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the magnetorotational instability (mri) is considered to be a promising mechanism to amplify the magnetic field in fast-rotating protoneutron stars. in contrast to accretion discs, radial buoyancy driven by entropy and lepton fraction gradients is expected to have a dynamical role as important as rotation and shear. we investigate the poorly known impact of buoyancy on the non-linear phase of the mri, by means of three-dimensional numerical simulations of a local model in the equatorial plane of a protoneutron star. the use of the boussinesq approximation allows us to utilize a shearing box model with clean shearing periodic boundary conditions, while taking into account the buoyancy driven by radial entropy and composition gradients. we find significantly stronger turbulence and magnetic fields in buoyantly unstable flows. on the other hand, buoyancy has only a limited impact on the strength of turbulence and magnetic field amplification for buoyantly stable flows in the presence of a realistic thermal diffusion. the properties of the turbulence are, however, significantly affected in the latter case. in particular, the toroidal components of the magnetic field and of the velocity become even more dominant with respect to the poloidal ones. furthermore, we observed in the regime of stable buoyancy the formation of long-lived coherent structures such as channel flows and zonal flows. overall, our results support the ability of the mri to amplify the magnetic field significantly even in stably stratified regions of protoneutron stars. | numerical simulations of the magnetorotational instability in protoneutron stars - i. influence of buoyancy |
we propose the formation of massive pristine dark-matter (dm) haloes with masses of ∼108 m⊙, due to the dynamical effects of frequent mergers in rare regions of the universe with high baryonic streaming velocity relative to dm. since the streaming motion prevents gas collapse into dm haloes and delays prior star formation episodes, the gas remains metal-free until the halo virial temperatures ≳2 × 104 k. the minimum cooling mass of dm haloes is boosted by a factor of ∼10-30 because frequent major mergers of haloes further inhibit gas collapse. we use monte carlo merger trees to simulate the dm assembly history under a streaming velocity of twice the root-mean-square value, and estimate the number density of massive dm haloes containing pristine gas as ≃10-4 cmpc-3. when the gas infall begins, efficient lyα cooling drives cold streams penetrating inside the halo and feeding a central galactic disc. when one stream collides with the disc, strong shock forms a dense and hot gas cloud, where the gas never forms h2 molecules due to effective collisional dissociation. as a result, a massive gas cloud forms by gravitational instability and collapses directly into a massive black hole (bh) with m• ∼ 105 m⊙. almost simultaneously, a galaxy with m_{\star,tot}∼ 10^6 m_⊙ composed of population iii stars forms in the nuclear region. if the typical stellar mass is as high as ∼100 m⊙, the galaxy could be detected with the james webb space telescope even at z ≳ 15. these massive seed bhs would be fed by continuous gas accretion from the host galaxy, and grow to be bright quasars observed at z ≳ 6. | massive black hole and population iii galaxy formation in overmassive dark-matter haloes with violent merger histories |
we present results from multi-wavelength simultaneous x-ray and radio observations of the black hole x-ray binary v404 cyg in quiescence. our coverage with nustar provides the very first opportunity to study the x-ray spectrum of v404 cyg at energies above 10 kev. the unabsorbed broadband (0.3-30 kev) quiescent luminosity of the source is 8.9 × 1032 erg s-1 for a distance of 2.4 kpc. the source shows clear variability on short timescales (an hour to a couple of hours) in the radio, soft x-ray, and hard x-ray bands in the form of multiple flares. the broadband x-ray spectra obtained from xmm-newton and nustar can be characterized with a power-law model having a photon index of γ = 2.12 ± 0.07 (90% confidence errors); however, residuals at high energies indicate spectral curvature significant at a 3σ confidence level with the e-folding energy of the cutoff as {20}-7+20 kev. such curvature can be explained using synchrotron emission from the base of a jet outflow. radio observations using the vla reveal that the spectral index evolves on very fast timescales (as short as 10 minutes), switching between optically thick and thin synchrotron emission, possibly due to instabilities in the compact jet or stochastic instabilities in the accretion rate. we explore different scenarios to explain this very fast variability. | characterizing x-ray and radio emission in the black hole x-ray binary v404 cygni during quiescence |
this review, based on lectures given at the 45th saas-fee advanced course "from protoplanetary disks to planet formation", introduces physical processes in protoplanetary disks relevant to accretion and the initial stages of planet formation. after a brief overview of the observational context, i introduce the elementary theory of disk structure and evolution, review the gas-phase physics of angular momentum transport through turbulence and disk winds, and discuss possible origins for the episodic accretion observed in young stellar objects. turning to solids, i review the evolution of single particles under aerodynamic forces, and describe the conditions necessary for the development of collective gas-particle instabilities. observations show that disks can exhibit pronounced large-scale structure, and i discuss the types of structures that may form from gas and particle interactions at ice lines, vortices and zonal flows, prior to the formation of large planetary bodies. i conclude with disk dispersal. | physical processes in protoplanetary disks |
the magnetorotational instability (mri) can be a powerful mechanism amplifying the magnetic field in core-collapse supernovae. whether initially weak magnetic fields can be amplified by this instability to dynamically relevant strengths is still a matter of debate. one of the main uncertainties concerns the process that terminates the growth of the instability. parasitic instabilities of both kelvin-helmholtz and tearing-mode type have been suggested to play a crucial role in this process, disrupting mri channel flows and quenching magnetic field amplification. we perform two-dimensional and three-dimensional sheering-disc simulations of a differentially rotating protoneutron star layer in non-ideal magnetohydrodynamics with unprecedented high numerical accuracy, finding that kelvin-helmholtz parasitic modes dominate tearing modes in the regime of large hydrodynamic and magnetic reynolds numbers, as encountered close to the surface of protoneutron stars. they also determine the maximum magnetic field stress achievable during the exponential growth of the mri. our results are consistent with the theory of parasitic instabilities based on a local stability analysis. to simulate the kelvin-helmholtz instabilities properly, a very high numerical resolution is necessary. using ninth-order spatial reconstruction schemes, we find that at least eight grid zones per mri channel are necessary to simulate the growth phase of the mri and reach an accuracy of ∼10 per cent in the growth rate, while more than ∼60 zones per channel are required to achieve convergent results for the value of the magnetic stress at mri termination. | termination of the magnetorotational instability via parasitic instabilities in core-collapse supernovae |
we investigate how the detectability of signatures of self-gravity in a protoplanetary disk depends on its temporal evolution. we run a one-dimensional model for secular timescales to follow the disk mass as a function of time. we then combine this with three-dimensional global hydrodynamics simulations that employ a hybrid radiative transfer method to approximate realistic heating and cooling. we simulate alma continuum observations of these systems and find that structures induced by the gravitational instability (gi) are readily detectable when q = m disk/m * ≳ 0.25 and r outer ≲ 100 au. the high accretion rate generated by gravito-turbulence in such a massive disk drains its mass to below the detection threshold in ∼104 years, or approximately 1% of the typical disk lifetime. therefore, disks with spiral arms detected in alma dust observations, if generated by self-gravity, must either be still receiving infall to maintain a high q value, or have just emerged from their natal envelope. detection of substructure in systems with lower q is possible, but would require a specialist integration with the most extended configuration over several days. this disfavors the possibility of gi-caused spiral structure in systems with q < 0.25 being detected in relatively short integration times, such as those found in the dsharp alma survey. we find no temporal dependence of detectability on dynamical timescales. | the temporal requirements of directly observing self-gravitating spiral waves in protoplanetary disks with alma |
recent discoveries of gas giant exoplanets around m-dwarfs from transiting and radial velocity surveys are difficult to explain with core-accretion models. we present here a homogeneous suite of 162 models of gravitationally unstable gaseous disks. these models represent an existence proof for gas giants more massive than 0.1 jupiter masses to form by the gas disk gravitational instability (gdgi) mechanism around m-dwarfs for comparison with observed exoplanet demographics and protoplanetary disk mass estimates for m-dwarf stars. we use the enzo 2.6 adaptive mesh refinement (amr) 3d hydrodynamics code to follow the formation and initial orbital evolution of gas giant protoplanets in gravitationally unstable gaseous disks in orbit around m-dwarfs with stellar masses ranging from 0.1 m ⊙ to 0.5 m ⊙. the gas disk masses are varied over a range from disks that are too low in mass to form gas giants rapidly to those where numerous gas giants are formed, therefore revealing the critical disk mass necessary for gas giants to form by the gdgi mechanism around m-dwarfs. the disk masses vary from 0.01 m ⊙ to 0.05 m ⊙ while the disk to star mass ratios explored the range from 0.04 to 0.3. the models have varied initial outer disk temperatures (10-60 k) and varied levels of amr grid spatial resolution, producing a sample of expected gas giant protoplanets for each star mass. broadly speaking, disk masses of at least 0.02 m ⊙ are needed for the gdgi mechanism to form gas giant protoplanets around m-dwarfs. | forming gas giants around a range of protostellar m-dwarfs by gas disk gravitational instability |
we present the discovery of two galaxy overdensities in the hubble space telescope udf: a proto-cluster, hudfj0332.4-2746.6 at z=1.84+/- 0.01, and a group, hudfj0332.5-2747.3 at z=1.90 ± 0.01. assuming viralization, the velocity dispersion of hudfj0332.4-2746.6 implies a mass of {{m}200}=(2.2+/- 1.8)× {{10}14} {{m}⊙ }, consistent with the lack of extended x-ray emission. neither overdensity shows evidence of a red sequence. about 50% of their members show interactions and/or disturbed morphologies, which are signatures of merger remnants or disk instability. most of their etgs have blue colors and show recent star formation. these observations reveal for the first time large fractions of spectroscopically confirmed star-forming blue etgs in proto-clusters at z≈ 2. these star-forming etgs are most likely among the progenitors of the quiescent population in clusters at more recent epochs. their mass-size relation is consistent with that of passive etgs in clusters at z∼ 0.7-1.5. if these galaxies are the progenitors of cluster etgs at these lower redshifts, their size would evolve according to a similar mass-size relation. it is noteworthy that quiescent etgs in clusters at z=1.8-2 also do not show any significant size evolution over this redshift range, contrary to field etgs. the etg fraction is ≲ 50%, compared to the typical quiescent etg fraction of ≈ 80% in cluster cores at z< 1. the fraction, masses, and colors of the newly discovered etgs imply that other cluster etgs will be formed/accreted at a later time. | star-forming blue etgs in two newly discovered galaxy overdensities in the hudf at z=1.84 and 1.9: unveiling the progenitors of passive etgs in cluster cores |
we observed the high-mass protostellar core g335.579-0.272 alma1 at ~200 au (0.″05) resolution with the atacama large millimeter/submillimeter array (alma) at 226 ghz (with a mass sensitivity of 5σ = 0.2 m ⊙ at 10 k). we discovered that at least a binary system is forming inside this region, with an additional nearby bow-like structure (≲1000 au) that could add an additional member to the stellar system. these three sources are located at the center of the gravitational potential well of the alma1 region and the larger mm1 cluster. the emission from ch3oh (and many other tracers) is extended (>1000 au), revealing a common envelope toward the binary system. we use ch2chcn line emission to estimate an inclination angle of the rotation axis of 26° with respect to the line of sight based on geometric assumptions and derive a kinematic mass of the primary source (protostar+disk) of 3.0 m ⊙ within a radius of 230 au. using sio emission, we find that the primary source drives the large-scale outflow revealed by previous observations. precession of the binary system likely produces a change in orientation between the outflow at small scales observed here and large scales observed in previous works. the bow structure may have originated from the entrainment of matter into the envelope due to the widening or precession of the outflow, or, alternatively, an accretion streamer dominated by the gravity of the central sources. an additional third source, forming due to instabilities in the streamer, cannot be ruled out as a temperature gradient is needed to produce the observed absorption spectra. | digging into the interior of hot cores with alma (dihca). ii. exploring the inner binary (multiple) system embedded in g335 mm1 alma1 |
we carry out two-fluid, two-dimensional global hydrodynamic simulations to test whether protostellar infall can trigger the rossby wave instability (rwi) in protoplanetry disks. our results show that infall can trigger the rwi and generate vortices near the outer edge of the mass landing on the disk (i.e., centrifugal radius). we find that the rwi is triggered under a variety of conditions, although the details depend on the disk parameters and the infall pattern. the common key feature of triggering the rwi is the steep radial gradient of the azimuthal velocity induced by the local increase in density at the outer edge of the infall region. vortices form when the instability enters the nonlinear regime. in our standard model where self-gravity is neglected, vortices merge together to a single vortex within ∼20 local orbital times, and the merged vortex survives for the remaining duration of the calculation (>170 local orbital times). the vortex takes part in outward angular momentum transport, with a reynolds stress of ≲10-2. our two-fluid calculations show that vortices efficiently trap dust particles with stopping times of the order of the orbital time, locally enhancing the dust to gas ratio for particles of the appropriate size by a factor of ∼40 in our standard model. when self-gravity is considered, however, vortices tend to be impeded from merging and may eventually dissipate. we conclude it may well be that protoplanetary disks have favorable conditions for vortex formation during the protostellar infall phase, which might enhance early planetary core formation. | are protoplanetary disks born with vortices? rossby wave instability driven by protostellar infall |
the phenomenological disc instability model has been successful in reproducing the observed light curves of dwarf nova outbursts by invoking an enhanced shakura-sunyaev α parameter ∼0.1-0.2 in outburst compared to a low value ∼0.01 in quiescence. recent thermodynamically consistent simulations of magnetorotational instability (mri) turbulence with appropriate opacities and equation of state for dwarf nova accretion discs have found that thermal convection enhances α in discs in outburst, but only near the hydrogen ionization transition. at higher temperatures, convection no longer exists and α returns to the low value comparable to that in quiescence. in order to check whether this enhancement near the hydrogen ionization transition is sufficient to reproduce observed light curves, we incorporate this mri-based variation in α into the disc instability model, as well as simulation-based models of turbulent dissipation and convective transport. these mri-based models can successfully reproduce observed outburst and quiescence durations, as well as outburst amplitudes, albeit with different parameters from the standard disc instability models. the mri-based model light curves exhibit reflares in the decay from outburst, which are not generally observed in dwarf novae. however, we highlight the problematic aspects of the quiescence physics in the disc instability model and mri simulations that are responsible for this behaviour. | dwarf nova outbursts with magnetorotational turbulence |
observations of the near-infrared excess object g2/dso increased attention toward the galactic center and its vicinity. the predicted flaring event in 2014 and the outcome of the intense monitoring of the supermassive black hole in the center of our galaxy did not fulfill all predictions about a significantly enhanced accretion event. subsequent observations addressed the question concerning the nature of the object because of its compact shape, especially during its periapse in 2014. theoretical approaches have attempted to answer the contradictory behavior of the object, resisting the expected dissolution of a gaseous cloud due to tidal forces in combination with evaporation and hydrodynamical instabilities. however, assuming that the object is instead a dust-enshrouded young stellar object seems to be in line with the predictions of several groups and observations presented in numerous publications. here we present a detailed overview and analysis of the observations of the object that have been performed with sinfoni (vlt) and we provide a comprehensive approach to clarify the nature of g2/dso. we show that the tail emission consists of two isolated and compact sources with different orbital elements for each source rather than an extended and stretched component as it appeared in previous representations of the same data. considering our recent publications, we propose that the monitored dust-enshrouded objects are remnants of a dissolved young stellar cluster whose formation was initiated in the circumnuclear disk. this indicates a shared history, which agrees with our analysis of the d- and x-sources. | the apparent tail of the galactic center object g2/dso |
context. protoplanetary discs are cold, dense, and weakly ionised environments that witness planetary formation. among these discs, transition discs (tds) are characterised by a wide cavity (up to tens of au) in the dust and gas distribution. despite this lack of material, a considerable fraction of tds are still strongly accreting onto their central star, possibly indicating that a mechanism is driving fast accretion in td cavities.aims: the presence of radially extended `dead zones' in protoplanetary discs has recently revived interest in magnetised disc winds (mdws), where accretion is driven by a large magnetic field extracting angular momentum from the disc. we propose that tds could be subject to similar disc winds, and that these could naturally explain the fast-accreting and long-lived cavities inferred in tds.methods: we present the results of the first 2.5d global numerical simulations of tds harbouring mdws using the pluto code. we imposed a cavity in the gas distribution with various density contrasts, and considered a power-law distribution for the large-scale magnetic field strength. we assume the disc is weakly ionised and is therefore subject to ambipolar diffusion, as expected in this range of densities and temperatures.results: we find that our simulated tds always reach a steady state with an inner cavity and an outer `standard' disc. these models also maintain an approximately constant accretion rate through the entire structure, reaching 10−7 m⊚ yr−1 for typical surface density values. the mdw launched from the cavity is more magnetised and has a significantly larger lever arm (up to 10) than the mdw launched from the outer disc. the material in the cavity is accreted at sonic velocities, and the cavity itself is rotating at 70% of the keplerian velocity due to the efficient magnetic braking imposed by the mdw. overall, our cavity matches the dynamical properties of an inner jet emitting disc (jed) and of magnetically arrested discs (mads) in black-hole physics. finally, we observe that the cavity is subject to recurring accretion bursts that may be driven by a magnetic rayleigh-taylor instability of the cavity edge.conclusions: some strongly accreting tds could be the result of magnetised wind sculpting protoplanetary discs. kinematic diagnostics of the disc or the wind (orbital velocity, wind speeds, accretion velocities) could disentangle classic photo-evaporation from mdw models. | magnetised winds in transition discs. i. 2.5d global simulations |
recent results by martin et al. showed in 3d smoothed particle hydrodynamics simulations that tilted discs in binary systems can be unstable to the development of global, damped kozai-lidov (kl) oscillations in which the discs exchange tilt for eccentricity. we investigate the linear stability of kl modes for tilted inviscid discs under the approximations that the disc eccentricity is small and the disc remains flat. by using 1d equations, we are able to probe regimes of large ratios of outer to inner disc edge radii that are realistic for binary systems of hundreds of astronomical unit separations and are not easily probed by multidimensional simulations. for order unity binary mass ratios, kl instability is possible for a window of disc aspect ratios h/r in the outer parts of a disc that roughly scale as (nb/n)2 ≲ h/r ≲ nb/n, for binary orbital frequency nb and orbital frequency n at the disc outer edge. we present a framework for understanding the zones of instability based on the determination of branches of marginally unstable modes. in general, multiple growing eccentric kl modes can be present in a disc. coplanar apsidal-nodal precession resonances delineate instability branches. we determine the range of tilt angles for unstable modes as a function of disc aspect ratio. unlike the kl instability for free particles that involves a critical (minimum) tilt angle, disc instability is possible for any non-zero tilt angle depending on the disc aspect ratio. | kozai-lidov disc instability |
context. in the disc-mediated accretion scenario for the formation of the most massive stars, high densities and high accretion rates could induce gravitational instabilities in the disc, forcing it to fragment and produce companion objects.aims: we investigate the effects of inclination and spatial resolution on the observable kinematics and stability of discs in high-mass star formation.methods: we studied a high-resolution 3d radiation-hydrodynamic simulation that leads to the fragmentation of a massive disc. using radmc-3d we produced 1.3 mm continuum and ch3cn line cubes at different inclinations. the model was set to different distances, and synthetic observations were created for alma at 80 mas resolution and noema at 0.4''.results: the synthetic alma observations resolve all fragments and their kinematics well. the synthetic noema observations at 800 pc with linear resolution of 300 au are able to resolve the fragments, while at 2000 pc with linear resolution of 800 au only a single structure slightly elongated towards the brightest fragment is observed. the position-velocity (pv) plots show the differential rotation of material best in the edge-on views. a discontinuity is seen at a radius of 250 au, corresponding to the position of the centrifugal barrier. as the observations become less resolved, the inner high-velocity components of the disc become blended with the envelope and the pv plots resemble rigid-body-like rotation. protostellar mass estimates from pv plots of poorly resolved observations are therefore overestimated. we fit the emission of ch3cn (12k-11k) lines and produce maps of gas temperature with values in the range of 100-300 k. studying the toomre stability of the discs, we find low q values below the critical value for stability against gravitational collapse at the positions of the fragments and in the arms connecting the fragments for the resolved observations. for the poorly resolved observations we find low q values in the outskirts of the disc. therefore, although we could not resolve any of the fragments, we are able to predict that the disc is unstable and fragmenting. this conclusion is valid regardless of our knowledge about the inclination of the disc.conclusions: these synthetic observations reveal the potential and limitations of studying discs in high-mass star formation with current (millimetre) interferometers. while the extremely high spatial resolution of alma reveals objects in extraordinary detail, rotational structures and instabilities within accretion discs can also be identified in poorly resolved observations. | disc kinematics and stability in high-mass star formation. linking simulations and observations |
ultraluminous x-ray sources (ulxs) are our best laboratories for studying extreme super-eddington accretion. most studies of these objects are of relatively persistent sources; however, there is growing evidence to suggest a large fraction of these sources are transient. here we present a sample of five newly reported transient ulxs in the galaxies ngc 4945, ngc 7793, and m81 serendipitously discovered in swift/xrt observations. swift monitoring of these sources have provided well-sampled lightcurves, allowing for us to model the lightcurves with the disk-instability model of hameury & lasota, which implies durations of 60-400 days and that the mass-accretion rate through the disk is close to or greater than the eddington rate. of the three source regions with prior hubble space telescope imaging, color-magnitude diagrams of the potential stellar counterparts show varying ages of the possible stellar counterparts. our estimation of the rates of these sources in these three galaxies is 0.4-1.3 yr-1. we find that, while persistent ulxs dominate the high end of galaxy luminosity functions, the number of systems that produce ulx luminosities are likely dominated by transient sources. | a new sample of transient ultraluminous x-ray sources serendipitously discovered by swift/xrt |
disc accretion rate onto low mass protostar fu ori suddenly increased hundreds of times 85 yr ago and remains elevated to this day. we show that the sum of historic and recent observations challenges existing fu ori models. we build a theory of a new process, extreme evaporation (ee) of young gas giant planets in discs with midplane temperatures of ≳ 30 000 k. such temperatures are reached in the inner 0.1 au during thermal instability bursts. in our 1d time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. we use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. we show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a ~o(104) yr with radius of ~10rj. we show that their ee rates are $\gtrsim 10^{-5} {\rm {\rm m}_{\odot }}$ yr-1; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for ~o(100) yr. we find that a ~6 jupiter mass planet evaporating in a disc fed at a time-averaged rate of $\sim 10^{-6} {\rm {\rm m}_{\odot }}$ yr-1 appears to explain all that we currently know about fu ori accretion outburst. more massive planets and/or planets in older less massive discs do not experience ee process. future fuor modelling may constrain planet internal structure and evolution of the earliest discs. | extreme evaporation of planets in hot thermally unstable protoplanetary discs: the case of fu ori |
we present results of our analysis of up to 15 yr of photometric data from eight am cvn systems with orbital periods between 22.5 and 26.8 min. our data have been collected from the goto, ztf, pan-starrs, asas-sn, and catalina all-sky surveys and amateur observations collated by the aavso. we find evidence that these interacting ultracompact binaries show a similar diversity of long-term optical properties as the hydrogen accreting dwarf novae. we found that am cvn systems in the previously identified accretion disc instability region are not a homogenous group. various members of the analysed sample exhibit behaviour reminiscent of z cam systems with long superoutbursts (sos) and standstills, su uma systems with regular, shorter sos, and nova-like systems that appear only in a high state. the addition of tess full frame images of one of these systems, kl dra, reveals the first evidence for normal outbursts appearing as a precursor to sos in an am cvn system. our results will inform theoretical modelling of the outbursts of hydrogen deficient systems. | evidence that short-period am cvn systems are diverse in outburst behaviour |
context. the observed signatures of winds from x-ray binaries are broadly consistent with thermal winds, which are driven by x-ray irradiation of the outer accretion disc. thermal winds produce mass outflow rates that can exceed the accretion rate in the disc.aims: we aim to study the impact of thermal wind mass loss on the stability and lightcurves of black hole x-ray binaries subject to the thermal-viscous instability driving their outbursts. strong mass loss could stop outbursts early, as proposed for the 2015 outburst of v404 cyg.methods: we used an analytical model for thermal (compton) wind mass loss as a function of radius, x-ray spectrum, and luminosity that was calibrated against numerical simulations. we also estimated the fraction of the x-rays, emitted close to the compact object, that are scattered back to the outer disc in the wind. scattering in the thermal wind couples irradiation to the disc size and inner mass accretion rate. the disc evolution equations were modified to include this wind mass loss and the varying irradiation fraction.results: scattering in the strong wind expected of long porb systems enhances the irradiation heating of the outer disc, keeping it stable against the thermal-viscous instability. this accounts very well for the existence of persistently bright systems with large discs, such as cyg x-2, 1e 1740.7-2942, or grs 1758-258. mass loss from the thermal wind shortens the outburst, as expected, but it is insufficient in explaining the rapid decay timescale of black-hole x-ray binary outbursts. however, including the wind-related varying irradiation fraction produces lightcurves with plateaus in long porb systems like gro j1655-40. plateau lightcurves may be a dynamical signature of enhanced irradiation due to scattering in thermal winds.conclusions: mass loss due to thermal winds is not a major driver for the outburst dynamics up to luminosities of 0.1 - 0.2 ledd. higher luminosities may produce stronger mass loss but studying them is complicated since the wind becomes opaque. magnetic winds, which extract angular momentum with little mass loss, seem more promising to explain the fast decay timescales generically seen in black-hole x-ray binaries. thermal winds can play an important role in the outburst dynamics through the varying irradiation heating. this may be evidenced by relating changes in wind properties, x-ray spectra, or luminosity with changes in the optical emission that traces the outer disc. simulations should enable more accurate estimates of the dependence of the irradiation onto the disc as a function of irradiation spectrum, radius, and disc wind properties. | the impact of thermal winds on the outburst lightcurves of black hole x-ray binaries |
we present a new zoom-in hydrodynamical simulation, `erisbh', which features the same initial conditions, resolution, and sub-grid physics as the close milky way-analogue `eris' (guedes et al. 2011), but it also includes prescriptions for the formation, growth and feedback of supermassive black holes. this enables a detailed study of black hole evolution and the impact of active galactic nuclei (agn) feedback in a late-type galaxy. at z = 0, the main galaxy of erisbh hosts a central black hole of 2.6 × 106 m⊙, which correlates to the bulge mass and the galaxy's central velocity dispersion similarly to what is observed in the milky way and in pseudobulges. during its evolution, the black hole grows mostly through mergers with black holes brought in by accreted satellite galaxies and very little by gas accretion (due to the modest amount of gas that reaches the central regions). agn feedback is weak and it affects only the central 1-2 kpc. yet, it limits the growth of the bulge, which results in a rotation curve that, in the inner ∼ 10 kpc, is flatter than that of eris. we find that erisbh is more prone to instabilities than eris, due to its smaller bulge and larger disc. at z ∼ 0.3, an initially small bar grows to be of a few disc scalelengths in size. the formation of the bar causes a small burst of star formation in the inner few hundred pc, provides new gas to the central black hole and causes the bulge to have a boxy/peanut morphology by z = 0. | black hole starvation and bulge evolution in a milky way-like galaxy |
the origin of the spins of giant planets is an open question in astrophysics. as planets and stars accrete from discs, if the specific angular momentum accreted corresponds to that of a keplerian orbit at the surface of the object, it is possible for planets and stars to be spun-up to near-break-up speeds. however, accretion cannot proceed on to planets and stars in the same way that accretion proceeds through the disc. for example, the magneto-rotational instability cannot operate in the region between the nearly keplerian disc and more slowly rotating surface because of the sign of the angular velocity gradient. through this boundary layer where the angular velocity sharply changes, mass and angular momentum transport is thought to be driven by acoustic waves generated by global supersonic shear instabilities and vortices. we present the first study of this mechanism for angular momentum transport around rotating stars and planets using 2d vertically integrated moving-mesh simulations of ideal hydrodynamics. we find that above rotation rates of ~0.4-0.6 times the keplerian rate at the surface the rate at which angular momentum is transported inwards through the boundary layer by waves decreases by ~1-3 orders of magnitude depending on the gas sound speed. we also find that the accretion rate through the boundary layer decreases commensurately and becomes less variable for faster rotating objects. our results provide a purely hydrodynamic mechanism for limiting the spins of accreting planets and stars to factors of a few less than the break-up speed. | on the terminal spins of accreting stars and planets: boundary layers |
gravitationally unstable accretion disks emerge in a variety of astrophysical contexts—giant planet formation, fu orioni outbursts, feeding of active galactic nuclei, and the origin of pop iii stars. when a gravitationally unstable disk is unable to cool rapidly, it settles into a quasi-stationary, fluctuating gravitoturbulent state, in which its toomre q remains close to a constant value {{q}0}∼ 1. here we develop an analytical formalism describing the evolution of such a disk, which is based on the assumptions of q={{q}0} and local thermal equilibrium. our approach works in the presence of additional sources of angular momentum transport (e.g., mri), as well as external irradiation. thermal balance dictates a unique value of the gravitoturbulent stress {{α }gt} driving disk evolution, which is a function of the local surface density and angular frequency. we compare this approach with other commonly used gravitoturbulent viscosity prescriptions, which specify the explicit dependence of stress {{α }gt} on toomre q in an ad hoc fashion, and identify the ones that provide consistent results. we nevertheless argue that our q={{q}0} approach is more flexible, robust, and straightforward and should be given preference in applications. we illustrate this with a couple of analytical calculations—locations of the snow line and of the outer edge of the dead zone in a gravitoturbulent protoplanetary disk—which clearly show the simplicity and versatility of the q={{q}0} approach. | viscosity prescription for gravitationally unstable accretion disks |
the existence of giant planets on wide orbits ($\stackrel{\gt }{_\sim }100\rm ~au$) challenge planet formation theories; the core accretion scenario has difficulty in forming them, whereas the disc instability model forms an overabundance of them that is not seen observations. we perform n-body simulations investigating the effect of close stellar encounters (≤1200 au) on systems hosting wide-orbit giant planets and the extent at which such interactions may disrupt the initial wide-orbit planet population. we find that the effect of an interaction on the orbit of a planet is stronger for high-mass, low-velocity perturbers, as expected. we find that due to just a single encounter there is a $\sim 17~{{\ \rm per\ cent}}$ chance that the wide-orbit giant planet is liberated in the field, a $\sim 10~{{\ \rm per\ cent}}$ chance it is scattered significantly outwards, and a $\sim 6~{{\ \rm per\ cent}}$ chance it is significantly scattered inwards. moreover, there is a $\sim 21~{{\ \rm per\ cent}}$ chance that its eccentricity is excited to e > 0.1, making it more prone to disruption in subsequent encounters. the results strongly suggest that the effect of even a single stellar encounter is significant in disrupting the primordial wide-orbit giant planet population; in reality the effect will be even more prominent, as in a young star-forming region more such interactions are expected to occur. we conclude that the low occurrence rate of wide-orbit planets revealed by observational surveys does not exclude the possibility that such planetary systems are initially abundant, and therefore the disc-instability model may be a plausible scenario for their formation. | on the survivability of a population of gas giant planets on wide orbits |
context. angular momentum transport in accretion discs is often believed to be due to magnetohydrodynamic turbulence mediated by the magnetorotational instability (mri). despite an abundant literature on the mri, the parameters governing the saturation amplitude of the turbulence are poorly understood and the existence of an asymptotic behaviour in the ohmic diffusion regime has not been clearly established.aims: we investigate the properties of the turbulent state in the small magnetic prandtl number limit. since this is extremely computationally expensive, we also study the relevance and range of applicability of the most common subgrid scale models for this problem.methods: unstratified shearing box simulations are performed both in the compressible and incompressible limits, with a resolution up to 800 cells per disc scale height. this is the highest resolution ever attained for a simulation of mri turbulence. different magnetic field geometry and a wide range of dimensionless dissipative coefficients are considered. we also systematically investigate the relevance of using large eddy simulations (les) in place of direct numerical simulations.results: in the presence of a mean magnetic field threading the domain, angular momentum transport converges to a finite value in the small pm limit. when the mean vertical field amplitude is such that β (the ratio between the thermal and magnetic pressure) equals 103, we find α ~ 3.2 × 10-2 when pm approaches zero. in the case of a mean toroidal field for which β = 100, we find α ~ 1.8 × 10-2 in the same limit. implicit les and the chollet-lesieur closure model both reproduce these results for the α parameter and the power spectra. a reduction in computational cost by a factor of at least 16 (and up to 256) is achieved when using such methods.conclusions: mri turbulence operates efficiently in the small pm limit provided there is a mean magnetic field. implicit les offers a practical and efficient means of investigation of this regime but should be used with care, particularly in the case of a vertical field. the chollet-lesieur closure model is perfectly suited for simulations done with a spectral code. | angular momentum transport and large eddy simulations in magnetorotational turbulence: the small pm limit |
low-frequency quasi-periodic oscillations, or lfqpos, are ubiquitous in black hole x-ray binaries and provide strong constraints on the accretion-ejection processes. although several models have been proposed, none has been proven to reproduce all observational constraints, and no consensus has emerged so far. we make the conjecture that disks in binaries are threaded by a large-scale vertical magnetic field that splits it into two radial zones. in the inner jet-emitting disk (jed), a near equipartition field allows driving powerful self-collimated jets, while beyond a transition radius, the disk magnetization is too low and a standard accretion disk (sad) is settled. in a series of papers, this hybrid jed-sad disk configuration has been shown to successfully reproduce most multiwavelength (radio and x-rays) observations, as well as the concurrence with the lfqpos for the archetypal source gx 339-4. we first analyze the main qpo scenarios provided in the literature: (1) a specific process occurring at the transition radius, (2) the accretion-ejection instability, and (3) the solid-body lense-thirring disk precession. we recall their main assumptions and shed light on some severe theoretical issues that question the capability of reproducing lfqpos. we then argue that none of these models can be operating under jed-sad physical conditions. we finally propose an alternative scenario according to which lfqpos are the disk response to an instability triggered in the jets near a magnetic recollimation zone. this situation can account for most of the type c qpo phenomenology and is consistent with the global behavior of black hole binaries. this nondestructive jet instability remains to be calculated, however. if this instability is numerically confirmed, then it might also naturally account for the jet wobbling phenomenology seen in various accreting sources such as compact objets and young forming stars. | are low-frequency quasi-periodic oscillations in accretion flows the disk response to jet instability? |
turbulent radiation flow is ubiquitous in many physical systems where light–matter interaction becomes relevant. photon bubble instabilities, in particular, have been identified as a possible source of turbulent radiation transport in astrophysical objects such as massive stars and black hole accretion disks. here, we report on the experimental observation of a photon bubble instability in cold atomic gases, in the presence of multiple scattering of light. two different regimes are identified, namely, the growth and formation of quasi-static structures of depleted atom density and increased photon number, akin to photon bubbles in astrophysical objects, and the destabilisation of these structures in a second regime of photon bubble turbulence. a two-fluid theory is developed to model the coupled atom–photon gas and to describe both the saturation of the instability in the regime of quasi-static bubbles and the low-frequency turbulent phase associated with the growth and collapse of photon bubbles inside the atomic sample. we also employ statistical dimensionality reduction techniques to describe the low-dimensional nature of the turbulent regime. the experimental results reported here, along with the theoretical model we have developed, may shed light on analogue photon bubble instabilities in astrophysical scenarios. our findings are consistent with recent analyses based on spatially resolved pump–probe measurements. | quasi-static and dynamic photon bubbles in cold atom clouds |
we use the galform semi-analytical model of galaxy formation and the planck-millennium simulation to investigate the origins of stellar mass in galaxies and their spheroids. we compare the importance of mergers and disc instabilities, as well as the starbursts that they trigger. we find that the fraction of galaxy stellar mass formed ex situ (i.e. through mergers; fex) increases sharply from m* = 1011 m⊙ upwards, reaching 80 per cent at m* = 1011.3 m⊙. the massive end of the fex-m* relation does not evolve with redshift, in disagreement with other models. for low-mass galaxies we find larger ex situ contributions at z = 0 than in other models (7-12 per cent), with a decrease towards higher redshifts. major mergers contribute roughly half of the ex situ mass, with minor mergers and smooth accretion of satellites both accounting for ≈25 per cent, almost independent of stellar mass and redshift. mergers dominate in building up high-mass (m*, sph > 1011 m⊙) and low-mass (m*, sph < 108.5 m⊙) spheroids. disc instabilities and their associated starbursts dominate for intermediate-mass spheroids (108.5 < m*, sph < 1011 m⊙) at z = 0. the mass regime where pseudo-bulges dominate is in agreement with observed pseudo-bulge fractions, but the peak value in the pseudo-bulge fraction predicted by galform is likely too high. starbursts induced by disc instabilities are the dominant channel for spheroid growth at all redshifts, while merger-induced starbursts are relatively negligible, except at very high redshifts (z > 5). | the buildup of galaxies and their spheroids: the contributions of mergers, disc instabilities, and star formation |
it is likely that young protostellar disks undergo a self-gravitating phase. such systems are characterized by the presence of a spiral pattern that can be either in a quasi-steady state or in a nonlinear unstable condition. this spiral wave affects both the gas dynamics and kinematics, resulting in deviations from the keplerian rotation. recently, a lot of attention has been devoted to kinematic studies of planet-forming environments, and we are now able to measure even small perturbations of velocity field (≲1% of the keplerian speed) thanks to high spatial and spectral resolution observations of protostellar disks. in this work, we investigate the kinematic signatures of gravitational instability: we perform an analytical study of the linear response of a self-gravitating disk to a spiral-like perturbation, focusing our attention on the velocity field perturbations. we show that unstable disks have clear kinematic imprints into the gas component across the entire disk extent, due to the gi spiral wave perturbation, resulting in deviations from keplerian rotation. the shape of these signatures depends on several parameters, but they are significantly affected by the cooling factor: by detecting these features, we can put constraints on protoplanetary disk cooling. | investigating protoplanetary disk cooling through kinematics: analytical gi wiggle |
local models of gaseous accretion discs have been successfully employed for decades to describe an assortment of small-scale phenomena, from instabilities and turbulence, to dust dynamics and planet formation. for the most part, they have been derived in a physically motivated but essentially ad hoc fashion, with some of the mathematical assumptions never made explicit nor checked for consistency. this approach is susceptible to error, and it is easy to derive local models that support spurious instabilities or fail to conserve key quantities. in this paper we present rigorous derivations, based on an asympototic ordering, and formulate a hierarchy of local models (incompressible, boussinesq and compressible), making clear which is best suited for a particular flow or phenomenon, while spelling out explicitly the assumptions and approximations of each. we also discuss the merits of the anelastic approximation, emphasizing that anelastic systems struggle to conserve energy unless strong restrictions are imposed on the flow. the problems encountered by the anelastic approximation are exacerbated by the disc's differential rotation, but also attend non-rotating systems such as stellar interiors. we conclude with a defence of local models and their continued utility in astrophysical research. | local models of astrophysical discs |
x-ray transients, such as accreting neutron stars, periodically undergo outbursts, thought to be caused by a thermal-viscous instability in the accretion disc. usually outbursts of accreting neutron stars are identified when the accretion disc has undergone an instability, and the persistent x-ray flux has risen to a threshold detectable by all sky monitors on x-ray space observatories. here, we present the earliest known combined optical, uv, and x-ray monitoring observations of the outburst onset of an accreting neutron star low-mass x-ray binary (lmxb) system. we observed a significant, continuing increase in the optical i'-band magnitude starting on july 25, 12 d before the first x-ray detection with swift/xrt and nicer (august 6), during the onset of the 2019 outburst of sax j1808.4-3658. we also observed a 4 d optical to x-ray rise delay, and a 2 d uv to x-ray delay, at the onset of the outburst. we present the multiwavelength observations that were obtained, discussing the theory of outbursts in x-ray transients, including the disc instability model, and the implications of the delay. this work is an important confirmation of the delay in optical to x-ray emission during the onset of outbursts in lmxbs, which has only previously been measured with less sensitive all sky monitors. we find observational evidence that the outburst is triggered by ionization of hydrogen in the disc. | enhanced optical activity 12 d before x-ray activity, and a 4 d x-ray delay during outburst rise, in a low-mass x-ray binary |
negative superhumps (nshs) are signals a few percent shorter than the orbital period of a binary star and are considered to originate from the reverse precession of the tilted disk. based on tess photometry, we find nine new cataclysmic variable stars (cvs) with nshs. three (asas j1420, tz per, and v392 hya) of these stars similar to ah her still have nshs during dwarf nova outbursts, and the nsh amplitude varies with the outburst. the variation in the radius of the accretion disk partially explains this phenomenon. however, it does not explain the rebound of the nsh amplitude after the peak of the outburst and the fact that the nsh amplitude of the quiescence is sometimes not the largest, and it is necessary to combine the disk instability model (dim) and add other ingredients. therefore, we suggest that the variation of nsh amplitude with outburst can be an essential basis for studying the origin of nshs and improving the dim. the six ( asassn-v j1137, asassn-v j0611, 2mass j0715, lamost j0925, asassn-17qj, and ztf18acakuxo) remaining stars have been poorly studied, and for the first time we determine their orbital periods, nshs and superorbital signal (sor) periods. the nsh periods and amplitudes of asassn-v j1137 and asassn-17qj vary with the sor, and based on the comparison of the observations with the theory, we suggest that a single change in tilted disk angle does not explain the observations of the sor and that other ingredients need to be considered as well. | nine new cataclysmic variable stars with negative superhumps |
ah her is a z cam-type dwarf nova with an orbital period of ~0.258 days. dwarf nova oscillations and long-period dwarf nova oscillations have been detected, but no quasiperiodic oscillations (qpos) and negative superhumps (nshs) have been found. we investigated the association between nshs, qpos, and outbursts of ah her based on transiting exoplanet survey satellite photometry. we find for the first time nshs with a period of 0.24497(1) days in ah her, and trace the variation in the amplitude and period of nshs with the outburst. the amplitude of the nshs is the most significant at quiescence, weakening as the outburst rises, becoming undetectable at the top, rebounding and weakening at the plateau, and strengthening again as the outburst declines. the variation in the accretion disk radius can explain the nsh amplitude variation except for the plateau, so we suggest that the relationship between the nsh amplitude and outburst can be used as a window to study the accretion disk instability and the origin of nshs. in addition, we find periodic variations in the amplitude, maxima, and shape of the nshs ranging from 2.33(2) to 2.68(5) days, which may be related to the precession of the tilted disk. finally, we find qpos at the top of ah her's long outburst with ~2800 s similar to those of hs 2325+8205, suggesting that the presence of qpos at the top of z cam's long outburst may be a general phenomenon. | evolution of negative superhumps, quasiperiodic oscillations, and outbursts in the z cam-type dwarf nova ah her |
context. the physics of early stellar evolution (e.g. accretion processes) is often not properly included in the calculations of pre-main-sequence models, leading to insufficient model grids and hence systematic errors in the results.aims: we aim to investigate current and improved approaches for the asteroseismic modelling of pre-main-sequence δ scuti stars.methods: we calculated an extensive grid of pre-main-sequence models including the early accretion phase and used the resulting equilibrium models as input to calculate theoretical frequency spectra. these spectra were used to investigate different approaches in modelling echelle diagrams to find the most reliable methods. by applying petersen diagrams, we present a simple algorithm to extract echelle diagrams from observed pulsation frequencies.results: we show that model grids with insufficient input physics and imperfect modelling approaches lead to underestimated uncertainties and systematic errors in the extracted stellar parameters. our re-discussion of hd 139614 leads to different stellar parameters than the ones derived by murphy et al. (2021, mnras, 502, 1633). we performed a model comparison between this previous investigation and our results by applying the akaike and bayesian information criteria. while the results with regard to our ten-dimensional model are inconclusive, they show (very) strong evidence of a six-dimensional model with fixed accretion parameters (leading to almost identical stellar parameters to those of the ten-dimensional model) to be preferred over the model applied by murphy et al. (2021, mnras, 502, 1633). in general, our modelling approach can provide narrow constraints on the stellar parameters (i.e. δr ∼ 0.05 r⊙, δlog g ≲ 0.01, and δm⋆ ∼ 0.1 m⊙).conclusions: the extensively tested modelling approaches and automatic extraction of echelle diagrams should allow us to study many more pre-main-sequence δ scuti stars in the future and lead to reliable stellar parameters. | pulsational instability of pre-main-sequence models from accreting protostars. ii. modelling echelle diagrams of δ scuti stars without rotational splitting |
theories of planet formation predict that low-mass stars should rarely host exoplanets with masses exceeding that of neptune. we used radial velocity observations to detect a neptune-mass exoplanet orbiting lhs 3154, a star that is nine times less massive than the sun. the exoplanet’s orbital period is 3.7 days, and its minimum mass is 13.2 earth masses. we used simulations to show that the high planet-to-star mass ratio (>3.5 × 10−3) is not an expected outcome of either the core accretion or gravitational instability theories of planet formation. in the core-accretion simulations, we show that close-in neptune-mass planets are only formed if the dust mass of the protoplanetary disk is an order of magnitude greater than typically observed around very low-mass stars. | a neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models |
we revisit the conditions present in supermassive discs (smds) formed by the merger of gas-rich, metal-enriched galaxies at redshift z ~ 10. we find that smds naturally form hydrostatic cores which go through a rapidly accreting supermassive star phase, before directly collapsing into massive black holes via the general relativistic instability. the growth and collapse of the cores occurs within ~5 × 105 yr from the formation of the smd, producing bright electromagnetic, neutrino and gravitational wave transients with a typical duration of a few minutes and, respectively, a typical flux and a typical strain amplitude at earth of ~10-8 erg s-1 cm-2 and ~4 × 10-21. we provide a simple fitting formula for the resulting black hole masses, which range from a few 106 to 108 m⊙ depending on the initial smd configuration. crucially, our analysis does not require any specific assumption on the thermal properties of the gas, nor on the angular momentum loss mechanisms within the smd. led by these findings, we argue that the merger-driven scenario provides a robust pathway for the rapid formation of supermassive black holes at z > 6. it provides an explanation for the origin of the brightest and oldest quasars without the need of a sustained growth phase from a much smaller seed. its smoking gun signatures can be tested directly via multimessenger observations. | direct collapse of exceptionally heavy black holes in the merger-driven scenario |
the main problems of nonvacuum numerical relativity, compact binary mergers and stellar collapse, involve hydromagnetic instabilities and turbulent flows, so that kinetic energy at small scales leads to mean effects at large scale that drive the secular evolution. notable among these effects is momentum transport. we investigate two models of this transport effect, a relativistic navier-stokes system and a turbulent mean stress model, that are similar to all of the prescriptions that have been attempted to date for treating subgrid effects on binary neutron star mergers and their aftermath. our investigation involves both stability analysis and numerical experimentation on star and disk systems. we also begin the investigation of the effects of particle and heat transport on postmerger simulations. we find that correct handling of turbulent heating is crucial for avoiding unphysical instabilities. given such appropriate handling, the evolution of a differentially rotating star and the accretion rate of a disk are reassuringly insensitive to the choice of prescription. however, disk outflows can be sensitive to the choice of method, even for the same effective viscous strength. we also consider the effects of eddy diffusion in the evolution of an accretion disk and show that it can interestingly affect the composition of outflows. | comparison of momentum transport models for numerical relativity |
context. it is now widely accepted that most ultraluminous x-ray sources (ulxs) are binary systems whose large (above 1039 erg s-1) apparent luminosities are explained by super-eddington accretion onto a stellar-mass compact object. many of the ulxs, especially those containing magnetized neutron stars, are highly variable; some exhibit transient behaviour. large luminosities might imply large accretion discs that could be therefore prone to the thermal-viscous instability known to drive outbursts of dwarf novae and low-mass x-ray binary transient sources.aims: the aim of this paper is to extend and generalize the x-ray transient disc-instability model to the case of large (outer radius larger than 1012 cm) accretion discs and apply it to the description of systems with super-eddington accretion rates at outburst and, in some cases, super-eddington mass transfer rates.methods: we have used our disc-instability-model code to calculate the time evolution of the accretion disc and the outburst properties.results: we show that, provided that self-irradiation of the accretion disc is efficient even when the accretion rate exceeds the eddington value, possibly due to scattering back of the x-ray flux emitted by the central parts of the disc on the outer portions of the disc, heating fronts can reach the disc's outer edge generating high accretion rates. we also provide analytical approximations for the observable properties of the outbursts. we have successfully reproduced the observed properties of galactic transients with large discs, such as v404 cyg, as well as some ulxs such as m51 xt-1. our model can reproduce the peak luminosity and decay time of eso 243-39 hlx-1 outbursts if the accretor is a neutron star.conclusions: observational tests of our predicted relations between the outburst duration and decay time with peak luminosity would be most welcome. | models of ultraluminous x-ray transient sources |
we propose a framework for understanding the fragmentation criterion for self-gravitating discs which, in contrast to studies that emphasize the `gravoturbulent' nature of such discs, instead focuses on the properties of their quasi-regular spiral structures. within this framework there are two evolutionary paths to fragmentation: (i) collapse on the free-fall time, which requires that the ratio of cooling time to dynamical time (β) < 3 and (ii) quasi-static collapse on the cooling time at a rate that is sufficiently fast that fragments are compact enough to withstand disruption when they encounter spiral features in the disc. we perform 2d grid simulations which demonstrate numerically converged fragmentation at β < 3 (in good agreement with paardekooper, baruteau & meru, and others) and argue that this is a consequence of the fact that such simulations smooth the gravitational force on the scale h, the scaleheight of the disc. such simulations thus only allow fragmentation via route (i) above since they suppress the quasi-static contraction of fragments on scales <h; the inability of fragments to contract to significantly smaller scales then renders them susceptible to disruption at the next spiral arm encounter. on the other hand, 3d simulations indeed show fragmentation at higher β via route (ii). we derive an analytic prediction of fragmentation by route (ii) when β ≲ 12, based on the requirement that fragments must contract sufficiently to withstand disruption by spiral arms. we also discuss the necessary numerical requirements on both grid-based and smoothed particle hydrodynamics codes if they are to model fragmentation via route (ii). | dependence of fragmentation in self-gravitating accretion discs on small-scale structure |
context. the hierarchical process of star formation has so far mostly been studied on scales from thousands of au to parsecs, but the smaller sub-1000 au scales of high-mass star formation are still largely unexplored in the submillimeter regime.aims: we aim to resolve the dust and gas emission at the highest spatial resolution to study the physical properties of the densest structures during high-mass star formation.methods: we observed the high-mass hot core region g351.77-0.54 with the atacama large millimeter array with baselines extending out to more than 16 km. this allowed us to dissect the region at sub-50 au spatial scales.results: at a spatial resolution of 18/40 au (depending on the distance), we identify twelve sub-structures within the inner few thousand au of the region. the brightness temperatures are high, reaching values greater 1000 k, signposting high optical depth toward the peak positions. core separations vary between sub-100 au to several 100 and 1000 au. the core separations and approximate masses are largely consistent with thermal jeans fragmentation of a dense gas core. due to the high continuum optical depth, most spectral lines are seen in absorption. however, a few exceptional emission lines are found that most likely stem from transitions with excitation conditions above 1000 k. toward the main continuum source, these emission lines exhibit a velocity gradient across scales of 100-200 au aligned with the molecular outflow and perpendicular to the previously inferred disk orientation. while we cannot exclude that these observational features stem from an inner hot accretion disk, the alignment with the outflow rather suggests that it stems from the inner jet and outflow region. the highest-velocity features are found toward the peak position, and no hubble-like velocity structure can be identified. therefore, these data are consistent with steady-state turbulent entrainment of the hot molecular gas via kelvin-helmholtz instabilities at the interface between the jet and the outflow.conclusions: resolving this high-mass star-forming region at sub-50 au scales indicates that the hierarchical fragmentation process in the framework of thermal jeans fragmentation can continue down to the smallest accessible spatial scales. velocity gradients on these small scales have to be treated cautiously and do not necessarily stem from disks, but may be better explained with outflow emission. studying these small scales is very powerful, but covering all spatial scales and deriving a global picture from large to small scales are the next steps to investigate. the data are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/621/a122 | high-mass star formation at sub-50 au scales |
the local properties of turbulence driven by the magnetorotational instability (mri) in rotating, shearing flows are studied in the framework of a shearing-box model. based on numerical simulations, we propose that the mri-driven turbulence comprises two components: the large-scale shear-aligned strong magnetic field and the small-scale fluctuations resembling magnetohydrodynamic (mhd) turbulence. the energy spectrum of the large-scale component is close to k-2, whereas the spectrum of the small-scale component agrees with the spectrum of strong mhd turbulence k-3/2. while the spectrum of the fluctuations is universal, the outer-scale characteristics of the turbulence are not; they depend on the parameters of the system, such as the net magnetic flux. however, there is remarkable universality among the allowed turbulent states - their intensity v0 and their outer scale λ0 satisfy the balance condition v0/λ0 ∼ dω/dln r, where dω/dln r is the local orbital shearing rate of the flow. finally, we find no sustained dynamo action in the pm = 1 zero net-flux case for reynolds numbers as high as 45 000, casting doubts on the existence of an mri dynamo in the pm ≤ 1 regime. | on the nature of magnetic turbulence in rotating, shearing flows |
we carry out three dimensional radiation hydrodynamical simulations of gravitationally unstable discs to explore the movement of mass in a disc following its initial fragmentation. we find that the radial velocity of the gas in some parts of the disc increases by up to a factor of ≈10 after the disc fragments, compared to before. while the movement of mass occurs in both the inward and outward directions, the inwards movement can cause the inner spirals of a self-gravitating disc to become sufficiently dense such that they can potentially fragment. this suggests that the dynamical behaviour of fragmented discs may cause subsequent fragmentation to occur at smaller radii than initially expected, but only after an initial fragment has formed in the outer disc. | triggered fragmentation in self-gravitating discs: forming fragments at small radii |
aims: it is well known that low-mass young stellar objects (lmysos) gain a significant portion of their final mass through episodes of very rapid accretion, with mass accretion rates up to ṁ∗ ∼ 10−4 m⊙ yr−1. recent observations of high-mass young stellar objects (hmysos) with masses m∗ ≳ 10 m⊙ uncovered outbursts with accretion rates exceeding ṁ∗ ∼ 10−3 m⊙ yr−1. here, we examine which scenarios proposed in the literature so far to explain accretion bursts of lmysos can also apply to the episodic accretion in hmysos.methods: we utilise 1d time-dependent models of protoplanetary discs around hmysos to study burst properties.results: we find that discs around hmysos are much hotter than those around their low-mass cousins. as a result, a much more extended region of the disc is prone to the thermal hydrogen ionisation and magnetorotational activation instabilities. the former, in particular, is found to be ubiquitous in a very wide range of accretion rates and disc viscosity parameters. the outbursts triggered by these instabilities, however, always have too low of an ṁ∗ and are one to several orders of magnitude too long compared to those observed from hmysos to date. on the other hand, bursts generated by tidal disruptions of gaseous giant planets formed by the gravitational instability of the protoplanetary discs yield properties commensurate with observations, provided that the clumps are in the post-collapse configuration with planet radius rp ≳ 10 jupiter radii. furthermore, if observed bursts are caused by disc ionisation instabilities, then they should be periodic phenomena with the duration of the quiescent phase comparable to that of the bursts. this may yield potentially observable burst periodicity signatures in the jets, the outer disc, or the surrounding diffuse material of massive hmysos. bursts produced by disruptions of planets or more massive objects are not expected to be periodic phenomena, although multiple bursts per protostar are possible.conclusions: observations and modelling of episodic accretion bursts across a wide range of young stellar object (yso) masses is a new promising avenue to break the degeneracy between models of episodic accretion in ysos. | accretion bursts in high-mass protostars: a new test bed for models of episodic accretion |
we propose that the accretion discs fueling active galactic nuclei (agn) are supported vertically against gravity by a strong toroidal (φ-direction) magnetic field that develops naturally as the result of an accretion disc dynamo. the magnetic pressure elevates most of the gas carrying the accretion flow at r to large heights z ≳ 0.1r and low densities, while leaving a thin dense layer containing most of the mass - but contributing very little accretion - around the equator. we show that such a disc model leads naturally to the formation of a broad emission-line region through thermal instability. extrapolating to larger radii, we demonstrate that local gravitational instability and associated star formation are strongly suppressed compared to standard disc models for agn, although star formation in the equatorial zone is predicted for sufficiently high mass supply rates. this new class of accretion disc models thus appears capable of resolving two longstanding puzzles in the theory of agn fueling: the formation of broad emission-line regions and the suppression of fragmentation thought to inhibit accretion at the required rates. we show that the disc of stars that formed in the galactic center a few million years ago could have resulted from an episode of magnetically elevated accretion at ≳ 0.1 of the eddington limit. | magnetically elevated accretion discs in active galactic nuclei: broad emission-line regions and associated star formation |
binary black holes in which both spins are aligned with the binary's 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 [d. gerosa et al., phys. rev. lett. 115, 141102 (2015), 10.1103/physrevlett.115.141102]. 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 toward precise endpoints. we then present an analytic scheme to locate these final configurations and confirm them with numerical integrations. namely, unstable up-down binaries approach mergers with the two spins coaligned 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. as a by-product, we obtain new generic results on binary black hole spin-orbit resonances at 2nd post-newtonian order. 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 counteralignment, as might be the case for stellar-mass black holes embedded in the accretion disk of a supermassive black hole. | endpoint of the up-down instability in precessing binary black holes |
young protoplanetary discs and the outer radii of active galactic nuclei may be subject to gravitational instability and, as a consequence, fall into a 'gravitoturbulent' state. while in this state, appreciable angular momentum can be transported; alternatively, the gas may collapse into bound clumps, the progenitors of planets or stars. in this paper, we numerically characterize the properties of 3d gravitoturbulence, focusing especially on its dependence on numerical parameters (resolution, domain size) and its excitation of small-scale dynamics. via a survey of vertically stratified shearing-box simulations with pluto and rodeo, we find (a) evidence that certain gravitoturbulent properties are independent of horizontal box size only when the box is larger than ≃40h, where h is the scaleheight, (b) at high resolution, small-scale isotropic turbulence appears off the mid-plane around z ≃ 0.5-1h and (c) this small-scale dynamics results from a parametric instability, involving the coupling of inertial waves with a large-scale axisymmetric epicyclic mode. this mode oscillates at a frequency close to ω and is naturally excited by gravitoturbulence via a non-linear process to be determined. the small-scale turbulence we uncover has potential implications for a wide range of disc physics, e.g. turbulent saturation levels, fragmentation, turbulent mixing and dust settling. | gravitoturbulence and the excitation of small-scale parametric instability in astrophysical discs |
context. the hard-to-soft state transition of the outbursts in x-ray binaries (xrbs) is triggered by the rising of the mass accretion rate as a result of the disk instability. the hard x-ray transition luminosity is found to be tightly correlated to the soft x-ray peak luminosity in the soft state, the physical origin of which is still a mystery.aims: in order to explain the observed correlation between the hard x-ray transition luminosity and the soft x-ray peak luminosity in the soft state, we construct a magnetic disk-outflow model for the state transition in xrbs.methods: we assumed that the large-scale magnetic field in the outer thin disk is formed through an inverse cascade of the field generated by the small-scale dynamo, which is then advected by the inner advection-dominated accretion flow (adaf). the advected field accelerates a fraction of the gas in the adaf into the outflows. we calculated the transition luminosity of an adaf that is driven by these magnetic outflows, which vary with the mass accretion rate of the outer disk.results: during the outbursts, the heating front moves inward, and the field strength at the heating front of the outer disk is proportional to the accretion rate of the disk. much angular angular momentum of the inner adaf is carried away by the outflows for a stronger magnetic field, which leads to a high radial velocity of the adaf. this increases the critical mass accretion rate of the adaf with the field strength, and it therefore leads to a correlation between transition luminosity and the peak luminosity in the thermal state. we found that the values of the viscosity parameter α of the neutron star xrbs are systematically higher for those of the black hole (bh) xrbs (α ∼ 0.05−0.15 for bhs, and α ∼ 0.15−0.4 for neutron stars). our model predicts that the transition luminosity may be higher than the peak luminosity provided α is sufficiently high, which is able to explain a substantial fraction of outbursts in bhxrbs that do not reach the thermally dominant accretion state. | magnetic accretion disk-outflow model for the state transition in x-ray binaries |
context. vela x-1 is the prototype of the classical super-giant high mass x-ray binary systems. recent continuous and long monitoring campaigns revealed a large hard x-rays variability amplitude with strong flares and off-states. this activity has been interpreted by invoking clumpy stellar winds and/or magnetic gating mechanisms.aims: we investigate whether the observed behavior may be explained by unstable hydrodynamic flows close to the neutron star instead of by the more exotic phenomena.methods: we used the hydrodynamic code vh-1 to simulate the flow of the stellar wind with high temporal resolution and to compare the predicted accretion rate with the observed light curves.results: the simulation results are similar to the observed variability. off-states are predicted with a duration of 5 to 120 min corresponding to transient low-density bubbles forming around the neutron star. oscillations of the accretion rate with a typical period of ~6800 s are generated in our simulations and observed. they correspond to the complex motion of a bow shock, moving either toward or away from the neutron star. flares are also produced by the simulations up to a level of 1037 erg/s.conclusions: we have qualitatively reproduced the hard x-ray variations observed in vela x-1 with hydrodynamic instabilities predicted by a simple model. more sophisticated phenomena, such as clumpy winds or the magnetic gating mechanism, are not excluded, but are not required either to explain the basic phenomenology. | origin of the x-ray off-states in vela x-1 |
the assembly of the milky way bulge is an old topic in astronomy, one now in a period of renewed and rapid development. that is due to tremendous advances in observations of bulge stars, motivating observations of both local and high-redshift galaxies, and increasingly sophisticated simulations. the dominant scenario for bulge formation is that of the milky way as a nearly pure disk galaxy, with the inner disk having formed a bar and buckled. this can potentially explain virtually all bulge stars with [fe/h] ≳ -1.0, which comprise 95% of the stellar population. the evidence is the incredible success in n-body models of this type in making non-trivial, non-generic predictions, such as the rotation curve and velocity dispersion measured from radial velocities, and the spatial morphologies of the peanut/x-shape and the long bar. the classical bulge scenario, whereby the bulge formed from early dissipative collapse and mergers, remains viable for stars with [fe/h] ≲ -1.0 and potentially a minority of the other stars. a classical bulge is expected from λ-cdm cosmological simulations, can accentuate the properties of an existing bar in a hybrid system, and is most consistent with the bulge abundance trends such as [mg/fe], which are elevated relative to both the thin and thick disks. finally, the clumpy-galaxy scenario is considered, as it is the correct description of most milky way precursors given observations of high-redshift galaxies. simulations predict that these star-forming clumps will sometimes migrate to the centres of galaxies where they may form a bulge, and galaxies often include a bulge clump as well. they will possibly form a bar with properties consistent with those of the milky way, such as the exponential profile and metallicity gradient. given the relative successes of these scenarios, the milky way bulge is plausibly of composite origin, with a classical bulge and/or inner halo numerically dominant for stars with [fe/h] ≲ -1.0, a buckling thick disk for stars with - 1.0 ≲ [fe/h]] ≲ -0.50 perhaps descended from the clumpy-galaxy phase, and a buckling thin disk for stars with [fe/h] ≳ -0.50. overlaps from these scenarios are uncertain throughout. | was the milky way bulge formed from the buckling disk instability, hierarchical collapse, accretion of clumps, or all of the above? |
the ubiquity of earth- to super-earth-sized planets found very close to their host stars has motivated in situ formation models. in particular, inside-out planet formation is a scenario in which planets coalesce sequentially in the disk, at the local gas pressure maximum near the inner boundary of the dead zone. the pressure maximum arises from a decline in viscosity, going from the active innermost disk (where thermal ionization yields high viscosities via the magnetorotational instability [mri]) to the adjacent dead zone (where the mri is quenched). previous studies of the pressure maximum, based on α-disk models, have assumed ad hoc values for the viscosity parameter α in the active zone, ignoring the detailed mri physics. here we explicitly couple the mri criteria to the α-disk equations, to find steady-state solutions for the disk structure. we consider both ohmic and ambipolar resistivities, a range of disk accretion rates (10-10-10-8 m ⊙ yr-1), stellar masses (0.1-1 m ⊙), and fiducial values of the non-mri α-viscosity in the dead zone (α dz = 10-5 to 10-3). we find that (1) a midplane pressure maximum forms radially outside the dead zone inner boundary; (2) hall resistivity dominates near the inner disk midplane, perhaps explaining why close-in planets do not form in ∼50% of systems; (3) x-ray ionization can compete with thermal ionization in the inner disk, because of the low steady-state surface density there; and (4) our inner disks are viscously unstable to surface density perturbations. | inside-out planet formation. v. structure of the inner disk as implied by the mri |
based on the framework of kaniadakis' statistics and its related kinetic theory, the jeans instability for self-gravitational systems in the background of eddington-inspired born-infield (eibi) gravity is revisited. a dispersion relation generalizing the jeans modes is derived by modifying the maxwellian distribution to a family of power law distributions parameterized by the κ parameter. it is established that the κ-deformed kaniadakis distribution has significant effects on the jeans modes of the collisionless eibi-gravitational systems. and as expected, in the limitation κ → 0, the corresponding results for maxwellian case are recovered. the related result in the present work is valuable for the investigations involving the fields of astrophysics such as neutron stars, accretion disks, and relevant plasma physics, etc. *project supported by the national natural science foundation of china (grant nos. 11763006 and 11863004) and the fund from the jiangxi provincial key laboratory of fusion and information control (grant no. 20171bcd40005). | jeans gravitational instability with κ-deformed kaniadakis distribution in eddington-inspired born-infield gravity |
in outburst sources, quasi-periodic oscillation (qpo) frequency is known to evolve in a certain way: in the rising phase, it monotonically goes up until a soft intermediate state is achieved. in the propagating oscillatory shock model, oscillation of the compton cloud is thought to cause qpos. thus, in order to increase qpo frequency, the compton cloud must collapse steadily in the rising phase. in decline phases, the exact opposite should be true. we investigate cause of this evolution of the compton cloud. the same viscosity parameter that increases the keplerian disk rate also moves the inner edge of the keplerian component, thereby reducing the size of the compton cloud and reducing the cooling timescale. we show that cooling of the compton cloud by inverse comptonization is enough for it to collapse sufficiently so as to explain the qpo evolution. in the two-component advective flow configuration of chakrabarti-titarchuk, centrifugal force-induced shock represents the boundary of the compton cloud. we take the rising phase of 2010 outburst of galactic black hole candidate h 1743-322 and find an estimation of variation of the α parameter of the sub-keplerian flow to be monotonically rising from 0.0001 to 0.02, well within the range suggested by magnetorotational instability. we also estimate the inward velocity of the compton cloud to be a few meters per second, which is comparable to what is found in several earlier studies of our group by empirically fitting the shock locations with the time of observations. | is compton cooling sufficient to explain evolution of observed quasi-periodic oscillations in outburst sources? |
we present new narrow-band hα imaging for 24 nearby edge-on galaxies in the continuum halos in nearby galaxies—an evla survey (chang-es). we use the images in conjunction with the wide-field infrared survey explorer 22 μm imaging of the sample to estimate improved star formation rates (sfrs) using the updated recipe from vargas et al. we explore correlations between the updated star formation properties and radio continuum scale heights, scale lengths, and diameters, measured in krause et al. we find a newly discovered correlation between sfr and radio scale height that did not exist using mid-infrared (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 extended ultraviolet disk feature, as traced by the galaxy evolution explorer near-ultraviolet imaging. we theorize that the star formation feature arose due to gravitational instability within gas from an accretion event. new hα images from this work can be found at the chang-es data release website, https://www.queensu.ca/changes. | chang-es. xvii. hα imaging of nearby edge-on galaxies, new sfrs, and an extreme star formation region—data release 2 |
ax j1745.6-2901 is a low-mass x-ray binary with an accreting neutron star, showing clear evidence for highly ionized absorption. strong ionized fe kα and kβ absorption lines are always observed during the soft state, whereas they disappear during the hard state. we computed photoionization stability curves for the hard and the soft states, under different assumptions on the adopted spectral energy distributions and the physical parameters of the plasma. we observe that the ionized absorber always lies on a stable branch of the photoionization stability curve during the soft state, while it becomes unstable during the hard state. this suggests that photoionization instability plays a key role in defining the observable properties of the ionized absorber. the same process might explain the disappearance of the high ionization absorber/wind during the hard state in other accreting neutron stars and black holes. | photoionization instability of the fe k absorbing plasma in the neutron star transient ax j1745.6-2901 |
self-gravitating discs are believed to play an important role in astrophysics in particular regarding the star and planet formation process. in this context, discs subject to an idealized cooling process, characterized by a cooling time-scale β expressed in unit of orbital time-scale, have been extensively studied. we take advantage of the riemann solver and the 3d godunov scheme implemented in the code ramses to perform high-resolution simulations, complementing previous studies that have used smoothed particle hydrodynamics (sph) or 2d grid codes. we observe that the critical value of β for which the disc fragments is consistent with most previous results, and is not well converged with resolution. by studying the probability density function of the fluctuations of the column density (σ-pdf), we argue that there is no strict separation between the fragmented and the unfragmented regimes but rather a smooth transition with the probability of apparition of fragments steadily diminishing as the cooling becomes less effective. we find that the high column density part of the σ-pdf follows a simple power law whose slope turns out to be proportional to β and we propose an explanation based on the balance between cooling and heating through gravitational stress. our explanation suggests that a more efficient cooling requires more heating implying a larger fraction of dense material which, in the absence of characteristic scales, results in a shallower scale-free power law. we propose that the gravitational cascade proceeds in two steps, first the formation of a dense filamentary spiral pattern through a sequence of quasi-static equilibrium triggered by the viscous transport of angular momentum, and second the collapse alongside these filaments that eventually results in the formation of bounded fragments. | a two-step gravitational cascade for the fragmentation of self-gravitating discs |
we investigate the disk formation process in the tng50 simulation, examining the profiles of sfr surface density (σsfr), gas inflow and outflow, and the evolution of the angular momentum of inflowing gas particles. the tng50 galaxies tend to have larger star-forming disks, and they also show larger deviations from exponential profiles in σsfr when compared to real galaxies in the mapping nearby galaxies at apo survey. the stellar surface density of tng50 galaxies show good exponential profiles, which is found to be the result of strong radial migration of stars over time. however, this strong radial migration of stars in the simulation produces flatter age profiles in tng50 disks compared to observed galaxies. the star formation in the simulated galaxies is sustained by a net gas inflow, and this gas inflow is the primary driver for the cosmic evolution of star formation, as expected from simple gas-regulator models of galaxies. there is no evidence for any significant loss of angular momentum for the gas particles after they are accreted on to the galaxy, which may account for the large disk sizes in the tng50 simulation. adding viscous processes to the disks, such as the magnetic stresses from magnetorotational instability proposed by wang & lilly, will likely reduce the sizes of the simulated disks and the tension with the sizes of real galaxies, and this may produce more realistic exponential profiles. | the formation of star-forming disks in the tng50 simulation |
context. to better understand the formation of high-mass stars, it is fundamental to investigate how matter accretes onto young massive stars, how it is ejected, and how all this differs from the low-mass case. the massive protocluster g31.41+0.31 is the ideal target to study all these processes because observations at millimeter and centimeter wavelengths have resolved the emission of the main core into at least four massive dust continuum sources, named a, b, c, and d, within 1″ or 0.018 pc, and have identified signatures of infall and several outflows associated with the core.aims: we study the interplay between infall and outflow in g31.41+0.31, at a spatial resolution of a few 100 au, by investigating their properties and their possible impact on the core.methods: we carried out molecular line observations of typical high-density tracers, such as ch3cn or h2co, and shock and outflow tracers, such as sio, with alma at 1.4 mm that achieved an angular resolution of ~0.''09 (~340 au).results: the observations reveal inverse p cygni profiles in ch3cn and h2co toward the four sources embedded in the main core, suggesting that all of them are undergoing collapse. the infall rates estimated from the redshifted absorption are on the order of ~10−2 m⊙ yr−1. the individual infall rates imply that the accretion timescale of the main core is an order of magnitude smaller than its rotation timescale. this confirms that rotating toroids such as the g31 main core are non-equilibrium, transient collapsing structures that need to be constantly replenished with fresh material from a large-scale reservoir. for sources b, c, and d the infall could be accelerating inside the sources, while for source a the presence of a second emission component complicates the interpretation. the sio observations have revealed the presence of at least six outflows in the g31.41+0.31 star-forming region, and suggest that each of the four sources embedded in the main core drives a molecular outflow. the outflow rates are on the order of ~10−5-10−4 m⊙ yr−1, depending on the sio abundance. the feedback of the outflows appears sufficient to sustain the turbulence in the core and to eventually disrupt the core or prevent its further collapse. the mass accretion rates onto the individual sources, estimated from the highest values of the outflow mass loss rates, are on the order of 10−4 m⊙ yr−1. the difference of about two orders of magnitude between the accretion rates and the infall rates suggests that the central stars and the jets and outflows might not be efficient in removing disk material, which could lead to gravitational instabilities, fragmentation of the disk, and episodic accretion events.conclusions: infall and outflows are simultaneously present in all four sources embedded in the high-mass star-forming core g31.41+0.31. this indicates that these sources are still actively accreting and have not reached their final mass yet, which confirms the youth of this massive core. | the sharp alma view of infall and outflow in the massive protocluster g31.41+0.31 |
rapidly growing black holes are surrounded by accretion disks that make them the brightest objects in the universe. their brightness is known to be variable, but the causes of this are not implied by simple disk models and still debated. due to the small size of accretion disks and their great distance, there are no resolved images addressing the puzzle. in this work, we study the dependence of their variability on luminosity, wavelength and orbital/thermal timescale. we use over 5,000 of the most luminous such objects with light curves of almost nightly cadence from >5 years of observations by the nasa/atlas project, which provides 2 billion magnitude pairs for a structure function analysis. when time is expressed in units of orbital or thermal timescale in thin-disk models, we find a universal structure function, independent of luminosity and wavelength, supporting the model of magneto-rotational instabilities as a main cause. over a >1 dex range in time, the fractional variability amplitude follows log (a /a0 ) ≃1 /2 ×log (δ t /tth ) . deviations from the universality may hold clues as to the structure and orientation of disks. | universality in the random walk structure function of luminous quasi-stellar objects |
aims: exor-type objects are protostars that display powerful uv-optical outbursts caused by intermittent and powerful events of magnetospheric accretion. these objects are not yet well investigated and are quite difficult to characterize. several parameters, such as plasma stream velocities, characteristic densities, and temperatures, can be retrieved from present observations. as of yet, however, there is no information about the magnetic field values and the exact underlying accretion scenario is also under discussion.methods: we use laboratory plasmas, created by a high power laser impacting a solid target or by a plasma gun injector, and make these plasmas propagate perpendicularly to a strong external magnetic field. the propagating plasmas are found to be well scaled to the presently inferred parameters of exor-type accretion event, thus allowing us to study the behaviour of such episodic accretion processes in scaled conditions.results: we propose a scenario of additional matter accretion in the equatorial plane, which claims to explain the increased accretion rates of the exor objects, supported by the experimental demonstration of effective plasma propagation across the magnetic field. in particular, our laboratory investigation allows us to determine that the field strength in the accretion stream of exor objects, in a position intermediate between the truncation radius and the stellar surface, should be of the order of 100 g. this, in turn, suggests a field strength of a few kilogausses on the stellar surface, which is similar to values inferred from observations of classical t tauri stars. | inferring possible magnetic field strength of accreting inflows in exor-type objects from scaled laboratory experiments |
we have obtained alma band 7 observations of the fu ori outburst system at a 0.″6 × 0.″5 resolution to measure the link between the inner disk instability and the outer disk through submillimeter continuum and molecular line observations. our observations detect continuum emission that can be well-modeled by two unresolved sources located at the position of each binary component. the interferometric observations recover the entire flux reported in previous single-dish studies, ruling out the presence of a large envelope. assuming that the dust is optically thin, we derive disk dust masses of 2 × 10-4 m⊙ and 8× {10}-5 m⊙ for the north and south components, respectively. we place limits on the disks’ radii of r < 45 au. we report the detection of molecular emission from 12co(3-2), hco+(4-3), and from hcn(4-3). the 12co appears widespread across the two binary components and is slightly more extended than the continuum emission. the denser gas tracer hco+ peaks close to the position of the southern binary component, while hcn appears to be peaked at the position of the northern component. this suggests that the southern binary component is embedded in denser molecular material, consistent with previous studies that indicate a heavily reddened object. at this angular resolution, any interaction between the two unresolved disk components cannot be disentangled. higher-resolution images are vital for understanding the process of star formation via rapid accretion fu ori-type episodes. | the early alma view of the fu ori outburst system |
we present a detailed study of v899 mon (a new member in the fuors/exors family of young low-mass stars undergoing outburst), based on our long-term monitoring of the source starting from 2009 november to 2015 april. our optical and near-infrared photometric and spectroscopic monitoring recorded the source transitioning from its first outburst to a short-duration quiescence phase (<1 yr), and then returning to a second outburst. we report here the evolution of the outflows from the inner region of the disk as the accretion rate evolved in various epochs. our high-resolution (r ∼ 37,000) optical spectrum could resolve interesting clumpy structures in the outflow traced by various lines. change in far-infrared flux was also detected between two outburst epochs. based on our observations, we constrained various stellar and envelope parameters of v899 mon, as well as the kinematics of its accretion and outflow. the photometric and spectroscopic properties of this source fall between classical fuors and exors. our investigation of v899 mon hints at instability associated with magnetospheric accretion being the physical cause of the sudden short-duration pause of the outburst in 2011. it is also a good candidate to explain similar short-duration pauses in outbursts of some other fuors/exors sources. | v899 mon: an outbursting protostar with a peculiar light curve, and its transition phases |
ss cyg has long been recognized as the prototype of a group of dwarf novae that show only outbursts. however, this object has entered a quite anomalous event in 2021, which at first appeared to be standstill, i.e., an almost constant luminosity state observed in z cam-type dwarf novae. this unexpected event gives us a great opportunity to reconsider the nature of standstill in cataclysmic variables. we have observed this anomalous event and its forerunner, a gradual and simultaneous increase in the optical and x-ray flux during quiescence, through many optical telescopes and the x-ray telescopes nicer and nustar. we have not found any amplification of the orbital hump during quiescence before the anomalous event, which suggests that the mass transfer rate did not significantly fluctuate on average. the estimated x-ray flux was not enough to explain the increment of the optical flux during quiescence via x-ray irradiation of the disk and the secondary star. it would be natural to consider that viscosity in the quiescent disk was enhanced before the anomalous event, which increased mass accretion rates in the disk and raised not only the optical flux but also the x-ray flux. we suggest that enhanced viscosity also triggered the standstill-like phenomenon in ss cyg, which is considered to be a series of small outbursts. the inner part of the disk would always stay in the outburst state and only its outer part would be unstable against the thermal-viscous instability during this phenomenon, which is consistent with the observed optical color variations. this scenario is in line with our x-ray spectral analyses which imply that the x-ray-emitting inner accretion flow became hotter than usual and vertically expanded, and that it became denser and was cooled down after the onset of the standstill-like state. | on the nature of the anomalous event in 2021 in the dwarf nova ss cygni and its multi-wavelength transition |
by using a large, highly obscured ( ${n}_{{\rm{h}}}\gt {10}^{23}\ {\mathrm{cm}}^{-2}$ ) active galactic nucleus (agn) sample (294 sources at z ∼ 0-5) selected from detailed x-ray spectral analyses in the deepest chandra surveys, we explore distributions of these x-ray sources in various optical/infrared/x-ray color-color diagrams and their host-galaxy properties, aiming at characterizing the nuclear obscuration environment and the triggering mechanism of highly obscured agns. we find that the refined infrared array camera (irac) color-color diagram fails to identify the majority of x-ray-selected, highly obscured agns, even for the most luminous sources with $\mathrm{log}\,{l}_{{\rm{x}}}(\mathrm{erg}\ {{\rm{s}}}^{-1})\gt 44$ . over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of ${f}_{24\mu {\rm{m}}}/{f}_{r}\gt 1000$ and r - k > 4.5 criteria, implying complex origins and conditions for the obscuring materials that are responsible for the heavy x-ray obscuration. the average star formation rate (sfr) of highly obscured agns is similar to that of stellar mass- (m*-) and z-controlled normal galaxies, while a lack of quiescent hosts is observed for the former. partial correlation analyses imply that highly obscured agn activity (traced by ${l}_{{\rm{x}}}$ ) appears to be more fundamentally related to m*, and no dependence of ${n}_{{\rm{h}}}$ on either m* or sfr is detected. morphology analyses reveal that 61% of our sources have a significant disk component, while only ∼27% of them exhibit irregular morphological signatures. these findings together point toward a scenario where secular processes (e.g., galactic-disk instabilities), instead of mergers, are most probable to be the leading mechanism that triggers accretion activities of x-ray-selected, highly obscured agns. | piercing through highly obscured and compton-thick agns in the chandra deep fields. ii. are highly obscured agns the missing link in the merger-triggered agn-galaxy coevolution models? |
disks of gas accreting onto supermassive black holes, powering active galactic nuclei (agn), can capture stars from nuclear star clusters or form stars in situ via gravitational instability. the density and thermal conditions of these disks can result in rapid accretion onto embedded stars, dramatically altering their evolution in comparison to stars in the interstellar medium. theoretical models predict that, when subjected to sufficiently rapid accretion, fresh gas replenishes hydrogen in the cores of these stars as quickly as it is burned into helium, reaching a quasi-steady state. such massive, long-lived ("immortal") stars may be capable of dramatically enriching agn disks with helium, and would increase the helium abundance in agn broad-line regions relative to that in the corresponding narrow-line regions and hosts. we investigate how the helium abundance of agn disks alters the evolution of stars embedded therein. we find, in agreement with analytical arguments, that stars at a given mass are more luminous at higher helium mass fractions, and so undergo more radiation-driven mass loss. we further find that embedded stars tend to be less massive in disks with higher helium mass fractions, and that immortal stars are less common in such disks. thus, disk composition can alter the rates of electromagnetic and gravitational wave transients as well as further chemical enrichment by embedded stars. | the influence of disk composition on the evolution of stars in the disks of active galactic nuclei |
turbulent radiation flow is commonplace in systems with strong, incoherent, light-matter interactions. in astrophysical contexts, photon bubble turbulence is considered a key mechanism behind enhanced radiation transport, and its importance has been widely asserted for a variety of high energy objects such as accretion disks and massive stars. here, we show that analogous conditions to those of dense astrophysical objects can be obtained in large clouds of cold atoms, prepared in a laser-cooling experiment, driven close to a sharp electronic resonance. by accessing the spatially-resolved atom density, we are able to identify a photon bubble instability and the resulting regime of photon bubble turbulence. we also develop a theoretical model describing the coupled dynamics of both photon and atom gases, which accurately describes the statistical properties of the turbulent regime. this study thus opens the possibility of simulating radiation-dominated astrophysical systems in cold atom experiments. | photon bubble turbulence in cold atom gases |
this white paper submitted for 2020 decadal assessment of plasma science concerns the importance of multi-spacecraft missions to address fundamental questions concerning plasma turbulence. plasma turbulence is ubiquitous in the universe, and it is responsible for the transport of mass, momentum, and energy in such diverse systems as the solar corona and wind, accretion discs, planet formation, and laboratory fusion devices. turbulence is an inherently multi-scale and multi-process phenomenon, coupling the largest scales of a system to sub-electron scales via a cascade of energy, while simultaneously generating reconnecting current layers, shocks, and a myriad of instabilities and waves. the solar wind is humankind's best resource for studying the naturally occurring turbulent plasmas that permeate the universe. since launching our first major scientific spacecraft mission, explorer 1, in 1958, we have made significant progress characterizing solar wind turbulence. yet, due to the severe limitations imposed by single point measurements, we are unable to characterize sufficiently the spatial and temporal properties of the solar wind, leaving many fundamental questions about plasma turbulence unanswered. therefore, the time has now come wherein making significant additional progress to determine the dynamical nature of solar wind turbulence requires multi-spacecraft missions spanning a wide range of scales simultaneously. a dedicated multi-spacecraft mission concurrently covering a wide range of scales in the solar wind would not only allow us to directly determine the spatial and temporal structure of plasma turbulence, but it would also mitigate the limitations that current multi-spacecraft missions face, such as non-ideal orbits for observing solar wind turbulence. some of the fundamentally important questions that can only be addressed by in situ multipoint measurements are discussed. | [plasma 2020 decadal] disentangling the spatiotemporal structure of turbulence using multi-spacecraft data |
we model the intermediate-mass black hole hlx-1, using the hubble space telescope, xmm-newton and swift. we quantify the relative contributions of a bluer component, function of x-ray irradiation, and a redder component, constant and likely coming from an old stellar population. we estimate a black hole mass {≈ } (2^{+2}_{-1}) × 10^4 m_{⊙}, a spin parameter a/m ≈ 0.9 for moderately face-on view and a peak outburst luminosity ≈0.3 times the eddington luminosity. we discuss the discrepancy between the characteristic sizes inferred from the short x-ray time-scale (r ∼ a few 1011 cm) and from the optical emitter (r √{cos θ } ≈ 2.2 × 10^{13} cm). one possibility is that the optical emitter is a circumbinary disc; however, we disfavour this scenario because it would require a very small donor star. a more plausible scenario is that the disc is large but only the inner annuli are involved in the x-ray outburst. we propose that the recurrent outbursts are caused by an accretion-rate oscillation driven by wind instability in the inner disc. we argue that the system has a long-term-average accretion rate of a few per cent 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 yr (much longer than the binary period, which we estimate as ∼10 d). the oscillation that dominated the system in the last decade is now damped such that the accretion rate no longer reaches the level required to trigger a transition. finally, we highlight similarities between disc winds in hlx-1 and in the galactic black hole v404 cyg. | outbursts of the intermediate-mass black hole hlx-1: a wind-instability scenario |
estimates of the accretion rate in symbiotic recurrent novae (rne) often fall short of theoretical expectations by orders of magnitude. this apparent discrepancy can be resolved if the accumulation of mass by the white dwarf (wd) is highly sporadic, and most observations are performed during low states. here we use a re-analysis of archival data from the digital access to a sky century @harvard survey to argue that the most recent nova eruption in symbiotic rn t crb, in 1946, occurred during—and was therefore triggered by—a transient accretion high state. based on similarities in the optical light curve around 1946 and the time of the prior eruption, in 1866, we suggest that the wd in t crb accumulates most of the fuel needed to ignite the thermonuclear runaways (tnrs) during accretion high states. a natural origin for such states is dwarf-nova like accretion-disk instabilities, which are expected in the presumably large disks in symbiotic binaries. the timing of the tnrs in symbiotic rne could thus be set by the stability properties of their accretion disks. t crb is in the midst of an accretion high state like the ones we posit led to the past two nova eruptions. combined with the approach of the time at which a tnr would be expected based on the 80 yr interval between the prior two novae (2026 ± 3), the current accretion high state increases the likelihood of a tnr occurring in t crb in the next few years. | increasing activity in t crb suggests nova eruption is impending |
gravitational waves can be emitted by accretion discs if they undergo instabilities that generate a time varying mass quadrupole. in this work we investigate the gravitational signal generated by a thick accretion disc of 1 m⊙ around a static supermassive black hole of 106 m⊙, assumed to be formed after the tidal disruption of a solar type star. this torus has been shown to be unstable to a global non-axisymmetric hydrodynamic instability, the papaloizou-pringle instability, in the case where it is not already accreting and has a weak magnetic field. we start by deriving analytical estimates of the maximum amplitude of the gravitational wave signal, with the aim to establish its detectability by the laser interferometer space antenna (lisa). then, we compare these estimates with those obtained through a numerical simulation of the torus, made with a 3d smoothed particle hydrodynamics code. our numerical analysis shows that the measured strain is two orders of magnitude lower than the maximum value obtained analytically. however, accretion discs affected by the papaloizou-pringle instability may still be interesting sources for lisa, if we consider discs generated after deeply penetrating tidal disruptions of main-sequence stars of higher mass. | gravitational wave emission from unstable accretion discs in tidal disruption events |
we consider the parallaxes of sixteen cataclysmic variables and related objects that are included in the tgas catalogue, which is part of the gaia first data release, and compared these with previous parallax measurements. the parallax of the dwarf nova ss cyg is consistent with the parallax determination made using the vlbi, but with only one of the analyses of the hst fine guidance sensor (fgs) observations of this system. in contrast, the gaia parallaxes of v603 aql and rr pic are broadly consistent, but less precise than the hst/fgs measurements. the gaia parallaxes of ix vel, v3885 sgr, and ae aqr are consistent with, but much more accurate than the hipparcos measurements. we took the derived gaia distances and find that absolute magnitudes of outbursting systems show a weak correlation with orbital period. for systems with measured x-ray fluxes we find that the x-ray luminosity is a clear indicator of whether the accretion disc is in the hot and ionised or cool and neutral state. we also find evidence for the x-ray emission of both low and high state discs correlating with orbital period, and hence the long-term average accretion rate. the inferred mass accretion rates for the nova-like variables and dwarf novae are compared with the critical mass accretion rate predicted by the disc instability model. while we found agreement to be good for most systems there appears to be some uncertainty in the system parameters of ss cyg. our results illustrate how future gaia data releases will be an extremely valuable resource in mapping the evolution of cataclysmic variables. | distances of cataclysmic variables and related objects derived from gaia data release 1 |
we present high-time-resolution photometry and phase-resolved spectroscopy of the short-period ({p_orb}= 80.52 min) cataclysmic variable sdss j123813.73-033933.0, observed with the hubble space telescope (hst), the kepler/k2 mission, and the very large telescope (vlt). we also report observations of the first detected superoutburst. sdss j1238-0339 shows two types of variability: quasi-regular brightenings recurring every ≃8.5 h during which the system increases in brightness by {∼eq } 0.5 mag, and a double-hump quasi-sinusoidal modulation at the orbital period. the detailed k2 light curve reveals that the amplitude of the double-humps increases during the brightenings and that their phase undergoes a ≃90° phase shift with respect to the quiescent intervals. the hst data unambiguously demonstrate that these phenomena both arise from the heating and cooling of two relatively large regions on the white dwarf. we suggest that the double-hump modulation is related to spiral shocks in the accretion disc resulting in an enhanced accretion rate heating two localized regions on the white dwarf, with the structure of the shocks fixed in the binary frame explaining the period of the double humps. the physical origin of the 8.5 h brightenings is less clear. however, the correlation between the observed variations of the amplitude and phase of the double-humps with the occurrence of the brightenings is supportive of an origin in thermal instabilities in the accretion disc. | evidence for mass accretion driven by spiral shocks onto the white dwarf in sdss j123813.73-033933.0 |
we present fully general-relativistic numerical evolutions of self-gravitating tori around spinning black holes with dimensionless spin a /m =0.7 parallel or antiparallel to the disk angular momentum. the initial disks are unstable to the hydrodynamic papaloizou-pringle instability which causes them to grow persistent orbiting matter clumps. the effect of black hole spin on the growth and saturation of the instability is assessed. we find that the instability behaves similarly to prior simulations with nonspinning black holes, with a shift in frequency due to spin-induced changes in disk orbital period. copious gravitational waves are generated by these systems, and we analyze their detectability by current and future gravitational wave observatories for a large range of masses. we find that systems of 10 m⊙ —relevant for black hole-neutron star mergers—are detectable by cosmic explorer out to ∼300 mpc , while decigo (lisa) will be able to detect systems of 1000 m⊙ (105 m⊙ )—relevant for disks forming in collapsing supermassive stars—out to cosmological redshift of z ∼5 (z ∼1 ). computing the accretion rate of these systems we find that these systems may also be promising sources of coincident electromagnetic signals. | gravitational waves from disks around spinning black holes: simulations in full general relativity |
context. the vertical shear instability is one of several known mechanisms that are potentially active in the so-called dead zones of protoplanetary accretion disks. a recent analysis of the instability mechanism indicates that a subset of unstable modes shows unbounded growth - both as resolution is increased and when the nominal lid of the atmosphere is extended. this trend suggests that, possibly, the model system is ill-posed.aims: this research note both examines the energy content of these modes and questions the legitimacy of assuming separable solutions for a problem whose linear operator is fundamentally inseparable.methods: the reduced equations governing the instability are revisited and the generated solutions are examined using both the previously assumed separable forms and an improved non-separable solution form that is introduced in this paper.results: reconsidering the solutions of the reduced equations by using the separable form shows that, while the low-order body modes have converged eigenvalues and eigenfunctions (for both variations in the model atmosphere's vertical boundaries and radial numerical resolution). it is also confirmed that the corresponding high-order body modes and the surface modes indeed show unbounded growth rates. the energy contained in both the higher order body modes and surface modes diminishes precipitously due to the disk's gaussian density profile. most of the energy of the instability is contained in the low-order modes. an inseparable solution form is introduced to filter out the inconsequential surface modes, leaving only body modes (both low- and high-order ones). the analysis predicts a fastest growing mode with a specific radial length scale. the growth rates associated with the fundamental corrugation and breathing modes match the growth and length scales observed in previous nonlinear studies of the instability.conclusions: linear stability analysis of the vertical shear instability should be done assuming non-separable solutions, especially for settings involving boundaries in the radial direction. we also conclude that the surface modes are relatively inconsequential because of the little energy they contain, and are artifacts of imposing specific kinematic vertical boundary conditions in isothermals disk models. | linear analysis of the vertical shear instability: outstanding issues and improved solutions |
we present fully three-dimensional magnetohydrodynamic jet-launching simulations of a jet source orbiting in a binary system. we consider a time-dependent binary gravitational potential, and thus all tidal forces are experienced in the non-inertial frame of the jet-launching primary. we investigate systems with different binary separations, different mass ratios, and different inclinations between the disk plane and the orbital plane. the simulations run over a substantial fraction of the binary orbital period. all simulations show similar local and global non-axisymmetric effects, such as local instabilities in the disk and jet or in global features, such as disk spiral arms and warps, or a global realignment of the inflow-outflow structure. the disk accretion rate is higher than in axisymmetric simulations, most probably due to the enhanced angular momentum transport by spiral waves. the disk outflow leaves the roche lobe of the primary and becomes disturbed by tidal effects. while a disk-orbit inclination of 10° still allows for a persistent outflow, an inclination of 30° does not, suggesting a critical angle in between. for moderate inclination, we find an indication for jet precession, such that the jet axis starts to follow a circular pattern with an opening cone of ≃8°. simulations with different mass ratios indicate a change of timescales over which the tidal forces affect the disk-jet system. a large mass ratio (a massive secondary) leads to stronger spiral arms, higher (average) accretion, and a more pronounced jet-counter-jet asymmetry. | long-term simulation of mhd jet launching in an orbiting star-disk system |
horseshoe-shaped brightness asymmetries of several transitional discs are thought to be caused by large-scale vortices. anticyclonic vortices efficiently collect dust particles, therefore they can play a major role in planet formation. former studies suggest that the disc self-gravity weakens vortices formed at the edge of the gap opened by a massive planet in discs whose masses are in the range of 0.01 ≤ mdisc/m* ≤ 0.1. here, we present an investigation on the long-term evolution of the large-scale vortices formed at the viscosity transition of the discs' dead zone outer edge by means of two-dimensional hydrodynamic simulations taking disc self-gravity into account. we perform a numerical study of low-mass, 0.001 ≤ mdisc/m* ≤ 0.01, discs, for which cases disc self-gravity was previously neglected. the large-scale vortices are found to be stretched due to disc self-gravity even for low-mass discs with mdisc/m* ≳ 0.005, where initially the toomre q-parameter was ≲ 50 at the vortex distance. as a result of stretching, the vortex aspect ratio increases and a weaker azimuthal density contrast develops. the strength of the vortex stretching is proportional to the disc mass. the vortex stretching can be explained by a combined action of a non-vanishing gravitational torque caused by the vortex and the keplerian shear of the disc. self-gravitating vortices are subject to significantly faster decay than non-self-gravitating ones. we found that vortices developed at sharp viscosity transitions of self-gravitating discs can be described by a goodman - narayan - goldreich (gng) model as long as the disc viscosity is low, i.e. αdz ≤ 10-5. | vortex stretching in self-gravitating protoplanetary discs |
using the eris zoom-in cosmological simulation of assembly of a milky way analogue, we study the chemical enrichment of stars due to accretion of metal-enriched gas from the interstellar medium (ism) during the galaxy's development. we consider metal-poor and old stars in the galactic halo and bulge through the use of stellar orbits, gas density and metallicity distributions in eris. assuming spherically symmetric bondi-hoyle accretion, we find that halo and bulge stars accrete metals at the rate of about 10-24 and 10-22 m⊙ yr-1, respectively, at redshifts z ≲ 3, but this accretion rate increases roughly a hundred-fold to about 10-20 m⊙ yr-1 at higher redshifts due to increased gas density. bulge and halo stars accrete similar amounts of metals at high redshifts when kinematically distinct bulge and halo have not yet developed, and both sets of stars encounter a similar metal distribution in the ism. accretion alone can enrich main-sequence stars up to [fe/h] ∼ -2 in extreme cases, with the median enrichment level due to accretion of about [fe/h] ∼ -6 to -5. because accretion mostly takes place at high redshifts, it is α-enriched to [α/fe] ∼ 0.5. we find that accretive metal enrichment is sufficient to affect the predicted metallicity distribution function of halo stars at [fe/h] < -5. this can hinder attempts to infer natal chemical environment of metal-poor stars from their observed enrichment. peculiar enrichment patterns such as those predicted to arise from pair-instability supernovae could help in disentangling the natal and accreted metal content of stars. | chemical enrichment of stars due to accretion from the ism during the galaxy's assembly |
in zombie vortex instability (zvi), perturbations excite critical layers in stratified, rotating shear flow (as in protoplanetary disks (ppds)), causing them to generate vortex layers, which roll up into anticyclonic zombie vortices and cyclonic vortex sheets. the process is self-sustaining as zombie vortices perturb new critical layers, spawning a next generation of zombie vortices. here, we focus on two issues: the minimum threshold of perturbations that trigger self-sustaining vortex generation, and the properties of the late-time zombie turbulence on large and small scales. the critical parameter that determines whether zvi is triggered is the magnitude of the vorticity on the small scales (and not velocity); the minimum rossby number needed for instability is {{ro}}{crit}∼ 0.2 for β \equiv n/{{ω }}=2, where n is the brunt-väisälä frequency. while the threshold is set by vorticity, it is useful to infer a criterion on the mach number; for kolmogorov noise, the critical mach number scales with reynolds number: {{ma}}{crit}∼ {{ro}}{crit}{{re}}-1/2. in ppds, this is {{ma}}{crit}∼ {10}-6. on large scales, zombie turbulence is characterized by anticyclones and cyclonic sheets with typical rossby number ∼0.3. the spacing of the cyclonic sheets and anticyclones appears to have a “memory” of the spacing of the critical layers. on small scales, zombie turbulence has no memory of the initial conditions and has a kolmogorov-like energy spectrum. while our earlier work was in the limit of uniform stratification, we have demonstrated that zvi works for non-uniform brunt-väisälä frequency profiles that may be found in ppds. | zombie vortex instability. ii. thresholds to trigger instability and the properties of zombie turbulence in the dead zones of protoplanetary disks |
we expand the relativistic precession model to include nonequatorial and eccentric trajectories and apply it to quasi-periodic oscillations (qpos) in black hole x-ray binaries (bhxrbs) and associate their frequencies with the fundamental frequencies of the general case of nonequatorial (with carter's constant, $q\ne 0$ ) and eccentric ( $e\ne 0$ ) particle trajectories, around a kerr black hole. we study cases with either two or three simultaneous qpos and extract the parameters {e, rp, a, q}, where rp is the periastron distance of the orbit, and a is the spin of the black hole. we find that the orbits with $\left[q=0-4\right]$ should have e ≲ 0.5 and rp ∼ 2-20 for the observed range of qpo frequencies, where a ∈ [0, 1], and that the spherical trajectories {e = 0, $q\ne 0$ } with q = 2-4 should have rs ∼ 3-20. we find nonequatorial eccentric solutions for both m82 x-1 and groj 1655-40. we see that these trajectories, when taken together, span a torus region and give rise to a strong qpo signal. for two simultaneous qpo cases, we found equatorial eccentric orbit solutions for xtej 1550-564, 4u 1630-47, and grs 1915+105, and spherical orbit solutions for bhxrbs m82 x-1 and xtej 1550-564. we also show that the eccentric orbit solution fits the psaltis-belloni-klis correlation observed in bhxrb groj 1655-40. our analysis of the fluid flow in the relativistic disk edge suggests that instabilities cause qpos to originate in the torus region. we also present some useful formulae for trajectories and frequencies of spherical and equatorial eccentric orbits. | a geometric origin for quasi-periodic oscillations in black hole x-ray binaries |
we observed the 2015 july-august long outburst of v1006 cyg and established this object to be an su uma-type dwarf nova in the period gap. our observations have confirmed that v1006 cyg is the second established object showing three types of outbursts (normal, long normal, and superoutbursts) after tu men. we have succeeded in recording the growing stage of superhumps (stage a superhumps) and obtained a mass ratio of 0.26-0.33, which is close to the stability limit of tidal instability. this identification of stage a superhumps demonstrates that superhumps indeed slowly grow in systems near the stability limit, the idea first introduced by kato et al. (2014, pasj, 66, 90). the superoutburst showed a temporary dip followed by a rebrightening. the moment of the dip coincided with the stage transition of superhumps, and we suggest that stage c superhumps are related to the start of the cooling wave in the accretion disk. we interpret that the tidal instability was not strong enough to maintain the disk in the hot state when the cooling wave started. we propose that the properties commonly seen in the extreme ends of mass ratios (wz sge-type objects and long-period systems) can be understood as a result of weak tidal effect. | v1006 cygni: dwarf nova showing three types of outbursts and simulating some features of the wz sge-type behavior |
the neutron star (ns) low-mass x-ray binary (lmxb) the rapid burster (rb; mxb 1730-335) uniquely shows both type i and type ii x-ray bursts. the origin of the latter is ill-understood but has been linked to magnetospheric gating of the accretion flow. we present a spectral analysis of simultaneous swift, nustar and xmm-newton observations of the rb during its 2015 outburst. although a broad fe k line has been observed before, the high quality of our observations allows us to model this line using relativistic reflection models for the first time. we find that the disc is strongly truncated at 41.8^{+6.7}_{-5.3} gravitational radii (∼87 km), which supports magnetospheric type ii burst models and strongly disfavours models involving instabilities at the innermost stable circular orbit. assuming that the rb magnetic field indeed truncates the disc, we find b = (6.2 ± 1.5) × 108 g, larger than typically inferred for ns lmxbs. in addition, we find a low inclination (i = 29° ± 2°). finally, we comment on the origin of the comptonized and thermal components in the rb spectrum. | a strongly truncated inner accretion disc in the rapid burster |
v476 cyg (nova cyg 1920) is a bright, fast nova reaching a photographic magnitude of 2.0. using the zwicky transient facility (ztf) public database, i found that this nova is currently a dwarf nova with a cycle length of ~24 d. compared to other classical novae currently in dwarf nova-type states, outbursts of v476 cyg are rapidly rising and short with durations of a few days. based on the aavso observations, this nova was probably already in the dwarf nova-type phase in 2016, 96 years after the nova eruption. i found a possible orbital period of 0.1018002(6) d using the ztf data, which would place the object in the period gap. this supposed short orbital period appears to explain the features and faint absolute magnitudes of the observed dwarf nova outbursts. if this period is confirmed, v476 cyg is a classical nova with the shortest orbital period with distinct dwarf nova outbursts and in which a nova eruption was recorded in the modern era. i also compared with the outburst properties with v446 her (nova her 1960), which currently shows ss cyg-type outbursts. the transition to the dwarf nova-phase in v476 cyg occurred much earlier (~100 yr) than what has been supposed (~1000 yr) for classical novae below the period gap. v476 cyg would not only provide an ideal laboratory of the behavior of an irradiated accretion disk in which tidal instability is expected to work, but also an ideal laboratory of the effect of a massive white dwarf on dwarf nova outbursts. | v476 cyg (nova cyg 1920) is currently a dwarf nova -- first such an object in the period gap? |
taylor-couette flow (tcf) is often used as a simplified model for complex rotating flows in the interior of stars and accretion discs. the flow dynamics in these objects is influenced by magnetic fields. for example, quasi-keplerian flows in taylor-couette geometry become unstable to a travelling or standing wave in an external magnetic field if the fluid is conducting; there is an instability even when the flow is hydrodynamically stable. this magnetorotational instability leads to the development of chaotic states and, eventually, turbulence, when the cylinder rotation is sufficiently fast. the transition to turbulence in this flow can be complex, with the coexistence of parameter regions with spatio-temporal chaos and regions with quasi-periodic behaviour, involving one or two additional modulating frequencies. although the unstable modes of a periodic flow can be identified with floquet analysis, here we adopt a more flexible equation-free data-driven approach. we analyse the data from the transition to chaos in the magnetized tcf and identify the flow structures related to the modulating frequencies with dynamic mode decomposition; this method is based on approximating nonlinear dynamics with a linear infinite-dimensional koopman operator. with the use of these structures, one can construct a nonlinear reduced model for the transition. this article is part of the theme issue `taylor-couette and related flows on the centennial of taylor's seminal philosophical transactions paper (part 1)'. | transition to chaos and modal structure of magnetized taylor-couette flow |
the theory of instability of accretion disks about black holes, neutron stars, or protoplanets is revisited by means of the recent method of the spectral web. the cylindrical accretion disk differential equation is shown to be governed by the forward and backward doppler-shifted continuous alfvén spectra ${{\rm{\omega }}}_{{\rm{a}}}^{\pm }\equiv m{\rm{\omega }}\pm {\omega }_{{\rm{a}}}$ , where ω a is the static alfvén frequency. it is crucial to take nonaxisymmetry (m ≠ 0) and super-alfvénic rotation of the doppler frames (∣mω∣ ≫ ∣ω a∣) into account. the continua ${{\rm{\omega }}}_{{\rm{a}}}^{+}$ and ${{\rm{\omega }}}_{{\rm{a}}}^{-}$ then overlap, ejecting a plethora of super-alfvénic rotational instabilities (saris). in-depth analysis for small inhomogeneity shows that the two alfvén singularities reduce the extent of the modes to sizes much smaller than the width of the accretion disk. generalization for large inhomogeneity leads to the completely unprecedented result that, for mode numbers ∣k∣ ≫ ∣m∣, any complex ω in a wide neighborhood of the real axis is an approximate "eigenvalue." the difference with genuine eigenmodes is that the amount of complementary energy to excite the modes is tiny, ∣w com∣ ≤ c, with c the machine accuracy of the computation. this yields a multitude of two-dimensional continua of quasi-discrete modes: quasi-continuum saris. we conjecture that the onset of 3d turbulence in magnetized accretion disks is governed not by the excitation of discrete axisymmetric magnetorotational instabilities but by the excitation of modes from these two-dimensional continua of quasi-discrete nonaxisymmetric saris. | the super-alfvénic rotational instability in accretion disks about black holes |
we present the results of local, vertically stratified, radiation magnetohydrodynamic shearing-box simulations of magnetorotational instability (mri) turbulence for a (hydrogen poor) composition applicable to accretion disks in am cvn type systems. many of these accreting white dwarf systems are helium analogs of dwarf novae (dne). we utilize frequency-integrated opacity and equation-of-state tables appropriate for this regime to accurately portray the relevant thermodynamics. we find bistability of thermal equilibria in the effective-temperature, surface-mass-density plane typically associated with disk instabilities. along this equilibrium curve (i.e., the s-curve), we find that the stress to thermal pressure ratio α varied with peak values of ∼0.15 near the tip of the upper branch. similar to dne, we found enhancement of α near the tip of the upper branch caused by convection; this increase in α occurred despite our choice of zero net vertical magnetic flux. two notable differences we find between dn and am cvn accretion disk simulations are that am cvn disks are capable of exhibiting persistent convection in outburst, and ideal mhd is valid throughout quiescence for am cvns. in contrast, dne simulations only show intermittent convection, and nonideal mhd effects are likely important in quiescence. by combining our previous work with these new results, we also find that convective enhancement of the mri is anticorrelated with mean molecular weight. | convection enhances magnetic turbulence in am cvn accretion disks |
we study the evolution of close binary systems formed by a normal (solar composition), intermediate-mass-donor star together with a neutron star. we consider models including irradiation feedback and evaporation. these nonstandard ingredients deeply modify the mass-transfer stages of these binaries. while models that neglect irradiation feedback undergo continuous, long-standing mass-transfer episodes, models including these effects suffer a number of cycles of mass transfer and detachment. during mass transfer, the systems should reveal themselves as low-mass x-ray binaries (lmxbs), whereas when they are detached they behave as binary radio pulsars. we show that at these stages irradiated models are in a roche lobe overflow (rlof) state or in a quasi-rlof state. quasi-rlof stars have radii slightly smaller than their roche lobes. remarkably, these conditions are attained for an orbital period as well as donor mass values in the range corresponding to a family of binary radio pulsars known as "redbacks." thus, redback companions should be quasi-rlof stars. we show that the characteristics of the redback system psr j1723-2837 are accounted for by these models. in each mass-transfer cycle these systems should switch from lmxb to binary radio pulsar states with a timescale of approximately one million years. however, there is recent and fast growing evidence of systems switching on far shorter, human timescales. this should be related to instabilities in the accretion disk surrounding the neutron star and/or radio ejection, still to be included in the model having the quasi-rlof state as a general condition. | the quasi-roche lobe overflow state in the evolution of close binary systems containing a radio pulsar |
astrophysical objects possessing a material surface (white dwarfs, young stars, etc.) may accrete gas from the disc through the so-called surface boundary layer (bl), in which the angular velocity of the accreting gas experiences a sharp drop. acoustic waves excited by the supersonic shear in the bl play an important role in mediating the angular momentum and mass transport through that region. here we examine the characteristics of the angular momentum transport produced by the different types of wave modes emerging in the inner disc, using the results of a large suite of hydrodynamic simulations of the bls. we provide a comparative analysis of the transport properties of different modes across the range of relevant disc parameters. in particular, we identify the types of modes that are responsible for the mass accretion on to the central object. we find the correlated perturbations of surface density and radial velocity to provide an important contribution to the mass accretion rate. although the wave-driven transport is intrinsically non-local, we do observe a clear correlation between the angular momentum flux injected into the disc by the waves and the mass accretion rate through the bl. we find the efficiency of angular momentum transport (normalized by thermal pressure) to be a weak function of the flow mach number. we also quantify the wave-driven evolution of the inner disc, in particular the modification of the angular frequency profile in the disc. our results pave the way for understanding wave-mediated transport in future three-dimensional, magnetohydrodynamic studies of the bls. | boundary layers of accretion discs: wave-driven transport and disc evolution |
context. the equatorial accretion scenario, caused by the development of the rayleigh-taylor (rt) instability at the disk edge, was suggested by accurate three-dimensional magnetohydrodynamic (mhd) modelling, but no observational or experimental confirmation of such phenomena has been evidenced yet.aims: we studied the propagation of a laterally extended laser-generated plasma stream across a magnetic field and investigated if this kind of structure can be scaled to the case of equatorial `tongue' accretion channels in young stellar objects (ysos); if so, this would support the possibility of equatorial accretion in young accreting stars.methods: we conducted a scaled laboratory experiment at the pearl laser facility. the experiment consists in an optical laser pulse that is focused onto the surface of a teflon target. the irradiation of the target leads to the expansion of a hot plasma stream into the vacuum, perpendicularly to an externally applied magnetic field. we used a mach-zehnder interferometer to diagnose the plasma stream propagation along two axes, to obtain the three-dimensional distribution of the plasma stream.results: the laboratory experiment shows the propagation of a laterally extended laser-generated plasma stream across a magnetic field. we demonstrate that: (i) such a stream is subject to the development of the rt instability, and (ii) the stream, decomposed into tongues, is able to efficiently propagate perpendicular to the magnetic field. based on numerical simulations, we show that the origin of the development of the instability in the laboratory is similar to that observed in mhd models of equatorial tongue accretion in ysos.conclusions: as we verify that the laboratory plasma scales favourably to accretion inflows of ysos, our laboratory results support the argument in favour of the possibility of the rt-instability-caused equatorial tongue accretion scenario in the astrophysical case. | laboratory modelling of equatorial `tongue' accretion channels in young stellar objects caused by the rayleigh-taylor instability |
a sudden increase in the rate at which material reaches the most internal part of an accretion disk, i.e., the boundary layer, can change its structure dramatically. we have witnessed such a change for the first time in the symbiotic recurrent nova t crb. our analysis of xmm-newton, swift burst alert telescope (bat)/x-ray telescope (xrt)/ultraviolet optical telescope (uvot), and the american association of variable stars observers (aavso) v- and b-band data indicates that during an optical brightening event that started in early 2014 (δv ≈ 1.5) the following occurred: (i) the hard x-ray emission as seen with bat almost vanished; (ii) the xrt x-ray flux decreased significantly, while the optical flux remained high; (iii) the uv flux increased by at least a factor of 40 over the quiescent value; and (iv) the x-ray spectrum became much softer and a bright, new blackbody-like component appeared. we suggest that the optical brightening event, which could be a similar event to that observed about 8 years before the most recent thermonuclear outburst in 1946, is due to a disk instability. | dramatic change in the boundary layer in the symbiotic recurrent nova t coronae borealis |
the disk instability picture gives a plausible explanation for the behavior of soft x-ray transient systems if self-irradiation of the disk is included. we show that there is a simple relation between the peak luminosity (at the start of an outburst) and the decay timescale. we use this relation to place constraints on systems assumed to undergo disk instabilities. the observable x-ray populations of elliptical galaxies must largely consist of long-lived transients, as deduced on different grounds by piro & bildsten (2002). the strongly varying x-ray source hlx-1 in the galaxy eso 243-49 can be modeled as disk instability of a highly super-eddington stellar-mass binary similar to ss 433. a fit to the disk instability picture is not possible with an intermediate-mass black hole model for hlx-1. other recently identified super-eddington ulxs might be subject to disk instability. | x-ray transients: hyper- or hypo-luminous? |
recent work by levitan et al. has expanded the long-term photometric database for am cvn stars. in particular, their outburst properties are well correlated with orbital period and allow constraints to be placed on the secular mass transfer rate between secondary and primary if one adopts the disk instability model for the outbursts. we use the observed range of outbursting behavior for am cvn systems as a function of orbital period to place a constraint on mass transfer rate versus orbital period. we infer a rate ∼5× {{10}-9}{{m}⊙ } y{{r}-1}{{({{p}orb}/1000 s)}-5.2}. we show that the functional form so obtained is consistent with the recurrence time-orbital period relation found by levitan et al. using a simple theory for the recurrence time. also, we predict that their steep dependence of outburst duration on orbital period will flatten considerably once the longer orbital period systems have more complete observations. | constraining the physics of am canum venaticorum systems with the accretion disk instability model |
the radial transport, or drift, of dust has taken a critical role in giant planet formation theory. however, it has been challenging to identify dust drift pileups in the hard-to-observe inner disk. we find that the im lup disk shows evidence that it has been shaped by an episode of dust drift. using radiative transfer and dust dynamical modeling we study the radial and vertical dust distribution. we find that high dust drift rates exceeding 110 m ⊕ myr-1 are necessary to explain both the dust and co observations. furthermore, the bulk of the large dust present in the inner 20 au needs to be vertically extended, implying high turbulence (αz≳ 10-3) and small grains (0.2-1 mm). we suggest that this increased level of particle stirring is consistent with the inner dust-rich disk undergoing turbulence triggered by the vertical shear instability. the conditions in the im lup disk imply that giant planet formation through pebble accretion is only effective outside of 20 au. if such an early, high-turbulence inner region is a natural consequence of high dust drift rates, then this has major implications for understanding the formation regions of giant planets including jupiter and saturn. | a potential site for wide-orbit giant planet formation in the im lup disk |
context. most massive stars are located in multiple stellar systems. the modeling of disk fragmentation, a mechanism that may plausibly lead to stellar multiplicity, relies on parallel 3d simulation codes whose agreement remains to be evaluated.aims: cartesian adaptive-mesh refinement (amr) and spherical codes have frequently been used in the past decade to study massive star formation. we aim to study how the details of collapse and disk fragmentation depend on these codes.methods: using the cartesian amr code ramses within its self-gravity radiation-hydrodynamical framework, we compared disk fragmentation in a centrally condensed protostellar system to the findings of earlier studies performed on a grid in spherical coordinates using pluto.results: to perform the code comparison, two ramses runs were considered, effectively giving qualitatively distinct pictures. on the one hand, when allowing for unlimited sink particle creation with no initial sink, toomre instability and subsequent gas fragmentation leads to a multiple stellar system whose multiplicity is affected by the grid when triggering fragmentation and via numerically assisted mergers. on the other hand, using a unique, central, fixed-sink particle, a centrally-condensed system forms that is similar to that reported by pluto. hence, the ramses-pluto comparison was performed with the latter and an agreement between the two codes is found as to the first rotationally supported disk formation, the presence of an accretion shock onto it, and the first fragmentation phase. gaseous fragments form. the properties of the fragments (i.e., number, mass, and temperature) are dictated by local thermodynamics and are in agreement between the two codes given that the system has entered a highly nonlinear phase. over the simulations, the stellar accretion rate is made of accretion bursts and continuous accretion on the same order of magnitude. as a minor difference between both codes, the dynamics of the fragments causes the disk structure to be sub-keplerian in ramses, whereas it is found to be keplerian, thus reaching quiescence, in pluto. we attribute this discrepancy to the central star being twice less massive in ramses because of the different stellar accretion subgrid models in use - rather than the potential grid effects.conclusions: in a centrally condensed system, the agreement between ramses and pluto regarding many of the collapse properties and fragmentation process is good. in contrast, fragmentation occurring in the innermost region and given specific numerical choices (use of sink particles, grid, etc.) have a crucial impact when similar but smooth initial conditions are employed. these aspects prove more crucial than the choice of code, with regard to the system being multiple or centrally condensed. | disk fragmentation around a massive protostar: comparison of two 3d codes |
during their formation phase, stars gain most of their mass in violent episodic accretion events, such as observed in fu orionis (fuor) and exor stars. v346 normae is a well-studied fuor that underwent a strong outburst beginning around 1980. here, we report on photometric and spectroscopic observations, which show that the visual/near-infrared brightness has decreased dramatically between the 1990s and 2010 (δr ≈ 10.9 mag, δj ≈ 7.8 mag and δk ≈ 5.8 mag). the spectral properties of this fading event cannot be explained by variable extinction alone, but indicate a drop in accretion rate by two to three orders of magnitude. this is the first time that a member of the fuor class has been observed to switch to a very low accretion phase. remarkably, in the last few years (2011-2015) v346 nor has brightened again at all near-infrared wavelengths, indicating the onset of a new outburst event. the observed behaviour might be consistent with the clustered luminosity bursts that have been predicted by recent gravitational instability and fragmentation models for the early stages of protostellar evolution. given v346 nor's unique characteristics (concerning outburst duration, repetition frequency and spectroscopic diagnostics), our results also highlight the need to revisit the fuor/exor classification scheme. | v346 normae: first post-outburst observations of an fu orionis star |
non-linear evolution of the parametric instability of inertial waves inherent to eccentric discs is studied by way of a new local numerical model. mode coupling of tidal deformation with the disc eccentricity is known to produce exponentially growing eccentricities at certain mean-motion resonances. however, the details of an efficient saturation mechanism balancing this growth still are not fully understood. this paper develops a local numerical model for an eccentric quasi-axisymmetric shearing box which generalizes the often-used cartesian shearing box model. the numerical method is an overall second-order well-balanced finite volume method which maintains the stratified and oscillatory steady-state solution by construction. this implementation is employed to study the non-linear outcome of the parametric instability in eccentric discs with vertical structure. stratification is found to constrain the perturbation energy near the mid-plane and localize the effective region of inertial wave breaking that sources turbulence. a saturated marginally sonic turbulent state results from the non-linear breaking of inertial waves and is subsequently unstable to large-scale axisymmetric zonal flow structures. this resulting limit-cycle behaviour reduces access to the eccentric energy source and prevents substantial transport of angular momentum radially through the disc. still, the saturation of this parametric instability of inertial waves is shown to damp eccentricity on a time-scale of a thousand orbital periods. it may thus be a promising mechanism for intermittently regaining balance with the exponential growth of eccentricity from the eccentric lindblad resonances and may also help explain the occurrence of 'bursty' dynamics such as the superhump phenomenon. | non-linear hydrodynamic instability and turbulence in eccentric astrophysical discs with vertical structure |
short gamma-ray bursts (grbs) are thought to result from the merger of two neutron stars (nss) or an ns and a stellar mass black hole (bh). the final stages of the merger are generally accompanied by the production of one or more tidal "tails" of ejecta, which fall back onto the remnant-disk system at late times. using the results of a linear stability analysis, we show that if the material comprising these tails is modeled as adiabatic and the effective adiabatic index satisfies γ ≥ 5/3, then the tails are gravitationally unstable and collapse to form small-scale knots. we analytically estimate the properties of these knots, including their spacing along the tidal tail, the total number produced, and their effect on the mass return rate to the merger remnant. we perform hydrodynamical simulations of the disruption of a polytropic (with the polytropic and adiabatic indices γ equal), γ = 2 ns, by a bh and find agreement between the predictions of the linear stability analysis and the distribution of knots that collapse out of the instability. the return of these knots to the bh induces variability in the fallback rate, which can manifest as variability in the light curve of the grb and—depending on how rapidly the instability operates—the prompt emission. the late-time variability induced by the return of these knots is also consistent with the extended emission observed in some grbs. | variability in short gamma-ray bursts: gravitationally unstable tidal tails |
we study in detail the evolution of the 2015 outburst of gs 1354-64 (bw cir) at optical, uv and x-ray wavelengths using faulkes telescope south/las cumbres observatory global telescope network, small & moderate aperture research telescope system and swift. the outburst was found to stay in the hard x-ray state, albeit being anomalously luminous with a peak luminosity of lx > 0.15 ledd, which could be the most luminous hard state observed in a black hole x-ray binary. we found that the optical/uv emission is tightly correlated with the x-ray emission, consistent with accretion disc irradiation and/or a jet producing the optical emission. the x-ray spectra can be fitted well with a comptonization model, and show softening towards the end of the outburst. in addition, we detect a qpo in the x-ray light curves with increasing centroid frequency during the peak and decay periods of the outburst. the long-term optical light curves during quiescence show a statistically significant, slow rise of the source brightness over the 7 years prior to the 2015 outburst. this behaviour as well as the outburst evolution at all wavelengths studied can be explained by the disc instability model with irradiation and disc evaporation/condensation. | a `high-hard' outburst of the black hole x-ray binary gs 1354-64 |
the extent of the accretion disk in the low/hard state of stellar mass black hole x-ray binaries remains an open question. there is some evidence suggesting that the inner accretion disk is truncated and replaced by a hot flow, while the detection of relativistic broadened iron emission lines seems to require an accretion disk extending fully to the innermost stable circular orbit. we present comprehensive spectral and timing analyses of six nuclear spectroscopic telescope array and xmm-newton observations of gx 339-4 taken during outburst decay in the autumn of 2015. using a spectral model consisting of a thermal accretion disk, comptonized emission, and a relativistic reflection component, we obtain a decreasing photon index, consistent with an x-ray binary during outburst decay. although we observe a discrepancy in the inner radius of the accretion disk and that of the reflector, which can be attributed to the different underlying assumptions in each model, both model components indicate a truncated accretion disk that resiles with decreasing luminosity. the evolution of the characteristic frequency in fourier power spectra and their missing energy dependence support the interpretation of a truncated and evolving disk in the hard state. the xmm-newton data set allowed us to study, for the first time, the evolution of the covariance spectra and ratio during outburst decay. the covariance ratio increases and steeps during outburst decay, consistent with increased disk instabilities. | nustar and xmm-newton observations of the 2015 outburst decay of gx 339-4 |
v341 ara was recently recognized as one of the closest (d ≃ 150 pc) and brightest (v ≃ 10) nova-like cataclysmic variables. this unique system is surrounded by a bright emission nebula, likely to be the remnant of a recent nova eruption. embedded within this nebula is a prominent bow shock, where the system's accretion disc wind runs into its own nova shell. in order to establish its fundamental properties, we present the first comprehensive multiwavelength study of the system. long-term photometry reveals quasi-periodic, super-orbital variations with a characteristic time-scale of 10-16 d and typical amplitude of ≃1 mag. high-cadence photometry from thetransiting exoplanet survey satellite (tess) reveals for the first time both the orbital period and a 'negative superhump' period. the latter is usually interpreted as the signature of a tilted accretion disc. we propose a recently developed disc instability model as a plausible explanation for the photometric behaviour. in our spectroscopic data, we clearly detect antiphased absorption and emission-line components. their radial velocities suggest a high mass ratio, which in turn implies an unusually low white-dwarf mass. we also constrain the wind mass-loss rate of the system from the spatially resolved [o iii] emission produced in the bow shock; this can be used to test and calibrate accretion disc wind models. we suggest a possible association between v341 ara and a 'guest star' mentioned in chinese historical records in ad 1240. if this marks the date of the system's nova eruption, v341 ara would be the oldest recovered nova of its class and an excellent laboratory for testing nova theory. | bow shocks, nova shells, disc winds and tilted discs: the nova-like v341 ara has it all |
accreting binary black holes (bbhs) are multimessenger sources, emitting copious electromagnetic (em) and gravitational waves. one of their most promising em signatures is the light-curve modulation caused by a strong unique and extended azimuthal overdensity structure orbiting at the inner edge of the circumbinary disc (cbd), dubbed 'lump'. in this paper, we investigate the origin of this structure using 2d general-relativistic (gr) hydrodynamical simulations of a cbd in an approximate bbh space-time. first, we use the symmetric mass-ratio case to study the transition from the natural m = 2 mode to m = 1. the asymmetry with respect to m = 2 grows exponentially, pointing to an instability origin. we indeed find that the cbd edge is prone to a (magneto)hydrodynamical instability owing to the disc edge density sharpness: the rossby wave instability (rwi). the rwi criterion is naturally fulfilled at the cbd edge and we report the presence of vortices, which are typical structures of the rwi. the rwi is also at work in the asymmetric mass-ratio cases (from 0.1 to 0.5). however, the cbd edge sharpness decreases with a decreasing mass ratio, and so the lump. by proposing a scenario for this lump formation, our work further supports its existence in astrophysical cbds and potential source for an em signature of bbhs. finally, because the rwi is not caused by gr effects, it is also a robust candidate for the lump origin in cbds around non-compact objects, e.g. binary protostars. | on the origin of the lump in circumbinary discs |
the inner few parsecs of the milky way's galactic center contain the central accreting supermassive black hole, over a million stars, and multiple large gaseous structures. in the past, the structures at these length scales have generally been modeled independently of each other. it is consequently not well understood how these complex features interact with each other, nor how gas flows between the outer few parsecs and the inner subarcsecond region (1″ ≈ 0.04 pc). in this work, we present hydrodynamic simulations of the inner few parsecs of the galactic center that, for the first time, combine a realistic treatment of stellar winds and the circumnuclear disk (cnd) as they interact with the gravitational potential of the nuclear star cluster and sagittarius a*. we observe interactions of the stellar winds with the inner edge of the cnd, which leads to the growth of instabilities, induced accretion of cool gas from the inner edge of the disk, and the eventual formation of a small accretion disk of ~104-105 k within r ~ 0.1 pc. the formation of an inner disk qualitatively agrees with observations. this disk grows in radial extent and mass with time on ≳10 kyr timescales, with a growth rate of $m\propto {t}_{\mathrm{kyr}}^{3.5}$ . we discuss additional physical mechanisms not yet included in this work that can improve our model. | the inner 2 pc of sagittarius a*: simulations of the circumnuclear disk and multiphase gas accretion in the galactic center |
recent observations of the protoplanetary disc surrounding ab aurigae have revealed the possible presence of two giant planets in the process of forming. the young measured age of 1-4 myr for this system allows us to place strict time constraints on the formation histories of the observed planets. hence, we may be able to make a crucial distinction between formation through core accretion (ca) or the gravitational instability (gi), as ca formation time-scales are typically myr whilst formation through gi will occur within the first ≈104-105 yr of disc evolution. we focus our analysis on the 4-13mjup planet observed at r ≈ 30 au. we find ca formation time-scales for such a massive planet typically exceed the system's age. the planet's high mass and wide orbit may instead be indicative of formation through gi. we use smoothed particle hydrodynamic simulations to determine the system's critical disc mass for fragmentation, finding md,crit = 0.3 m⊙. viscous evolution models of the disc's mass history indicate that it was likely massive enough to exceed md,crit in the recent past; thus, it is possible that a young ab aurigae disc may have fragmented to form multiple giant gaseous protoplanets. calculations of the jeans mass in an ab aurigae-like disc find that fragments may initially form with masses 1.6-13.3mjup, consistent with the planets that have been observed. we therefore propose that the inferred planets in the disc surrounding ab aurigae may be evidence of planet formation through gi. | ab aurigae: possible evidence of planet formation through the gravitational instability |
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