow can spontaneously organize itself if the ionization fraction is low enough; in this case, accretion is halted and the disk exhibits axisymmetric structures, with possible consequences on planetary formation. as a second step, i study the launching of disk winds via a global model of stratified disk embedded in a warm atmosphere. this model is the first to compute non-ideal effects from a simplified chemical network in a global geometry. it reveals that the flow is essentially laminar, and that the magnetic field can adopt different global configurations, drastically affecting mass and magnetic flux transport through the disk. a new self-organization process is identified, also leading to the formation of axisymmetric structures, whereas the previous mechanism is discarded by the action of the wind. the properties of magnetothermal winds are examined for various disk magnetizations, allowing discrimination between magnetized and photoevaporative winds based upon their ejection efficiency. | large scale dynamics of protoplanetary discs |
we will present a set of three-dimensional, global, general relativistic radiation magnetohydrodynamic (grrmhd) simulations of geometrically thin radiation-pressure-dominated accretion disks around stellar-mass black hole. we test the hypothesis that strong magnetic fields can both drive accretion through the magneto-rotational instability and restore thermal stability to such disks. based on the results of these simulations, we find that the zero net-flux magnetic field configurations remain thermally unstable like its $\alpha$-viscosity counterpart, in our case collapsing vertically on the local thermal timescale and never fully recovering. the quadrupole and vertical magnetic field cases, on the other hand, stabilize and remain so (for many thermal timescales). the key stabilizing factor is the ability of specific field configurations to build up and sustain strong, $p_\mathrm{mag} > 0.5p_\mathrm{tot}$, toroidal fields near the midplane of the disk. we will talk about the reasons why certain configurations are able to do this effectively and others are not. | 3d global radiative grmhd models of strongly magnetized accretion flows |
due to the conservation of angular momentum contained in the parent molecular cloud, a protostar is inevitably surrounded by a flattened, centrifugally-supported circumstellar disk. for mass accretion to proceed, angular momentum transport must take place in such disks. canonically, the magnetorotational instability (mri) was considered to be the primary source of turbulence which could generate viscosity for accretion. however, several lines of reasoning now show that the observed turbulence in a typical protoplanetary disk is insufficient to drive the disk accretion. conversely, when all of the non-ideal magnetohydrodynamic effects are included, the simulations also point towards suppression of the mri. in this talk, i will present a global model of magnetic wind-driven accretion, based on numerical magnetohydrodynamics simulations of protoplanetary disks. we study long-term evolution of the disk and show that magnetocentrifugal winds are capable of transporting the angular momentum vertically, which results in sufficiently large accretion rates. i will also discuss the impact of wind-driven accretion on the inner disk structure and its implications for episodic accretion during star formation. | global model of magnetic wind-driven accretion in protoplanetary disks |
the observed gaps in the protoplanetary disk are often considered an imprint of planets orbiting around the central star. the width and the depth of the gaps depend on the mass of the planet along with the disk properties, for example, aspect ratio, viscosity, and particle size. an estimate of the masses and the location of the planets forming in the disk is crucial to distinguish between different planet formation scenarios like "core accretion" and "gravitational instability" models. we run a large number of two-dimensional hydrodynamic simulations in fargo3d using gpu clusters to compute the gaps induced by planets in a dusty disk for a wide parameter range. the dataset is then used to train a deep neural network to predict the planet mass from an observed disk gap in a protoplanetary disk. this machine learning technique provides a significant advantage over the existing empirical relations as our model can be trained for any number of relevant parameters and complex disk system. our trained neural network provides an accurate prediction of the planet mass for an observed gap in a (for example, hl tau) disk in much-reduced computing time. | using machine learning to infer planet mass from observed gap in protoplanetary disks |
accretion disks are observed around young stellar objects such as t tauri stars. in order to complete the star formation, particles in the disk need to loose angular momentum in order to be accreted into the central object. the magneto-rotational instability (mri) is probably the mechanism responsible for a magneto-hydrodynamic (mhd) turbulence that leads to disk accretion, which implies the disk particles to be coupled with the magnetic filed lines. as the temperature in the disk is low, we considered, besides the viscous heating mechanism often included in the models by means of the α - prescription, the damping of alfvén waves as an additional heating source. in particular, we show that the mechanism derived that couples the turbulent and non-linear damping mechanisms of alfvén waves proved to be very efficient, generating temperatures almost one order of magnitude higher than those mechanisms considered independently. | heating mechanisms in accretion disks around young stellar objects |
shear flows and magnetic fields are ubiquitous in astrophysical bodies such as stars and accretion discs. furthermore,the interaction between flows and magnetic field plays a key role in the dynamics of plasma fusion devices. typically,the flows and magnetic field are both sheared, and it is therefore a problem of fundamental importance to understandthe instabilities that may occur in such a system.in the absence of magnetic field, the linear stability of a viscous sheared flow is governed by the orr-sommerfeldequation; this is one of the classic problems of hydrodynamics. at the other limit, there are somewhat analogousinstabilities of a fluid of finite electrical conductivity containing a static sheared magnetic field. these are related tothe classical tearing modes that have received considerable attention in both the astrophysical and plasma physicsliterature.in general though, the fluid flow and the magnetic field will both be important players. previous studies have investigatedconfigurations which have served as models for systems such as the magnetotail and solar surges. while theseinvestigations have been fruitful, the prescription of the basic field and flow, while physically motivated, have beenchosen somewhat arbitrarily. it is therefore of interest to consider the instability problem within this more generalframework.motivated astrophysically, such as by the dynamics in the solar tachocline, here we consider a self-consistent problemin which both instabilities can occur. in particular, we consider the stability of equilibrium states arising from theshearing of a uniform magnetic field by a forced transverse flow. the problem is governed by three non-dimensionalparameters: the chandrasekhar number, and the flow and magnetic reynolds numbers. in opposite limits of parameterspace, we recover the predictions of the aforementioned classical problems. as we move through this three-dimensionalparameter space, a range of interactions are possible: we demonstrate the stabilisation of a purely hydrodynamicinstability through the magnetic field, show the existence of a joint instability outlining the physical mechanisms atplay, and demonstrate that under certain conditions, hydrodynamically-stable parallel shear flows lead to instabilitygrowth rates that exceed those of static tearing modes. to conclude, we elucidate the consequences of consideringthe linear stability of an evolving background state and show that a quasi-static approach may not be meaningful. inthese circumstances, it therefore becomes essential to perform a stability analysis of a time-varying basic state. | joint instabilities of sheared flows and magnetic fields |
x-ray binaries radiate brightly from radio to x-ray due to the accretion and ejection of matter in the system. there are complex, correlated flux variations that probe links between emitting components. studying the nature of accretion onto these compact objects sheds light on several broad fields in astrophysics. this is why multi-wavelength monitoring campaigns of these outbursts are becoming increasingly popular. here i briefly review the progression of multi-wavelength studies of x-ray binaries, focusing on long-term (day+ timescales) correlated behaviour between optical/infrared and x-ray emission. a tight power law correlation exists between optical/ir and x-ray fluxes in the hard state, which can be explained with a combination of disc and jet optical/ir emission. for most cases this is essentially an optical disc - x-ray corona relation. black hole systems are systematically optically brighter at a given x-ray luminosity compared to neutron stars. this is found to be due to different disc sizes, compact object masses, more jet emission for black holes, and a bolometric x-ray correction. the optical - x-ray correlation can be used to constrain the nature of the compact object for new x-ray transients. we present new optical/ir - x-ray correlations in several x-ray binaries (using x-ray swift, maxi, nicer and hxmt data), including maxi j0556-332, swift j1910.2-0546, maxi j1820+070 and maxi j1348-630. in sax j1808.4-3658 we find an optical - x-ray anti-correlation during its re-flare period, which is likely a positive correlation with a time delay. we demonstrate that using these correlations in conjunction with spectral energy distributions and colour-magnitude diagrams (and in some cases polarimetry and fast timing studies) can successfully isolate the different emitting components in order to measure accurate outer disc temperatures and jet synchrotron spectral indices. optical/ir monitoring of x-ray binaries is essential to these efforts; we have been monitoring $\sim$ 40-50 x-ray binaries with the faulkes telescopes / las cumbres observatory (lco) global robotic network for 1.5 decades. we present optical light curves and optical - x-ray correlations for several outbursts. we also find evidence for low level, variable accretion activity and long-term trends in quiescence in some systems. we will also introduce our new real-time optical monitoring pipeline, the "x-ray binary new early warning system (xb-news)", which aims to detect and announce new x-ray binary outbursts within a day of first optical detection. we are now detecting the early stages of these outbursts with our optical telescopes, before they become bright enough for x-ray detection. this allows us to trigger x-ray and multi-wavelength campaigns during the very early stages of outbursts, to constrain the outburst triggering mechanism. disc instability models predict that for x-ray binaries in quiescence, there should be a brightening of the optical flux prior to an x-ray outburst. tracking the x-ray variations of xrbs in quiescence is generally not possible, so optical monitoring provides the best means to measure the mass accretion rate variability between outbursts. multi-wavelength work of x-ray binaries is booming, but could still be in its infancy. with ska, lsst and various upcoming x-ray missions, the golden age may be yet to come. | long-term correlations between optical, infrared and x-ray observations of x-ray binaries |
the evolution of the kelvin-helmholtz instability (khi) and magnetohydrodynamic (mhd) wave emission is investigated at shear-flow boundaries of magnetized plasmas. while mhd wave emission has been suggested to be only possible during the nonlinear stages, we find that there is also significant wave emission during the khi's linear stages. these emitted mhd waves may have stronger impacts than khi surface waves since they can act to transport energy away from the local region of the shear flow. the removal of energy from the shear-flow region, instead of just the local redistribution considered in previous studies, and its propagation away from the interface could have major implications for the evolution of astrophysical objects characterized by fast plasma flow shears. | linear and nonlinear kelvin-helmholtz instability and magnetohydrodynamic wave emission in sheared astrophysical plasma flows |
massive stars (> 8 solar masses) form in turbulent, magnetized regions. because of their strong radiative pressure stopping accretion in 1d models, numerical efforts have been focused on radiative transfer. spherical symmetry has been broken in multidimensional simulations, and disk accretion (2d, yorke+02) then accretion via radiative rayleigh-taylor instabilities (3d, rosen+16), have emerged. i have been implementing a hybrid radiative transfer method in the ramses code to treat the stellar irradiation separately from the diffuse medium emission, allowing to capture better the radiation anisotropy and coupling with the gas. i will use it to investigate the main accretion mode during massive star formation (mignon-risse+20). i will then include turbulence and (non-ideal) mhd to determine the launching mechanisms of their bipolar outflows. i will finally provide observable quantities such as disk and outflows properties (e.g. opening angles) for comparison with alma observations. | high-mass turbulent core collapse with ambipolar diffusion and hybrid radiative transfer |
the embedded phase of star formation witnesses the birth of stars, the formation of circumstellar disks, and the launch of jets and outflows. it sets the stage for the disk and star evolution in the subsequent optically visible t tauri phase, largely determining disk and stellar masses and paving the way for planet assembly. this phase is hard to observe and theory can provide valuable insights into the processes that take place in the depth of collapsing clouds. i will overview processes that are linked with the disk early evolution, such as gravitational instability and its implications, variable protostellar accretion with episodic bursts and its effect on the disk dynamical and chemical evolution, and initial stages of dust growth. the nature of very low luminosity objects (vellos) will also be discussed. | the embedded phase of star formation : outflows, envelopes, first conditions of disk formation |
accretion disks around bhs are an under-studied potential gw source. the hydrodynamic papaloizou-pringle instability (ppi) can cause persistent orbiting matter clumps to grow and produce copious gws. via full numerical relativity simulations of self-gravitating disks, we have extended the understanding of these bh-disk systems in two new ways. first, we conducted the first-ever study of the ppi around spinning bhs (a / m = 0 . 7). we found that, in addition to slightly shifting orbital frequencies, prograde spin can reduce the accretion rate and extend gw signal lifetimes. systems of 10m⊙ - relevant for bhns mergers - could be detectable by cosmic explorer out to ~ 300 mpc, while decigo (lisa) could detect systems of 1000m⊙ (105m⊙) - relevant for disks forming in collapsing supermassive stars - out to cosmological redshift of z ~ 5 (z ~ 1). second, we investigated the impact of magnetic fields on the ppi. nsf grant phy-1912619. | nr simulations of ppi-unstable bh-disk systems: bh spin, magnetic fields, and gravitational wave detectability |
the liberation of gravitational binding energy to produce the photons that we observe from accreting systems remains a problem of fundamental importance in theoretical astrophysics. this problem is now becoming tractable thanks to modern computer codes which can simulate both the complex dynamics of magnetohydrodynamical (mhd) turbulence as well as the thermodynamics of radiative cooling. accretion disks around white dwarfs in binary systems have as yet received little attention from such codes, and yet it is these systems which provide the strongest observational constraints of ionized, electrically conducting accretion disks. much is known about how such disks are fueled by roche lobe overflow of the companion star, spectroscopy reveals the composition and physical state of the accreting matter, and eclipse mapping of some systems has even provided information on the spatial structure of the accretion disk. white dwarf accretion disks exhibit both aperiodic and periodic variability with properties that are shared by a broad range of accreting sources. thermal instabilities driven by ionization transitions also exist and the resulting outbursts provide the strongest observational constraints on mhd turbulent stresses. another distinct advantage of white dwarf accretion disks is that they have the smallest dynamic range of any accretion disk system in all of astrophysics: the fueling radius is only a factor of tens to hundreds times larger than the white dwarf radius itself. this makes them the most computationally tractable systems for simulating the entire global structure of the disk. we therefore propose to use the athena++ code to conduct global radiation mhd simulations of the most compact accreting white dwarf binaries in nature: the am cvn systems. these consist of a white dwarf being fed by a helium donor star. these systems exhibit a rich phenomenology of variability that is shared by other sources, including rapid periodic and aperiodic variability, superhumps, normal outbursts and superoutbursts. the fueling radius in these systems can be as small as 20 or so white dwarf radii. our simulations will be the first of any accretion disk in nature that incorporates the entire disk, from the fueling radius to the white dwarf boundary layer, and which includes self-consistent angular momentum transport by mhd turbulence and (possibly) spiral waves. they will also include (grey) radiation transport within the simulation itself, so that self-consistent light curves can be automatically produced. we will begin by simulating a compact, persistent (non-outbursting) system: kic 004547333, which has three years of high cadence kepler data, and which exhibits remarkably rich variability. with reasonable amounts of computer time, we can reach a global thermal equilibrium for the entire disk, although achieving global inflow equilibrium might still be challenging. a preliminary simulation of the growth of the accretion disk already shows interesting dynamics, as the disk is tilting out of the orbital plane and globally precessing. this behavior is promising to explain negative superhumps and also variable irradiation of the secondary star that may be necessary to explain superoutbursts. we will generate orbital phase resolved light curves from this simulation, analyze the causes of its dynamical behavior, and post process the simulation data to generate orbital phase resolved photon line and continuum spectra. we then propose to move to lower accretion rates and longer orbital periods, until we reach the helium ionization regime and trigger a cooling instability with the propagation of a cooling front. while following an entire outburst cycle will almost certainly not be feasible, understanding the thermodynamics of the cooling front and its interaction with mhd turbulence will be critical in advancing our understanding of outbursts in a wide variety of systems. | variability and spectra of global radiation mhd simulations of am cvn accretion disks |
it has been shown that disk fragmentation within several tens of au from the star is very difficult when the gravitational instability and cooling time criterion are considered. however, in the stochastic fragmentation scenario, things may be different. we investigate stochastic fragmentation in protoplanetary disks. in the stochastic fragmentation model, we consider the effects of the external irradiation on the fragmentation process. for the stochastic fragmentation, owing to the probability of generating bound fragments at longer cooling times relative to the critical value and the short collapsing times in the external irradiation region, the inner boundary of the fragmentation region extends inward to 19 au. we find that the required self-gravitating lifetime for fragmentation in the external irradiation region is far less than the self-gravitating lifetime of the disk. therefore, once a radius is in the external irradiation region, the fragmentation probability increases quickly to 1 after gravitational instability sets in, and fragmentation occurs within a short period of time. we also find that the self-gravitating lifetime required for fragmentation decreases significantly as the radius increases in the external irradiation region. | stochastic fragmentation in protoplanetary disks under external irradiation |
at all redshifts rest-frame ultraviolet morphologies tend to be patchy and clumpy or extremely compact in nature. these morphological signatures could result from either merger interactions between two or multiple systems that trigger star formation, cloud collapse via gravitational instabilities in a gaseous disk that is fed by cold gas spiraling inwards along filamentary structures, or another mechanism still to be determined. theoretical simulations of clumpy galaxy evolution suggest they could have evolved secularly through cold gas accretion onto rotating disks. clumps in disks could have migrated to the center of the potential well of a galaxy and combined to form a bulge, or, if gravitationally unstable, could have dissipated forming the disk component. we are exploring potential correlations amongst different morphological properties at intermediate-z which is pivotal in bridging observations at high-z to the local extragalactic universe. we will show how flocculent galaxies, starburst galaxies and compact groups of galaxies may resemble clumpy disks at intermediate redshifts in the rest-frame uv. | rest-frame ultraviolet morphologies: connecting local galaxies with the epoch of disk formation |
the intermediate palomar transient factory (iptf) has discovered a quasar the brightly-shining, active nucleus of a galaxy abruptly turning on in what appears to be the fastest such transition ever seen in such an object.a rapid transitionquasars are expected to show variations in brightness on timescales of hours to millions of years, but its not often that we get to study their major variability in real time! so far, weve discovered only a dozen changing-look quasars active galactic nuclei that exhibit major changes in their spectral class and brightness between observations. roughly half of these were quasars that turned on and half were quasars that turned off, generally on timescales of maybe 5 or 10 years.the dramatic change in spectrum of iptf 16bco between the archival sdss data from 2004 (bottom) and the follow-up spectroscopy from keck 2+deimos in 2016 (top). [adapted from gezari et al. 2017]in june 2016, however, a team of scientists led by suvi gezari (university of maryland) discovered iptf 16bco, a nuclear transient that wasnt there the last time palomar checked in 2012. a search through archival sloan digital sky survey and galex data in addition to some follow-up x-ray imaging and spectroscopic observations told the team what they needed to know: iptf 16bco is a quasar that only just turned on within the 500 days preceding the iptf observations.this source, in fact, is a 100-million-solar-mass black hole located at the center of a galaxy at a redshift of z= 0.237. in just over a year, the source changed classification from a galaxy with weak narrow-line emission to a quasar with characteristic strong, broad emission lines and a ten-fold increase in continuum brightness! what caused this sudden transition?instabilities at fault?iptf 16bco and the other known changing-look quasars with disappearing (red circles) and appearing (blue circles) broad-line emission. [adapted from gezari et al. 2017]gezari and collaborators used the large number of recent and archival observations of the galaxy to explore several scenarios that might be responsible for the rapid change in its brightness and spectral appearance. they found that the data disfavor variable obscuration by an absorber between us and the galaxy, microlensing of a background object, and tidal disruption of a star.instead, the authors conclude that the best-fitting explanation is one in which thegalaxys nucleus already had a preexisting accretion disk, but the disk recently developed an instability. that instability caused more gas to rapidly feed onto the black hole, bumping the accretion rate up a notch and resulting in the quasar suddenlybrightening.continued observations ofiptf 16bcowill certainly help us to better understand whats happening in this unusual source. in the meantime, itsrapid change of state pushes the limits of accretion disk theory and presents us with an intriguing challenge to our understanding of quasars.citations. gezari et al 2017 apj 835 144. doi:10.3847/1538-4357/835/2/144 | a quasar turns on |
we perform a large timescale core collapse simulation to explore the secular evolution of embedded protoplanetary discs. the secular evolution properties point towards evolved discs. the disc experiments three accretion phases where the magnetic braking is the main accretion driver in the pseudo-disc while self-gravity is the main accretion driver in the disc via toomre instability. | probing the secular evolution of embedded protoplanetary discs |
fu ori stars are a rare class of young stellar object, with less than two dozen examples known. yet the long-lasting fu ori outburst events play a prominent role in our understanding of stellar mass assembly at the individual star level. the currently favored model is that of an inner disk instability causing a factor of 100-10,000 increase in the disk-to-star accretion rate. ultraviolet observations are the most direct way to test the accretion disk scenario -- by measuring the hottest part of the inner disk and thereby determining physical parameters such as temperature t_max and temperature profile t(r). we propose here a complete stis+cos legacy spectrum for the prototype of the class, fu ori itself (11 orbits), plus a stis low-resolution nuv spectral survey for four additional fu ori outbursts (10 orbits). heating by viscous accretion will be tested by (1) comparing the new data on fu ori to a previous stis spectrum obtained 20 years ago, to look for evidence of disk cooling, and (2) comparing the maximum disk temperature across the five targets, which span a factor of 6 in luminosity. these tests can only be done in the nuv, which is sensitive to the hottest disk temperatures. the spectra will be interpreted using a new empirical disk model that has been applied successfully to longer wavelength observations of fu ori stars. this survey of extreme accretion disks provides an important complement to the ullyses dd spectroscopic legacy project, developed to evaluate magnetospheric accretion on typical t tauri stars. our program focuses on the extreme state of accretion physics for young stars, with importance for the evolution of the star and the disk. | the innermost regions of fu ori disks: a spectral legacy for hst/stis+cos |
in this paper, we analyze the so-called master equation of the linear backreaction of a plasma disk in the central object magnetic field, when small scale ripples are considered. this study allows to single out two relevant physical properties of the linear disk backreaction: (i) the appearance of a vertical growth of the magnetic flux perturbations; (ii) the emergence of sequence of magnetic field o-points, crucial for the triggering of local plasma instabilities. we first analyze a general fourier approach to the solution of the addressed linear partial differential problem. this technique allows to show how the vertical gradient of the backreaction is, in general, inverted with respect to the background one. instead, the fundamental harmonic solution constitutes a specific exception for which the background and the perturbed profiles are both decaying. then, we study the linear partial differential system from the point of view of a general variable separation method. the obtained profile describes the crystalline behavior of the disk. using a simple rescaling, the governing equation is reduced to the second-order differential whittaker equation. the zeros of the radial magnetic field are found by using the solution written in terms kummer functions. the possible implications of the obtained morphology of the disk magnetic profile are then discussed in view of the jet formation. | general features of the linear crystalline morphology of accretion disks |
we study the structure and temporal variabilty properties of the grb jets considering a magnetically arrested disk as their central engine. we numerically evolve the accretion disk around a kerr black hole using 3d general relativistic magnetohydrodynamic simulations. we consider two analytical equilibrium disk configurations, the fishbone-moncrief and chakrabarti solutions, as the initial conditions and impose poloidal magnetic fields upon them. the disk starts accreting due to the development of the magnetorotational instability and eventually develops to a magnetically arrested accretion disk state. we consider these models to be central engines of short and long-grbs, based on our initial conditions, and investigate the properties of the jets launched from these models. our models self-consistently produce structured jets with a hollow core up to $\sim$ 5 degrees. the jets from our simulations have an opening angle up to $\sim$ 11 degrees for the long-grb model and up to $\sim$ 25 degrees for the short-grb model. we also perform the time variability studies of the jets and provide an estimate of their minimum variability timescales. our models can be applied to the grb jets in the binary neutron star post-merger system or to the ultra-relativistic jets launched from collapsing stars. | modeling the grb jet properties with 3d general relativistic simulations of magnetically arrested accretion flows |
the atomic and molecular gas in galaxies comes from a combination of accretion and stellar reprocessing. accretion can occur either through mergers, by cooling of hot halo gas, or by cold flows, with the latter likely most important at early times. star formation occurs in cold molecular regions of this gas, so for many years it was suspected that the formation of molecular gas was a necessary step for star formation. recently, however, it has been understood that although molecules trace gravitationally collapsing, dense gas, they are not required for gas to cool and collapse to high densities. observed correlations between molecular abundance, other dense gas tracers, and star formation appear to be driven by this correlation, without causation necessarily being implied. the formation of molecular clouds from atomic gas, as well as subsequent star formation, relies on self-gravity resisted by shear, supernova-driven turbulence, diffuse uv heating, and magnetic fields. the interplay between these is primarily determined by gravitational instability of the combination of gas and stars (which produces spiral arms in disk galaxies), although turbulent compression can produce transient dense regions as well. | atomic and molecular phases of the interstellar medium |
many x-ray sources are now understood to be "black hole x-ray binaries'' in which a stellar remnant black hole either tidally "squeezes'' gas off a companion star, or pulls in some fraction the companion's wind. this gas can drain inward through a dense, thin disk characterized by thermalized radiation, or a sparse and radiatively-inefficient flow, or some combination of the two. observations at other energies often provide crucial information, but our primary tools to study accretion, especially closest to the black hole, are x-ray spectra and their time evolution. this evolution includes numerous behaviors spanning orders of magnitude in timescale and luminosity, and also hints at spatial structure since draining is generally faster at smaller radii. this includes variability at time-scales of weeks to months which remains difficult to explain despite an abundance of possible variability mechanisms since direct simulations covering the full spatial and temporal range remain impractical. after reviewing general aspects of accretion, i present both more and less familiar forms of longterm variability. based on these, i argue the problem involves finding a physical process (or combination) that can generate repeatable yet adjustable cycles in luminosity and evolution of low and high energy spectral components, while letting the ionization instability dominate conventional outbursts. specific models examined include: disks embedded in, and interacting with, hot, sparse flows, and another instability that quenches viscous-draining of the disk at more fundamental level. testing these theories, alone and in combination, motivates building a very general and simplified numerical model presented here. i find that two-phase flow models still predict excessive recondensation in lmc x-3 among other problems, while the viscosity-quenching instability may account for rapid drops and slow recoveries in disk accretion rate but also likely requires diffusivity orders of magnitude above the spitzer value. i also present complementary work on how total mass supply can vary (on day-week timescales) when the companion is irradiated by accretion-generated x-rays as the model is built with the intention (and features) to be used with more detailed disk models. | testing theories for longterm accretion variability in black hole x-ray binaries |
magneto-gas-dynamic (mgd) outflows from the accretion disks of t tauri stars with fossil large-scale magnetic fileld are investigated. we consider two mechanisms of the outflows: rise of the magnetic flux tubes (mft) formed in the regions of efficient generation of the toroidal magnetic fileld in the disk due to parker instability, and acceleration of particles in the current layer formed near the boundary between stellar magnetosphere and the accretion disk. structure of the disk is calculated using our mgd model of the accretion disks. we simulate dynamics of the mft in frame of slender flux tube approximation taking into account aerodynamic and turbulent drags, and radiative heat exchange with external gas. particle acceleration in the current layer is investigated on the basis of sweet-parker model of magnetic reconnection. our calculations show that the mft can accelerate to velocities up to 50 km s-1 causing periodic outflows from the accretion disks. estimations of the particle acceleration in the current layer are applied to interpret high-speed jets and x-rays observed in t tauri stars with the accretion disks. | outflows and particle acceleration in the accretion disks of young stars |
in the past decade, the atacama large millimeter/sub-millimeter array (alma) became the first telescope capable of collecting a myriad of highly-resolved protoplanetary disc observations, confirming the long-thought belief that protoplanetary discs are not featureless and instead are rich in structure. among the structures alma has observed are large-scale crescent-shaped features often at the edges of gaps or cavities in these discs. such features could potentially be explained by gap-opening planets generating dust-trapping vortices through the rossby wave instability (rwi) at one or both of their gap edges where a maximum in the disc's radial inverse vortensity profile develops. it remains an open question as to whether such vortices can survive for a long enough period of time to expect to observe even the relatively small number of crescent-shaped features found in observations. developing strong constraints on expected vortex lifetimes could potentially also constrain disc conditions and planet masses, properties that if known precisely would make it possible to study which disc conditions connect to which types of planets and to be able to connect the population of newly-formed planets in discs to the older population of planets detected more directly around main sequence stars. previous studies have demonstrated that various effects such as dust feedback, disc self-gravity, layered viscosity profiles, and sub-optimal disc aspect ratios can all drastically shorten vortex lifetimes. in this work, we explore yet another effect that can alter both the lifetimes and appearances of planet-induced vortices that has been largely neglected in previous studies, namely the growth of the planet. in the core accretion model, the preferred method for forming most gap-opening planets, the growth of the planet is not instantaneous, even in the runaway gas accretion phase when it accretes the bulk of its final mass. in spite of these potentially slow planetary growth timescales, previous computational studies of planet-induced vortices by others merely introduce a fully-grown planet into the disc on an unrealistically quick timescale of 10 to 100 planetary orbits. in our work, we study the effects of introducing the planet into our simulations on a slower, more realistic timescale with two different growth methods, namely prescribed growth and the more realistic process of a planetary core accreting its gaseous atmosphere directly from the disc. with realistic planet growth timescales, we find the planets highly preferentially induce vortices that are elongated, a stark contrast from the compact vortices that typically form with unrealistically quick planet growth. the underlying difference in the structure of these elongated vortices is that they do not develop a minimum rossby number < -0.15 that is needed for them to become compact. with a more elongated extent in the gas and a flatter pressure bump through the bulk of the vortex, we find the dust trapped in these elongated vortices circulates around nearly the entire azimuthal extent of the vortex instead of spiraling inwards to the center like in compact vortices. as result, elongated gas vortices should typically have elongated azimuthal extents and off-center peaks in alma observations, two features that distinguish them from compact vortices. double peaks are also possible while the dust is circulating through the middle of the vortex. while higher viscosities (a~ 10-4) are strong enough to cause these vortices to decay into rings, we find that shocks from the planet's spiral waves are more responsible for breaking up elongated vortices in lower-viscosity discs (a~ 10-5). as a result, elongated planet-induced vortices are much longer-lived in regions of a low-viscosity disc with larger aspect ratios due to the weaker shocks associated with the planet in these conditions. with lower aspect ratios, low-mass planets can still induce long-lived by asymmetries by causing the vortex to re-form. with higher-mass planets, however, the gap typically becomes too wide too quickly to maintain the prospect of the vortex re-forming beyond a relatively short amount of time. as a consequence of these effects and somewhat counterintuitively, lower-mass planets tend to produce longer-lived asymmetries than planets near jupiter's mass in discs with low to intermediate aspect ratios. overall, the long lifetimes we expect in the lower-mass planet cases do not seem to be consistent with the paucity of crescent-shaped features in protoplanetary disc observations, in particular those with two-sided gaps that are conventionally expected to arise from planets in a protoplanetary disc. this discrepancy adds support for the proposed mechanisms for shortening vortex lifetimes. nonetheless, the dust signatures we find with vortices induced by slowly-grown planets are a natural explanation for the elongated extent and off-center peak in hd 135344 b, one of the few discs with a crescent-shaped feature at the edge of a conventional two-sided gap. they can also potentially explain more bizarre signatures such as the two clumps in mwc 758, the separation between the dust and gas cavities in oph irs 48, the double peaks in hd 142527 or v1247 orionis. our work lays the foundation for understanding how planet-induced vortices could potentially constrain disc conditions and planet masses with better-resolved multi-wavelength observations in the future. | generating planet-induced vortices with slowly-growing gap-opening planets |
kepler light curves of short period dwarf novae have resparked interest in the nature of superoutbursts and led to the question: is the thermal-tidal instability needed, or can the plain vanilla version of the accretion disk limit cycle do the job all by itself? a detailed time-resolved study of an eclipsing su uma system during superoutburst onset should settle the question - if there is a dramatic contraction of the disk at superoutburst onset, osaki's thermal-tidal model would be preferred; if not, the plain disk instability model would be sufficient. i will present recent results that support the contention by osaki & kato that the time varying negative superhump frequencies can be taken as a surrogate for the outer disk radius variations. finally, it may be necessaryto look beyond the short period dwarf novae to gain perspective on the nature of embedded precursors in long outbursts. | recent developments on su uma stars - theory vs. observation |
the high metallicity of stars might favor planet formation either through nucleus accretion or disk instability. for this reason there is a great interest in the study of the atmospheres of stars that host planets. we present preliminary results of an analysis of the atmospheres of five of these stars. we determine chemical abundances of relevant elements and stellar parameters, such as effective temperature, surface gravity and microturbulence. these objects were observed with the casleo ebasim spectrograph. | abundancias químicas y parámetros físicos de estrellas con planetas |
this contribution presented high-resolution numerical simulations of the colliding wind system η carinae, showing accretion of the primary wind onto the secondary star close to periastron passage. we found that the stellar winds collide and develop instabilities, mainly the non-linear thin shell instability, and form filaments and clumps. we also found that a few days before periastron passage the dense filaments and clumps flow towards the secondary as a result of its gravitational attraction, and are then accreted onto the secondary. we ran our simulations for a conventional model of stellar masses, $m_1=120 \rm{m_\odot}$ and $m_2=30 \rm{m_\odot}$, and for the high-mass model, $m_1=170 \rm{m_\odot}$ and $m_2=80 \rm{m_\odot}$, that was proposed to fit better the history of giant eruptions in the 19$^{\rm th}$ century, as well as radial-velocity variations of spectral lines during recent spectroscopic events. the results of the simulations show that the accretion process is more pronounced in the high-mass model, and that the amount of mass accreted, as well as the duration of the accretion, are also fitted much better. our findings establish η car as the most massive binary system in the galaxy. as our simulations demonstrate, the presence of a binary companion can have a huge influence on the evolution of massive stars, especially at later stages where it may undergo giant episodes of mass loss. | accretion simulations of η carinae and implications for the evolution of massive binaries |
modern observational techniques are still not powerful enough to directly view planet formation, and so it is necessary to rely on theory. however, observations do give two important clues to the formation process. the first is that the most primitive form of material in interstellar space exists as a dilute gas. some of this gas is unstable against gravitational collapse, and begins to contract. because the angular momentum of the gas is not zero, it contracts along the spin axis, but remains extended in the plane perpendicular to that axis, so that a disk is formed. viscous processes in the disk carry most of the mass into the center where a star eventually forms. in the process, almost as a by-product, a planetary system is formed as well. the second clue is the time required. young stars are indeed observed to have gas disks, composed mostly of hydrogen and helium, surrounding them, and observations tell us that these disks dissipate after about 5 to 10 million years. if planets like jupiter and saturn, which are very rich in hydrogen and helium, are to form in such a disk, they must accrete their gas within 5 million years of the time of the formation of the disk. any formation scenario one proposes must produce jupiter in that time, although the terrestrial planets, which don't contain significant amounts of hydrogen and helium, could have taken longer to build. modern estimates for the formation time of the earth are of the order of 100 million years. to date there are two main candidate theories for producing jupiter-like planets. the core accretion (ca) scenario supposes that any solid materials in the disk slowly coagulate into protoplanetary cores with progressively larger masses. if the core remains small enough it won't have a strong enough gravitational force to attract gas from the surrounding disk, and the result will be a terrestrial planet. if the core grows large enough (of the order of ten earth masses), and the disk has not yet dissipated, then the planetary embryo can attract gas from the surrounding disk and grow to be a gas giant. if the disk dissipates before the process is complete, the result will be an object like uranus or neptune, which has a small, but significant, complement of hydrogen and helium. the main question is whether the protoplanetary core can grow large enough before the disk dissipates. a second scenario is the disk instability (di) scenario. this scenario posits that the disk itself is unstable and tends to develop regions of higher than normal density. such regions collapse under their own gravity to form jupiter-mass protoplanets. in the di scenario a jupiter-mass clump of gas can form—in several hundred years which will eventually contract into a gas giant planet. the difficulty here is to bring the disk to a condition where such instabilities will form. now that we have discovered nearly 3000 planetary systems, there will be numerous examples against which to test these scenarios. | planet formation |
during the lifetime of a galaxy, secular radial mass redistribution is expected to gradually build up a bulge and transform the hubble type from late to early. the dominant dynamical process responsible for this transformation is a collective instability mediated by density-wave collisionless shocks (zhang 1996, 1998, 1999). the ability of this new mechanism to secularly redistribute the stellar mass provides a general pathway for the formation and evolution of the majority of hubble types, ranging from late type disk galaxiess to disky ellipticals. atlas3dresults (cappellari et al. 2013) showed that spirals and s0s and disky ellipticals form a continuous trend of evolution which also coincides with the aging of the stellar population of galactic disks. the importance of stellar accretion is also revealed in the results of the cosmos team which showed that the evolution of the black-hole-mass/bulge-mass correlation since z = 1 was mainly due to the mass redistribution on pre-existing stellar disks which were already in place by z = 1 (cisternas et al. 2011). the weaker correlation between the masses of late-type bulges and agns observed at any given epoch in our view is a result of the quicker initial onset of accretion events in agn disks compared to that in galactic disks, since the dynamical timescale is shorter for smaller agn accretion disks. the same secular dynamical process can produce and maintain the well-known scaling relations and universal rotation curves of observed galaxies during their hubble-type transformation (zhang 2008), as well as reproduce many other observed structural and kinematic properties of galaxies such as the size-line-width relation of the interstellar medium and the age-velocity dispersion relation of solar neighborhood stars in our own galaxy. a by-product of this analysis is a powerful new method for locating the multiple corotation resonances in galaxies (zhang & buta 2007; buta & zhang 2009). the current work also highlights the connection between collective effects in galactic dynamics and nonequilibrium phase transition processes in other branches of physics such as fluid turbulence and spontaneous breaking of gauge symmetry in high-energy physics. the continuous build-up of the hubble sequence of galaxies through secular mass accretion also hints at the baryonic nature of galactic dark matter and poses challenges to the existing lcdm paradigm, since the well-known adiabatic compression process during baryonic mass inflow produced by secular evolution would lead to a concentration of the cold dark matter to the central region of early-type galaxies, which is not observed. | the role of collective effects and secular mass migration on galactic transformation |
we study the evolution of simulated galaxies in the presence of feedback from active galactic nuclei (agn). first, we present a study conducted with a semi-analytic model (sam) of galaxy formation and evolution that includes prescriptions for bulge growth and agn feedback due to galaxy mergers and disk instabilities. we find that with this physics included, our model is able to qualitatively reproduce a population of galaxies with the correct star-formation and morphological properties when compared with populations of observed galaxies out to z 3. we also examine the characteristic histories of galaxies with different star-formation and morphological properties in our model in order to draw conclusions about the histories of observed galaxies. next, we examine the structural properties of galaxies (morphology, size, surface density) as a function of distance from the "star-forming main sequence'' (sfms), the observed correlation between the star formation rates (sfrs) and stellar masses of star-forming galaxies. we find that, for observed galaxies, as we move from galaxies above the sfms (higher sfrs) to those below it (lower sfrs), there exists a nearly monotonic trend towards more bulge-dominated morphology, smaller radius, lower sfr density, and higher stellar density. we find qualitatively similar results for our model galaxies, again driven by our prescriptions for bulge growth and agn feedback. next, we conduct a study of the effect of agn feedback on the gas in individual galaxies using a suite of cosmological hydrodynamical simulations. we compare two sets of 24 galaxies with halo masses of 1012 - 1013.4 msun run with two different feedback models: one which includes stellar feedback via uv heating, stellar winds and supernovae, agn feedback via momentum-driven winds and x-ray heating, and metal heating via photoelectric heating and cosmic x-ray background heating from accreting black holes in background galaxies (mragn), and another model which is identical except that it does not include any agn feedback (noagn). we find that our agn feedback prescription acts both "ejectively,'' removing gas from galaxies in powerful outflows, and "preventatively'', suppressing the inflow of gas onto the galaxy. the histories of mragn galaxies are gas ejection-dominated, while the histories of noagn galaxies are gas recycling-dominated. this difference in gas cycles results in the quenching of star formation in mragn galaxies, while their noagn counterparts continue to form stars until z=0. finally, we examine how this change in the baryon cycle affects the metal content of mragn galaxies relative to noagn galaxies and find that a combination of gas removal from and metal injection into the hot gas halo results in higher average halo metallicities in mragn galaxies. | exploring the effect of active galactic nuclei on quenching, morphological transformation and gas flows with simulations of galaxy evolution |
generally it has been assumed that the presence of a fast (close to the escape velocity) accretion is an indication of the magnetospheric accretion. however, observations indicate that fast accretion also occurs even in a weakly magnetized stars like herbig ae stars, which poses a question about the picture of accretion we have developed. we performed 3d mhd simulations by using the athena++ code, and analyzed the accretion from an mri (magneto-rotational instability)-active disk onto a weakly magnetized star. as a result, we found that fast accretion to a high-latitude, which is similar to the magnetospheric accretion, is possible even without the stellar magnetosphere. our results suggest a possibility that stars without the magnetosphere can show a violent accretion behavior associated with x-ray activities. we will discuss the physics of the accretion on the basis of our simulations. | fast accretion into a weakly magnetized star |
we report the results of 1978-2019 time-series photometry of am cvn, the prototype of the "double-degenerate" or ultracompact class of cataclysmic variable. the star remained faithfully in the range v=14.10-14.25 throughout the ~1400 nights of observation, and flashed its familiar positive and negative superhumps, at periods near 1051 s and 1011 s respectively. these periods, arising from instabilities in the accretion disk, wander erratically in phase, on a timescale of weeks. the 1028.7322 s orbital signal is slightly weaker (~0.003 mag semiamplitude), but is phase-stable over many years. over a 27-year baseline, timings of orbital minimum show a period decrease of 17 3 s/year. this is consistent with evolution driven by gravitational radiation, but only if the secondary is a low-mass helium star, rather than a degenerate dwarf. | 40 years of periodic signals in the ultracompact binary am canum venaticorum |
giant planets have been discovered at large separations from the central star. a striking number of young circumstellar disks also have gas and/or dust gaps at large orbital separations, indicating the potential population of young planets there. but to form massive planets at large orbital separations quickly through core accretion, an early solid body to seed pebble and gas accretion is desirable. early protoplanetary disks are likely self-gravitating, and these gravitoturbulent disks have been shown to efficiently concentrate dust within spiral features. furthermore, the self-gravity of the gas will enhance settling towards the midplane providing more favorable conditions for dust to reach densities necessary for gravitational collapse. we run 3d local hydrodynamical simulations of gravitoturbulent disks with lagrangian dust particles to determine whether particle and gas self-gravity can lead to the formation of dense solid bodies which can serve as the starting points of later planet formation. when self-gravity between dust particles is included, solids of size st = 0.1 to 1 concentrate within the gravitoturbulent spiral features and collapse under their own self-gravity into dense clumps which can be up to several m⊕ in mass. the most efficiently drifting dust size st=1 forms the most massive clouds of particles, while smaller dust particles st=0.1 concentrate up to masses an order of magnitude lower. dust cloud masses are smaller by a factor of a few but more numerous, when the effect of dust backreaction onto the gas is included. crucially, gravitational collapse of dust occurs with the formation of spirals, when the gas disk has an instability parameter q ~ 1.5. this means the formation of solid planetesimals and embryos can happen without the gas disk being massive enough for fragmentation. the existence of large solid bodies at an early stage of the disk can accelerate the planet formation process, particularly at wide orbital separations, and potentially explain planets far away from the central stars and young disks with substructures. | the direct formation of planetary embryos in self-gravitating disks |
we model the intermediate-mass black hole hlx-1, using hst, xmm-newton, and swift. we quantify the relative contributions of a bluer optical component, function of x-ray irradiation, and a redder component, constant and likely coming from an old stellar population. we estimate a bh mass m ≈ 2 × 10^4 m_{⊙}, a spin parameter a/m ≈ 0.9, and a peak outburst luminosity l_x ≈ 0.3 l_{edd}. we suggest that the disk is large (r_{out} ≈ 2 × 10^{13} cm) but only the inner annuli (r ∼ a few 10^{11} cm) are involved in the x-ray outburst cycle. we propose that outbursts are due to accretion rate oscillations, caused by radiatively-driven disk outflows. we argue that the system has a long-term-average accretion rate of a few percent of eddington, just below the upper limit of the low/hard state; a wind-driven oscillation can trigger transitions to the high/soft state, with a recurrence period ∼1 year (much longer than the binary period, which we estimate as ∼10 days). the oscillation that dominated the system in the last decade is now damped. finally, we highlight similarities between disk winds in hlx-1 and in the galactic bh v404 cyg. | outbursts of the intermediate-mass black hole hlx-1: a wind instability scenario |
fingering convection is an important source of mixing in stars, which are low prandtl number fluids. quantifying transport by fingering convection is therefore crucial to a better understanding of stellar evolution. in the absence of magnetic fields, brown et al. have shown that fingering-induced fluxes measured in dns are well-predicted by a so-called ''parasitic'' model, in which the primary fingering instability is assumed to saturate due to the growth of secondary shear instabilities between the fingers. recent work extended this analysis to include a vertical magnetic field, and found that a similar parasitic model could account for the increase in turbulent fluxes with magnetic field strength observed in simulations. however, this analysis was limited to a small region of parameter space. here we show that lowering the magnetic prandtl number and exploring a broader range of stratifications reveals discrepancies between the parasitic saturation model and the dns results. we propose some explanations, and discuss implications of our findings for other systems where parasitic saturation models are used, such as the mri in accretion disks and gsf instability in stars. this work was funded by the national science foundation (nsf ast 1908338). | mhd effects on fingering convection in stars: the problem with parasites |
a widely accepted picture of an accretion flow in a soft spectral state x-ray binary system is a geometrically thin disk structure much alike the classic analytic solution of shakura and sunyaev. despite the fact that the analytic models are troubled by instabilities and miss important aspects of physics such as magnetic fields, they are successfully used as a framework for interpreting observational data through continuum spectral fitting. we present results of general relativistic radiative magnetohydrodynamic simulations of sub-eddington optically thick accretion on a stellar mass black hole with a mildly sub-eddington luminosity. we find the accretion flow stabilized by magnetic field, with a puffed-up optically thick region, resembling a warm corona surrounding denser disk core. we analyze the inner structure and properties of the puffy disk and compare it with analytical models. | grrmhd simulation of sub-eddington accretion onto stellar mass black hole |
accretion disc winds in x-ray binaries have been recently recognised to be a major ingredient of accretion. recent results indicate that they can carry away more matter than the one accreted onto the compact object, that their presence appears connected with the state of the accretion disc and with the absence of the jet. here we present the case of ax j1745.6-2901, a neutron star low mass x-ray binary showcasing intense ionised fe k absorption only during the soft state. thanks to the availability of a large number of simultaneous xmm-newton and nustar spectra, we accurately determine the x-ray spectral energy distribution and its variations between the states. we observe that the ionised absorber lies always on a stable branch of the photo-ionisation stability curve during the soft state, while it becomes unstable during the hard state. the same process might explain the disappearance of the high ionisation absorber/wind during the hard state in other systems. | photoionisation instability of winds in x-ray binaries |
disks of gas and dust around forming stars - circumstellar disks - last only a few million years. this is a very small fraction of the entire lifetime of sun-like stars, several billion years. nevertheless, by the time circumstellar disks dissipate stars complete building up their masses, giant planets finish accreting gas, and terrestrial bodies are nearly fully grown and ready for their final assembly to become planets. understanding the evolution of circumstellar disks are thus crucial in many contexts. using numerical simulations as the primary tool, my thesis has focused on the studies of various physical processes that can occur throughout the lifetime of circumstellar disks, from their formation to dispersal. chapters 2, 3, and 4 emphasize the importance of early evolution, during which time a forming star-disk system obtains mass from its natal cloud: the infall phase. in chapter 2 and 3, i have modeled episodic outbursts of accretion in protostellar systems resulting from disk instabilities - gravitational instability and magnetorotational instability. i showed that outbursts occur preferentially during the infall phase, because the mass addition provides more favorable conditions for gravitational instability to initiate the outburst cycle, and that forming stars build up a significant fraction of their masses through repeated short-lived, episodic outbursts. the infall phase can also be important for the formation of planets. recent alma observations revealed sets of bright and dark rings in circumstellar disks of young, forming stars, potentially indicating early formation of planets. in chapter 4, i showed that infall streams can create radial pressure bumps near the outer edge of the mass landing on the disk, from which vortices can form, collecting solid particles very efficiently to make initial seeds of planets. the next three chapters highlight the role of planets in setting the observational appearance and the evolution of circumstellar disks. when a planet forms in a disk, the gravitational interaction between the planet and disk can create structures, such as spiral arms and gaps. in chapter 5, i compared the disk structures formed by planetary companions in numerical simulations with the observed structures in the disk surrounding an 8 myr-old herbig ae star sao 206462. based on the experiments, i made predictions for the mass and position of a currently unrevealed planet, which can help guide future observations to search for more conclusive evidence for the existence of a planetary companion in the system. in chapter 6, i showed for the first time in global simulation domains that spiral waves, driven for instance by planets or gravitational instability, can be unstable due to resonant interactions with inertial modes, breaking into turbulence. in chapter 7, i showed that the spiral wave instability operates on the waves launched by planets and that the resulting turbulence can significantly stir up solid particles from the disk midplane. the stirring of solid particles can have influences on the observation appearance of the parent disk and on the subsequent assembly of planetary bodies in the disk. finally, in chapter 8, i investigated the dispersal of circumstellar disks via photoevaporative winds, finding that the photoevaporative loss alone, coupled with a range of initial angular momenta of protostellar clouds, can explain the observed decline of the disk frequency with increasing age. the findings and future possibilities are summarized in chapter 9. | studies of young, star-forming circumstellar disks |
x-ray transients, such as accreting neutron stars, can periodically undergo outbursts. the exact mechanism that causes the onset of outburst is unknown, but it is theorised that these kinds of outbursts are caused by a thermal-viscous instability in the accretion disk. usually outbursts of accreting neutron stars are caught when the accretion disk has already undergone an instability, and the persistent flux has risen to a threshold detectable by all sky monitors on x-ray space observatories. in this presentation i will present the earliest known combined optical and x-ray monitoring observations of the lead-up and onset of outburst in an accreting neutron star system. we monitored the accretion-powered millisecond pulsar sax j1808.4-3658 during the lead-up to the august 2019 outburst of the source using the neil gehrels swift observatory, the 2-m faulkes telescope south (at siding spring, australia) and the las cumbres observatory (lco) network of 1-m robotic telescopes. we began our monitoring program 3 weeks before the outburst officially began, and observed a 12 day delay between the first optical rise and the first x-ray detection of the source. in this presentation i will discuss the implications of a 12 day delay between the optical and x-ray brightening, and relate this to the theory of outburst in soft x-ray transients such as sax j1808.4-3658, including the disk instability model. optical emission is thought to be produced by either the companion star or the outer disk, and x-ray emission is thought to be produced by the inner disk, where matter from the disk is transferred onto the neutron star. theoretical calculations using the accretion rate and radius indicate that the viscous timescale of the disk in this system is approximately 12 days, coinciding with the optical to x-ray emission delay we observed.this work provides the earliest multiwavelength observations of an accreting neutron star coming into outburst that we are aware of, and is an important confirmation of the expected delay in optical to x-ray emission during the onset of outburst in soft x-ray transients. it also provides a new early indication of the onset of outburst in x-ray transients. | capturing a pulsar powering up |
over the past decade direct imaging searches for self-luminous giant planets have uncovered an unexpected population of young planetary-mass companions on extremely wide orbits (>100 au). the masses of these companions typically straddle the deuterium burning limit, throwing into question whether these are high mass planets or low mass brown dwarfs. these wide-separation companions pose significant challenges to three possible formation routes, namely core accretion, disk instability, and turbulent fragmentation. in this talk i will describe a program to directly test these three competing formation mechanisms for wide-separation planets by measuring their rotation rates using nirspec/keck nir high-resolution spectroscopy. rotation rates of young gas giant planets provide a unique window into planet accretion histories, and can be used to test how they formed. with our initial sample of rotation rates we placed the first constraints on the spin distribution and angular momentum evolution in the planetary-mass regime. with our expanded sample we aim to test new trends with rotation rate, such as how they vary with separation and mass ratio between the companion and host star, and trace these back to potential formation histories. | the world is spinning: constraining the origin of gas giants using planetary spin |
we present results from two-dimensional, general relativistic, radiation, magnetohydrodynamic (grrmhd) numerical simulations of radiation-pressure-dominated, shakura-sunyaev thin disks accreting onto a stellar-mass, schwarzschild black hole. in previous work, we showed that such disks are thermally unstable. here we test the idea that strong magnetic fields may stabilize the disk. we consider three different magnetic field configurations: a uniform, vertical magnetic field; a single, radially extended dipole field contained within the disk; and a series of alternating, small poloidal loops, distributed along the disk midplane. all three configurations start with initially weak magnetic fields. differential winding (the so-called alpha dynamo) should increase the magnetic pressure relative to the gas pressure in both the vertical and radial field cases. the question is whether the fields can reach sufficient strength to stabilize the disks thermally before other processes (magnetic reconnection or the parker instability) saturate their growth. | testing the role of strong magnetic fields in stabilizing radiation-pressure-dominated thin accretion disks |
the driving mechanism of protostellar outflows and jets and their effects on the star formation process obtained from recent theoretical and numerical studies are described. low-velocity outflows are driven by an outer region of the circumstellar disk, while high-velocity jets are driven near an inner edge of the disk. the disk angular momentum is effectively transferred by magnetic effects in the outflow and jet driving regions where the magnetic field is well coupled with neutral gas. on the other hand, in a high density gas region of the disk (or intermediate region), the magnetic field dissipates and is decoupled from neutral gas. thus, in such a magnetically inactive region, no outward flow appears and the disk angular momentum is not effectively transferred by magnetic effects. therefore, in the disk intermediate region, the disk surface density continues to increase and gravitational instability occurs and produce a non-axisymmetric (or spiral) structure. after spiral arms sufficiently develop, the disk angular momentum is transferred by gravitational torque and a large amount of the disk mass accretes onto the central protostar from the circumstellar disk. the episodic accretion induces time-variable high-velocity jets. the jets do not significantly contribute to a dynamical evolution of the protostar and circumstellar disk, while the low-velocity outflows can eject a large fraction of the infalling gas and determine the final stellar mass. | protostellar jets and outflows in low-mass star formation |
accretion discs around neutron stars are bright in x-rays. this emission has long been known to vary with energy, revealing the material composition of the disc. but also to vary the with a multitude of periods, revealing geometric structures in the disc. understanding how, exactly, these spectral and temporal signatures couple is crucial for understanding the dynamics of the accretion process through extremely curved space-time. providing excellent spectral and timing capabilities, nicer is well suited for this task. using nicer observations of aquila x-1 we find, for the first time in a neutron star system, that instabilities in the accretion disc have a two-component spectrum, with the low energy variability driving leading the correlated signals at higher energies. this contribution presents these new results, and their implications for the accretion process in neutron star and black hole systems. | spectral-timing of aquila x-1 |
we present a numerical analysis of the spin evolution of neutron stars in low-mass x-ray binaries, trying to explain the discrepancy in the spin period distribution between observations of millisecond pulsars and theoretical results. in our calculations, we take account of possible effects of radiation pressure and irradiation-induced instability on the structure of the disk, and the evolution of the mass transfer rate, respectively. we report the following results: (1) the radiation pressure in the accretion disk leads to a slight increase of spin periods, and the variation of mass transfer rate caused by the neutron star irradiation can shorten the spin-down phase of evolution. (2) the calculated results of the model combining radiation pressure and irradiation show that the accretion is strongly limited by the radiation pressure in the high mass transfer phase. (3) the accreted mass and fastness parameter can affect the number of systems in the equilibrium state. | evolution of the spin periods of neutron stars in low mass x-ray binaries |
we present a detailed study of the circular geodesic motion of neutral test particles on the equatorial plane of a spherically symmetric scalarized neutron star (ns). we also examined the stability criteria for massive and massless particles for the said ns by computing the effective potential. we compute the radii of innermost stable circular orbit (isco), marginally bound circular orbit (mbco) and circular photon orbit (cpo).we also derive the paczy'{n}ski-witta potential which is so called the pseudo-newtonian potential which is very crucial to analyze the accretion disk properties. by analyzing the null circular geodesics we compute the lyapunov exponent for the scalarized ns. moreover, we show that in the eikonal limit, the real and imaginary parts of the quasi normal modes (qnm) of the scalarized ns could be determined in terms of the frequency of the ns and instability time scale of the unstable circular photon geodesics. | paczy'{n}ski-witta potential form of scalarized neutron star |
the two favored mechanisms suggested for forming gas giants are disk instability and core accretion. the latter is the generally accepted mechanism on short orbits. according to this model, one would expect to observe a positive correlation between the transit depth of gas giants and the metallicity of the host star. however, a negative correlation was reported between kepler’s q1-q12 gas giant candidates and the stellar metallicity. even though this correlation is extremely weak, at the -1.17 sigma, it challenges the theory of planet formation. my work involves revising this correlation now that the number of kepler's candidates/confirmed has increased. but large-scale surveys, such as kepler, are subject to selection effects and biases. these biases should be quantified and accounted for in the statistical analysis in order to best understand the correlation. this work reflects the importance of statistical analysis in detecting and characterizing exoplanets, especially in the era of large-scale surveys. such analysis will lead to a greater understanding of planet formation. | revising the transit depth-metallicity correlation of kepler's giant candidates |
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