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i will present observational theoretical work on the formation bulges and pseudo-bulges in galaxies at high and low redshifts. specifically, using the cosmological zoom-in simulations vela, i will show how typical galaxies on the star-forming main sequence at redshift 2 can form a bulge component through rapid gas inflows, triggered by mergers, counter-rotating accretion streams, or violent disk instabilities. a comparison with our sinfoni-hst observations of stellar mass and star-formation rate profiles support this picture. in the second part, i will present how bulges and pseudo-bulges assemble in the new cosmological simulation illustristng. in particular, i discuss which physical processes are important to set the diversity of bulges in milky way like halos. | bulge formation scenarios …perspectives from a high-z observer |
we model the optical/uv emission of the intermediate-mass black hole hlx-1, using three sets of hst observations (from 2010, 2012, 2013), together with xmm-newton and swift data. 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. by combining optical and x-ray data, in particular around its state transitions, we estimate a bh mass 2 x 10^4 msun, a spin parameter a/m 0.9 for a moderately face-on view, and a peak outburst luminosity 0.3 times the eddington luminosity. we discuss the discrepancy between the characteristic size inferred from the short x-ray timescale (r a few 10^{11} cm) and the characteristic size of the irradiated optical emitter (r sqrt[cos theta] 2 x 10^{13} cm). one possibility is that the optical emitter is a circumbinary disk; however, we disfavour this scenario because it would require a very small donor star. a more plausible scenario is that the disk is large but only the inner annuli are involved in the x-ray outburst cycle. we propose that the recurrent outbursts are caused by an accretion-rate oscillation driven by wind instability in the inner disk. we argue that the system has a long-term-average accretion rate of a few percent of eddington, just below the upper limit of the low/hard state; a wind-driven oscillation can trigger transitions to the high/soft state, with a recurrence period 1 year (much longer than the binary period, which we estimate as 10 days). the oscillation that dominated the system in the last decade is now damped such that the accretion rate more rarely reaches the level required to trigger a state transition. finally, we highlight similarities between the role of disk winds in hlx-1 and in the galactic bh v404 cyg. | explaining the outbursts of the intermediate-mass bh hlx-1 as a wind-driven instability |
redback millisecond pulsars (msps; hereafter redbacks) are a subpopulation of eclipsing msps in close binaries.the formation processes of these systems are not clear. the three pulsars showing transitions between rotation- andaccretion-powered states belong to both redbacks and transient low-mass x-ray binaries (lmxbs), suggesting apossible evolutionary link between them. through binary evolution calculations, we show that the accretion disksin almost all lmxbs are subject to the thermal-viscous instability during certain evolutionary stages, and theparameter space for the disk instability covers the distribution of known redbacks in the orbital period-companionmass plane. we accordingly suggest that the abrupt reduction of the mass accretion rate during quiescence oftransient lmxbs provides a plausible way to switch on the pulsar activity, leading to the formation of redbacks, ifthe neutron star has been spun up to be an energetic msp. we investigate the evolution of redbacks, taking intoaccount the evaporation feedback, and discuss its possible influence on the formation of black widow msps. | from transient low-mass x-ray binaries to redback millisecond pulsars |
recurring outbursts associated with matter flowing onto compact stellar remnants (such as black holes, neutron stars and white dwarfs) in close binary systems provide constraints on the poorly understood accretion process. taken as a whole, the light-curves of these outbursts are shaped by the efficiency of angular-momentum (and thus mass) transport in the accretion discs in these systems, which has traditionally been encoded in the α-viscosity parameter. numerical simulations of the magneto-rotational instability that is believed to be the physical mechanism behind this transport yield values of α of roughly 0.1-0.2, consistent with values inferred from observations of accreting white dwarfs. however, equivalent viscosity parameters have never been estimated for discs around neutron stars or black-holes (i.e., low-mass x-ray binaries) before. we have combined theory, advanced bayesian statistical techniques, and the observed x-ray light-curves to build an innovative methodology, which for the first time, measures the α-viscosity parameter in low-mass x-ray binary discs. here we report the results of applying this methodology to the galactic black-hole low-mass x-ray binary population. we discuss how these results suggest that strong accretion disc winds, with the ability to remove a substantial fraction of the disc mass, must exist in all accretion states throughout outbursts of accreting stellar-mass black-holes. | strong disc winds traced throughout outbursts in black-hole x-ray binaries |
the exponential shape of radial surface brightness profiles of disk galaxies has been observed and known for decades. however, the physical mechanism of its formation is not well understood. disturbances from bars and spiral arms, viscous accretion of gas, and interaction with surrounding galaxies can account for an exponential disk, but these existing theories all have limitations. experiments with the n-body simulation code gadget-2 show that exponential profiles can form out of various initial stellar density distributions in a disk containing massive scattering centers, which in a real galaxy can be massive clouds or stellar clusters. the timescale of the profile evolution is influenced by scattering center properties including mass, orbital radius, and spatial number density. cold disks with local gravitational instabilities can trigger temporary phase mixing and violent relaxation, which accelerate profile changes towards an exponential. this may be responsible for fast establishment of exponential profiles in disturbed dwarf galaxies. | formation of exponential profiles from stellar scattering investigated with n-body simulations |
atacama large millimeter/submillimeter array (alma) has revealed that the dust disk around the young star hl tau consists of multiple nearly concentric rings and gaps (alma partnership et al. 2015). the well separated rings indicate that the dust disk is geometrically thin, suggesting that the turbulence in the gas disk is weak, at least near the outer edge of the disk. on the other hand, the strong brgamma emission from this system points to strong gas accretion from the disk to the star, with the accretion rate of 10-7 msun yr-1 (beck et al. 2010). the simplest accretion model where the accretion is assumed to be steady and driven only by turbulence does not explain the strong gas accretion and the significant settling of the millimeter-sized dust particles simultaneously (pinte et al. 2016). here we show that additional gas accretion powered by magnetic disk winds could resolve this paradox. recent magnetohydrodynamic (mhd) simulations show that the magnetic diffusivity arising from weak ionization suppresses mhd turbulence. yet accretion takes place as winds launched from the disk surface accelerate via magnetic forces, extracting orbital angular momentum (e.g., bai & stone 2013; simon et al. 2013; zhu et al. 2015). based on the results of the recent mhd simulations, we construct a simple, one-dimensional magnetic accretion model where we incorporate the angular momentum transport due to both mhd turbulence and winds as well as the turbulent diffusion of dust particles along the disk's vertical direction. we find that our model simultaneously reproduces three important observational features of the hl tau disk--the high accretion rate onto the central star, the absence of evidence for gravitational instability, and the geometrically thin subdisk of millimeter-sized dust particles--when the strength of the magnetic field driving the disk wind falls within a certain range. direct observations of gas outflow from the disk surface and/or surface gas inflow toward the central star will provide further constraints on the mechanism of protoplanetary disk accretion. | testing magnetic accretion models against alma observations of the hl tau disk |
the importance of major merging at z>1 remains in question despite the rapid buildup and development of massive galaxies at these epochs. new theories and observations suggest that non-merging processes like disk instabilities may be even more important than previously thought at assembling bulges, producing clumps, and inducing morphological disturbances that may be misinterpreted as the product of major merging. we propose a novel search for tidal features on a complete sample of nearly 6000 massive z>1 galaxies from candels. our novel approach will use improved residual maps from multi-component fits to archival wfc3 images, and a new machine learning classification pipeline to robustly identify transient and faint tidal signatures: the hallmark sign of major merging. we will repeat this analysis on synthetic mock images to thoroughly quantify the impacts of cosmological dimming, and calibrate the observability timescale of tidal feature detections. our study will yield (1) definitive census of massive z>1 galaxies with merger tidal signatures from candels, (2) critically important calibrations for converting tidal detections into merger rates, (3) two novel software pipelines, (4) new stringent tidal-based merger rates that will provide critical tests of theoretical predictions on the relative importance of ex-situ (hierarchical merging) vs. in-situ (gas accretion, cooling, and star formation) assembly of massive galaxies. | mining candels for tidal features to measure major merging at cosmic high noon |
the formation of directly imaged planets at wide separations is unclear and has become a major unanswered question in the field of exoplanets. core accretion plus dynamical scattering, in situ formation through disk instability, and turbulent cloud fragmentation all leave dynamical imprints on the orbits of these planets. we will obtain new astrometry for known planets and a control sample of brown dwarf companions spanning 10-200 au to compare the orbital eccentricity distributions of these two populations. if planets formed in disks then they should have relatively circular orbits, whereas brown dwarfs should have a broader eccentricity distribution if formed like binary stars. | uncovering the formation of giant planets on wide orbits with astrometric orbit constraints |
a candidate companion to a very young star has been discovered in hst snapshot optical images. the object is projected at the outer radius of an edge-on protoplanetary disk and is aligned with the disk plane. keck lgs photometry results indicate the object has the same temperature as brown dwarf gq lupi b but with 10x less luminosity - consistent with a planetary mass companion. because the edge-on disk suppresses the light of the central star, the companion is uniquely accessible to follow-up studies with minimal starlight residuals. we propose hst/wfc3 imaging and spectroscopy of the system to 1) fully define the morphology of the disk scattered light, particularly at the disk outer edge near the companion; 2) search for halpha emission from the companion as evidence that it is actively accreting; and 3) complete spectral characterization of the companion using g141 spectroscopy. confirmation of a substellar spectrum, accretion, and disk interaction action would establish this object as a leading example of an accreting protoplanet at 100 au and offer support to models for planet formation by gravitational instability. | tracing interactions of a protoplanet with its circumstellar disk |
v630 cas and v1017 sgr are long-period cataclysmic variables withrare, long-duration, dwarf nova outbursts. the characteristics of theseoutbursts are likely shaped by the large physical size of the accretiondisks. they also appear to be luminous (> 10e33 ergs/s) x-ray sourcesin quiescence, severely challenging the disk instability model. herewe propose xmm-newton observations of these objects in quiescence toobtain x-ray spectra of sufficient quality to constrain the white dwarfmass and the accretion rate. we will thus test the hypothesis that alarge accretion disk can remain in quiescence while maintaining a highaccretion rate. we also propose to obtain fast uv photometry with theom, since uv flickering has emerged as a key diagnostic of accretionfor the related class of symbiotic stars. | v630 cas and v1017 sgr: probing accretion disks on intermediate scales |
we perform a systematic study of the effect of sub-grid physics, resolution and structural parameters on the fragmentation of gas-rich galaxy discs into massive star forming clumps due to gravitational instability. we use the state-of-the-art zoom-in cosmological hydrodynamical simulation argo (fiacconi et al. 2015) to set up the initial conditions of our models, and then carry out 26 high resolution controlled simulations of high-z galaxies using the gasoline2 code, which includes a modern, numerically robust sph implementation.we find that when blast-wave feedback is included, the formation of long-lived, gravitationally bound clumps requires disc gas fractions of at least 50% and massive discs, which should have vmax > 200 km/s at z ∼ 2, more massive than the typical galaxies expected at those redshifts.less than 50 myr after formation, clumps have stellar masses in the range 4 × 106 - 5 × 107 m⊙.formation of clumps with mass exceeding ∼108 m⊙ is a rare occurrence, since it requires mergers between multiple massive clumps, as we verified by tracing back in time the particles belonging to such clumps. such mergers happen after a few orbital times (∼200-300 myr), but normally clumps migrate inward and are tidally disrupted on shorter timescales.clump sizes are in the range 100-500 pc. we argue that giant clumps identified in observations (∼109 m⊙ and 1 kpc in size) might either have a different origin, such as minor mergers and clumpy gas accretion, or their sizes and masses may be overestimated due to resolution issues.using an analytical model, already developed to explain the fragmentation scale in gravitationally unstable 3d protoplanetary discs, we can predict fairly accurately the characteristic gaseous masses of clumps soon after fragmentation, when standard toome analysis becomes invalid.due to their modest size, clumps have little effect on bulge growth as they migrate to the center. in our unstable discs a small bulge can form irrespective of the presence of long-lived clumps, since it is triggered by efficient gas inflows due to global non-axisymmetric instabilities in the form of global spiral modes. | a lower fragmentation mass scale for clumps in high redshift galaxies: a systematic numerical study |
kepler revealed the common existence of tightly packed super earth systems around solartype stars, existing entirely inside the orbit ofour venus. those systems must be stable for the ages of their host stars (~10^9 years); their formation mechanism must provide interplanet spacings that permit longterm stability. if one postulates that most planetary systems form with tightlypacked inner planets, their current absence in some systems could be explained by the collisional destruction of the inner system after a period of metastability. we posit that our solar system also originally had a system of multiple planets interior to the orbit of venus. this would resolve a known issuethat the energy/angular momentum of our innerplanet system is best explained by accreting the current terrestrial planets from a disklimited to 0.71.1 au; in our picture the disk material closer to the sun also formed planets, but they have since been destroyed. by studyingthe orbital stability of systems like the known kepler systems, we demonstrate that orbital excitation and collisional destruction could beconfined to just the inner parts of the system. in this scenario, mercury is the final remnant of the inner system's destruction via aviolent multicollision (and/or hitandrun disruption) process. this would provide a natural explanation for mercury's unusually higheccentricity and orbital inclination; it also fits into the general picture of longtimescale secular orbital instability, with mercury’scurrent orbit being unstable on 5 gyr time scales. the common decade spacing of instability time scales raises the intriguing possibilitythat this destruction occurred roughly 0.6 gyr after the formation of our solar system and that the lunar cataclysm is a preserved record ofthis apocalyptic event that began when slow secular chaos generated orbital instability in our former superearthsystem. | consolidating and crushing exoplanets: did it happen here? |
accretion disks likely provide the conduit for fueling active galactic nuclei (agn), linking the black hole's immediate surroundings to the host galaxy's nuclear star cluster, and possibly beyond. yet detailed agn disk models fail to explain several of the most basic observational features of agn: how do the outer regions of the disk avoid stalling as a result of wholesale gravitational fragmentation? what regulates the amount of star formation that is inferred to accompany accretion in some agn? why is the broad emission line region a ubiquitous feature of luminous agn? what processes create and maintain the so-called "dusty torus"? analytic work suggests that vertical pressure support of the disk primarily by a toroidal magnetic field, rather than by gas or radiation pressure, can readily resolve these problems. and recent numerical simulations have indicated that such a strong toroidal field is the inevitable consequence of the magnetorotational instability (mri) when a disk accumulates a modest amount of net magnetic flux, thus providing a sound theoretical basis for strongly magnetized disks. we propose an analytic and computational study of such disks in the agn context, focusing on: (1) the basic physical properties of strongly magnetized agn disks. we will focus on the competition between field generation and buoyancy, improving on previous work by considering realistic equations of state, dissipative processes and radiative losses. we will use global simulations to test the limiting magnetic fields that can be produced by mridriven accretion disk dynamos and explore the driving mechanisms of disk winds and the resulting levels of mass, angular momentum and energy loss. (2) gravitational fragmentation and star formation in strongly magnetized disks. we will determine how a strong field reduces and regulates gravitational fragmentation, by both lowering the disk density and creating a stratified structure in which star formation near the equator can co-exist with accretion at large heights. using simulations, we will study fragmentation conditions, the clumpiness of stable agn disks, and the mass function of collapsed clumps. (3) physics of the broad emission line region and dusty torus . we will study the possible role of the strong toroidal field in promoting thermal instabilities to create dense lineemitting filaments, transporting them in height, and confining the line-emitting gas. extrapolating to slightly larger distances, we will examine whether the field can elevate dusty gas to heights at which it can reprocess a substantial fraction of the agn radiation. this study will establish a new theoretical framework for interpreting multi-wavelength observations of agn, involving nasa s infrared, ultraviolet and x-ray observatories as well as ground-based detectors. it addresses fundamental questions about how supermassive black holes interact with their galactic environments, as well as broader issues of feedback and black hole-galaxy co-evolution. | strongly magnetized accretion disks in active galactic nuclei |
the mass-metallicity-sfr (m-z-sfr) relation for galaxies gives insight into the accretion and outflow of gas. heightened accretion should increase disk turbulence and the corresponding jeans mass for gravitational instabilities, making star-formation clumpy, and it should also trigger star formation directly in large clumps where infalling clouds impact the disk. it follows that if the most irregular and clumpy galaxies are the most actively accreting, then they should be low-z outliers in the m-z-sfr relation. we propose to investigate for the first time whether any relationship exists between position on the m-z-sfr relation and clumpy morphology. we will use wfc3/ir grism observations from several large surveys in the candels fields to measure metallicities of intermediate redshift (1.3 | the role of galaxy morphology in the mass-metallicity-sfr relation |
tidal disruption events, which occur when a star is destroyed by the gravitational field of a supermassive black hole, are unique probes of the inner regions of galaxies. in this thesis we explore various stages of the tidal disruption process, in an attempt to relate the observable signatures of tidal disruption events to the properties of the disrupted star and the black hole. we use numerical techniques to study the long-term evolution of the debris streams produced from tidal disruption events, showing that they can be gravitationally unstable and, as a result of the instability, fragment into small-scale, localized clumps. the implications of this finding are discussed, and we investigate how the thermodynamic properties of the gas comprising the stream affect the nature of the instability. we derive an analytic model for the structure of tidally-disrupted, stellar debris streams, and we compare the predictions of our model to numerical results. we present a model for the accretion disk that forms from a tidal disruption event when the accretion rate surpasses the eddington limit of the supermassive black hole, showing that these disks are puffed up into quasispherical envelopes that are threaded by bipolar, relativistic jets. we compare the predictions of this model to observations of the jetted tidal disruption event swift j1644+57. finally, we derive, from the relativistic boltzmann equation, the general relativistic equations of radiation hydrodynamics in the viscous limit, which characterize the interaction between radiation and matter when changes in the fluid over the photon mean free path are small. our results demonstrate that, in contrast to previous works, a radiation-dominated fluid does in fact possess a finite bulk viscosity and a correction to the comoving energy density. using the general relativistic equations of radiation hydrodynamics in the viscous limit, we present two models to describe the interaction between a relativistic jet launched during a tidal disruption event and its surroundings. these models show that regions of very large shear that arise between the fast-moving out ow and the surrounding envelope possess fewer scatterers and a harder photon spectrum, meaning that observers looking "down the barrel of the jet" infer vastly different properties of the outflow than those who look off-axis. | the evolutionary pathways of tidal disruption events: from stars to debris streams, accretion disks, and relativistic jets |
more than half of all stars are thought to be in binary or multiple star systems. but how do these systems form? the misaligned spins of some binary protostars might provide a clue.two formation modelsits hard to tell how multiple-star systems form, since these systems are difficult to observe in their early stages. but based on numerical simulations, there are two proposed models for the formation of stellar binaries:turbulent fragmentationturbulence within a single core leads to multiple dense clumps. these clumps independently collapse to form stars that orbit each other.disk fragmentationgravitational instabilities in a massive accretion disk cause the formation of a smaller, secondary disk within the first, resulting in two stars that orbit each other.log column density for one of the authors simulated binary systems, just after the formation of two protostars. diamonds indicate the protostar positions. [adapted from offner et al. 2016]outflows as clueshow can we differentiate between these formation mechanisms? led by stella offner (university of massachusetts), a team of scientists has suggested that the key isto examine the alignment of the stars protostellar outflows jets that are often emitted from the poles of young, newly forming stars.naively, wed expect that disk fragmentation would produce binary stars with common angular momentum. as the stars spins would be aligned, they would therefore also launch protostellar jets that were aligned with each other. turbulent fragmentation, on the other hand, would cause the stars to have independent angular momentum. this would lead to randomly oriented spins, so the protostellar jets would be misaligned.snapshots from the authors simulations. left panel of each pair: column density; green arrows giveprotostellar spin directions. right panel: synthetic observations produced from the simulations; cyan arrows giveprotostellar outflow directions. [offner et al. 2016]simulations of fragmentationin order to better understand the alignment of protostellar outflows during binary formation, offner and collaborators conduct a series of numerical simulations of the process of turbulent fragmentation.the teams radiation-magnetohydrodynamics simulations start with a spherical core with random turbulent velocities within it. the simulations then follow the formation of seeds within the core, which accrete mass and eventually launch protostellar outflows.in total, offner and collaborators run twelve simulations, in which five produce single stars, five produce binaries, and two produce triplestar systems.comparison to observationscumulative density function of the angles between simulated binary pairs protostellar outflows. the black line is the masses data (observations of actual binaries). the alignments from the simulations are consistent with the real observational data. [offner et al. 2016]as a final step, the authors generate synthetic observations from their simulations, to demonstrate what the protostellar outflows would look like. they then compare these to real observations of outflow orientations in young binaries from a survey known as masses.statistical analysis shows that the protostellar jets in the authors simulations are consistent with being randomly aligned or misaligned. this confirms what we would expect since the systems formed at wide separations from separate gravitational collapse events and the alignment distribution is consistent with observations of binaries in masses.offner and collaborators work in this study indicates that the presence of misaligned binaries in observations supports turbulent fragmentation as the mechanism for binary formation. the authors caution, however, that were dealing with small-number statistics: masses consists of only 19 binary pairs. the next step is to obtain a larger sample of observations for comparison.citationstella s. r. offner et al 2016 apj 827 l11. doi:10.3847/2041-8205/827/1/l11 | exploring the birth of binary stars |
relatively evolved (~ 1 myr old) protostars with little residual natal envelope, but massive disks, are commonly assumed to be the sites of ongoing planet formation. critical to our study of these objects is information about the available mass reservior and dust structure, as they directly tie in to how much mass is available for planets as well as the modes of planet formation that occur (i.e., core-accretion vs. gravitational instability). millimeter-wave observations provide this critical information as continuum emission is relatively optically thin, allowing for mass estimates, and the availability of high-resolution interferometry, allowing structure constraints. we present high-resolution observations of the population of class ii protostars in the rho-ophiuchus cloud (d ~ 130 pc). our survey observed ~50 of these older protostars at 870µm, using the atacama large millimeter/submillimeter array (alma). out of these sources, there are ~10 transition disks, where we see a ring of dust emission surrounding the central protostar -- indicative of ongoing planet formation -- as well as many binary systems. both of these stages have implications for star and planet formation. we present results from both 1-d and 2-d disk modeling, where we try to understand disk substructure that might indicate on-going planet formation, in particular, transition disk cavities, disk gaps, and asymmetries in the dust emission. | an alma survey of planet forming disks in rho ophiuchus |
previously we demonstrated that collisions between clumps of gas in the circum-nuclear disc can reduce their angular momentum and set some of the clumps on a plunging trajectory towards the supermassive black hole. if the central luminosity is determined by the gas accretion mechanism, then there exists a certain range of accretion rate and efficiency that allow the thermal instability to sustain the mass inflow through the two-temperature medium. here we explore the stellar component of the nuclear star cluster which acts as an additional source of heating and contributes an additional energy input into the gaseous environment in the galactic center minispiral region. under these conditions we discuss the values of relevant parameters that can support or suppress the thermal instability. | influence of stellar component on the conditions for thermal instability in the galactic center minispiral region |
accretion is a ubiquitous process in astrophysics. in cases when the magnetic field is not too strong and a disk is formed, accretion can proceed through the mid plane all the way to the surface of the central compact object. unless that compact object is a black hole, a boundary layer will be formed where the accretion disk touches its surfaces. the boundary layer is both dynamically and observationally significant as up to half of the accretion energy is dissipated there.using a combination of analytical theory and computer simulations we show that angular momentum transport and accretion in the boundary layer is mediated by waves. this breaks with the standard astrophysical paradigm of an anomalous turbulent viscosity that drives accretion. however, wave-mediated angular momentum transport is a natural consequence of "sonic instability." the sonic instability, which we describe analytically and observe in our simulations, is a close cousin of the papaloizou-pringle instability. however, it is very vigorous in the boundary layer due to the immense radial velocity shear present at the equator.our results are applicable to accreting neutron stars, white dwarfs, protostars, and protoplanets. | astrophysical boundary layers: a new picture |
the study of am cvn stars - semi-detached he wd-wd binaries - has exploded in recent years thanks to long term light curves obtained by the palomar transient factory. systems are seen withbinary periods ranging from about 5 minutes to about an hour. am cvn stars are similar to dwarfnovae in that they can undergo accretion disk outbursts. systems with high dm/dt have steady disks in permanent outburst, whereas for very low dm/dt systems the disks are too cool to have outbursts.disk instability theory gives a specific prediction for the zone of instability, therefore by matchingthe observed zone with the theoretical one we constrain dm/dt(p_orb), the rate of mass transferversus orbital period. the inferred relation is consistent with expectations from stellar evolution.one also has predictions for the recurrence time for outbursts and outburst duration versus p_orbwhich can be compared to observations. | clues to the evolution of helium wd-wd binaries from the palomar transient factory |
multi-wavelength monitoring of accretion-instability (outburst)evolution of x-ray binaries is key to understanding the role of theblack hole event horizon, neutron star (ns) surface and magnetic fieldand the spin of the compact object, in the context of accretion and jetproduction. we propose to monitor one transient ns x-ray binary duringan accretion outburst. our aim is to track the evolution of the jet powerand the jet break to determine the size and magnetic field of the regionwhere particles are first accelerated to high energies. furthermore,with the unique capabilities of xmm-newton, we will explore withspectral-timing techniques the evolution of the causal relationshipbetween thermal/non-thermal x-ray components that are at the core ofthe extreme disk/jet changes that we are witnessing. | the evolving multi-wavelength spectrum of a transient neutron star x-ray binary |
we present numerical analysis of the spin evolution of the neutron stars in low-mass x-ray binaries, trying to explain the discrepancy in the spin period distribution between observations of millisecond pulsars and theoretical results. in our calculations, we take account of possible effect of radiation pressure, and irradiation-induced instability on the structure of the disk, and the evolution of the mass transfer rate, respectively. we report the following results: (1) radiation pressure leads to a slight increase of the spin periods, and irradiation-induced mass transfer cycles can shorten the spin-down phase of evolution. (2) the calculated results in the model combining radiation pressure and irradiation-induced mass transfer cycles show that accretion is strongly limited by radiation pressure in high mass transfer phase. (3) the accreted mass and the critical fastness parameter can affect the number of systems in equilibrium state. | evolution of the spin periods of neutron stars in low-mass x-ray binaries |
in recent years, the increasing evidence that a significant fraction of white dwarfs is accreting matter from a debris disk has triggered a significant scientific interest. its mere existence suggests that the planetary system which had formed around the star was able to survive all previous phases of stellar evolution, including those implying dramatic size changes as well as mass loss events of the central star. the computation of accretion rates provides us important information about the original planetary system. unfortunately the present estimations do not take into account a physical process that may happen when heavy material falls ontop a lighter one, generating turbulences that dilutes the accreted material. this process affects directly the computed accretion rates, since if it takes place, larger accretion rates become necessary in order to explain the amount of surface contamination observed. in this work we present the results of numerical simulations that show that this destabilizing physical process actually occurs. its impact on an accreting da white dwarf model is presented. | the ultimate fate of planetary systems |
it is believed that low-mass stars build a significant fraction of their total mass during short outbursts of enhanced accretion (up to 10e-4 msolar /yr). the most dramatic episodic accretion events known in young stellar objects (ysos) are fu ori and exor outbursts. fu ori objects are characterized by a sudden brightening of 5 magnitudes or more within one year and remain bright for decades. exor objects have lower amplitude outbursts on shorter timescales (months to years). here we present an alma 230 ghz (1.3 mm / band-6) mini-survey of 8 outbursting sources (three fu ori and ve exor objects) in orion with 1" (450 au) resolution. we present continuum, 12co, 13co, and c18o line images and derive dust and (when possible) gas disk masses. the disk masses derived from the line observations are systematically lower (by factors of 3-5) than those calculated from the continuum and adopting the standard gas-to-dust ratio of 100, which agrees with results on t tauri disks in taurus. after beam deconvolution, we nd that the disks are remarkably compact (r = 70-150 au). the 1.3 mm fuxes of the outbursting sources span over three orders of magnitude, but the fu ori objects are signi cantly brighter than the exor objects. the inferred disk masses for the brightest objects are > 0.1 msolar , rendering gravitational instability a likely outburst mechanism. on the other hand, the inferred disk masses for the faintest targets are ~ 1-5 mjup , and thus an alternative mechanism must be responsible for their outbursts. | alma early science observations of outbursting stellar systems:disk masses for fu ori and exor objects |
we present the results of a study of the neutron star low-mass x-ray binary 4u 1636-536. we have performed temporal analysis of all available rxte/asm, rxte/pca, swift/bat and maxi data collected until 2017.we have confirmed the previously discovered quasi-periodicity of 45 d present during 2004, and we have found it continued to 2006. at other epochs, the quasi-periodicity is only transient, and the quasi-period, if present, drifts from 32 to 72 days. we have applied a time-dependent accretion disc model to the interval with the significant x-ray quasi-periodicity. although 4u 1636-536 is persistent, the observed quasi-periodicity can be well modelled by the hydrogen thermal-ionization instability occurring in outer regions of the accretion disc. for our best model, the period and the amplitude of the theoretical light curve agree well with those observed. | long-term quasi-periodicity of 4u 1636-536 resulting from accretion disc instability |
dh tau b is a planetary-mass companion orbiting the t tauri star dh tau (~0.5 solar masses) with a projected separation of 340 au. with an age of ~2 myrs, it is one of the youngest planetary-mass companions discovered to date. dh tau b straddles the deuterium-burning limit with an estimated mass of 8-22 jupiter masses, and like many directly imaged companions it is unclear whether it formed like a star (i.e., via turbulent fragmentation) or like a planet (i.e., either disk instability or core accretion). to shed light on dh tau b's formation history, we obtain the first-ever measurement of rotational line broadening for this object using high-resolution (r ~25,000) near-infrared spectroscopy from keck/nirspec. we then compare dh tau b's rotation rate with previously established trends in mass and age for both bound and free-floating planetary-mass objects, and find that dh tau b reinforces existing trends in rotation rates from this population. | measuring the rotation rate of the planetary-mass companion dh tau b |
the central object in a low-mass x-ray binary (lmxb) accretes matter from its companion star mainly via roche lobe overflow; the keplerian disk, having high angular momentum and high viscosity becomes instrumental in overall accretion. in the same transient lmxb, the accumulation radius, from where matter rushes towards the black hole due to thermal-viscous instability, may be different in different outbursts. using rxte/asm long-term data and employing a simple timing analysis, we examine more than a dozen outbursts in transient lmxbs, exhibiting either multiple outbursts (gx 339-4 and h 1743-322) or sporadic outbursts (e.g. 4u 1543-47, etc.). it is found that the keplerian disk indeed varies in its size from an outburst to another; this, in turn, dictates the brightness of outbursts. both canonical and anomalous/failed outbursts can be distinguished from our results. outbursts, in general, are shown to fall into two types, based on four features observed by us. | does the geometry of keperian disk vary during the outbursts of a transient lmxb? |
magnetized disk winds and wind-driven accretion are an essential and intensively studied dispersion mechanism of protoplanetary disks. however, the stability of these mechanisms has yet to be adequately examined. this paper employs semi-analytic linear perturbation theories under non-ideal magnetohydrodynamics, focusing on disk models whose magnetic diffusivities vary by a few orders of magnitude from the disk midplane to its surface. linear modes are distinguished by their symmetry with respect to the midplane. these modes have qualitatively different growth rates: symmetric modes almost always decay, while at least one anti-symmetric mode always has a positive growth rate. this growth rate decreases faster than the keplerian angular velocity with cylindrical radius $r$ in the disk and scales as $r^{-2}$ in the fiducial disk model. the growth of anti-symmetric modes breaks the reflection symmetry across the disk equatorial plane, and may occur even in the absence of the hall effect. in the disk regions where fully developed anti-symmetric modes occur, accretion flows appear only on one side of the disk, while disk winds occur only on the other. this may explain the asymmetry of some observed protoplanetary-disk outflows. | non-ideal magnetohydrodynamic instabilities in protoplanetary disks: vertical modes and reflection asymmetry |
the collapse of interstellar gaseous clouds towards a protostar leads to the formation of accretion disks around the central star. such disks can be dynamically stable if they settle in an axisymmetric state. in this letter, we investigate the long-term stability of astrophysical viscous disks around various protostars. we apply an implicit numerical code which solves the equations of radiation hydrodynamics and treats turbulence-induced viscosity according to the $\alpha$-viscosity model. we show how the viscosity is related to the disk mass. a stability criterion to determine the maximum disk mass can be formulated. we analyse such instabilities for a variety of radial points with different orbital distances from the host star and discuss the feedback on the disk in the event of an unstable protoplanetary disk. additionally, we examine the critical disk-mass for disks with variable outer boundaries and compare them to observations of protostellar disks in the upper scorpius ob association and near the lupus complex. we derive an easily applicable method to obtain an estimate for maximum disk masses when the outer disk radius ins known. | mass limits for stationary protoplanetary accretion disks |
ringed protoplanetary disks, in the class ii phase of low-mass star formation when the envelope has mostly dispersed, have been found in abundance in recent years with high-resolution alma observations. these ringed disks have been often interpreted as evidence of ongoing planet formation. in the younger class 0 and i phases there are few examples of high resolution dust disk observations due to the challenge of the dense envelope surrounding the disk, and more often than not reveal spiral-like structures. however, these embedded stages may be when the first steps of planet formation occur, and studying ringed structures in these phases will constrain the initial conditions of planet formation. we have used alma 1.3 mm long-baseline dust continuum observations to study the class i protostar irs 63 with 7 au resolution and expose the detailed physical structure of a class i disk. the alma data indicate that concentric dust rings are present in the disk, revealing irs 63 is the youngest-known protostellar disk with multiple ringed dust substructures and demonstrating that these features are already present in the class i phase. the dust ring structures could arise via several mechanisms including rapid pebble growth near snowlines, magnetorotational instabilities, asymmetric accretion from the envelope to disk, or planet-disk interactions carving gaps in the disk. even if planets have not yet formed, dust rings in disks at such an early evolutionary stage could provide a stable environment for long enough time scales to grow planets. | rings in a young embedded disk: footholds of planet formation at early times |
young stars harbor a disk of gas and dust orbiting them. similarly, young planets are surrounded by a circumplanetary disk (cpd). cpds supply gas and dust to the growing planet, and they are also the birthplace of moons. if we are to understand how forming planets accrete materials, what chemical compositions accreting gas has, and how and under which environments moons form, it is crucial to study cpds. despite the importance, observing cpds has been very challenging due mainly to their small sizes – for a jupiter-mass planet orbiting a solar-mass star at 100 au as an example, the diameter of the cpd is about 5 aus, corresponding to 0.03 arcsec at the distance of nearby star-forming regions (~140 pc). thanks to the angular resolution and sensitivity it offers, alma has been rapidly changing the situation and providing exciting opportunities to directly detect and characterize cpds. in this talk, i will present the first discovery of a cpd in molecular line observations, around a young, forming exoplanet as 209b. these observations provide solely needed observational constraints on the thermo-physical properties of cpd for the first time. i will conclude by discussing the potential formation mechanism of as 209b in the framework of two main channels of giant planet formation, namely gravitational instability and core accretion. | witnessing the formation of giant planets: the first discovery of a circumplanetary disk in alma molecular line observations |
we present observations and analysis of the starburst, pacs-819, at z=1.45 ($m_*=10^{10.7}$ m$_{ \odot}$), using high-resolution ($0^{\prime \prime}.1$; 0.8 kpc) alma and multi-wavelength jwst images from the cosmos-web program. dissimilar to hst/acs images in the rest-frame uv, the redder nircam and miri images reveal a smooth central mass concentration and spiral-like features, atypical for such an intense starburst. through dynamical modeling of the co j=5--4 emission with alma, pacs-819 is rotation-dominated thus has a disk-like nature. however, kinematic anomalies in co and asymmetric features in the bluer jwst bands (e.g., f150w) support a more disturbed nature likely due to interactions. the jwst imaging further enables us to map the distribution of stellar mass and dust attenuation, thus clarifying the relationships between different structural components, not discernable in the previous hst images. the co j = 5 -- 4 and fir dust continuum emission are co-spatial with a heavily-obscured starbursting core (<1 kpc) which is partially surrounded by much less obscured star-forming structures including a prominent arc, possibly a tidally-distorted dwarf galaxy, and a clump, either a sign of an ongoing violent disk instability or a recently accreted low-mass satellite. with spatially-resolved maps, we find a high molecular gas fraction in the central area reaching $\sim3$ ($m_{\text{gas}}$/$m_*$) and short depletion times ($m_{\text{gas}}/sfr\sim$ 120 myrs) across the entire system. these observations provide insights into the complex nature of starbursts in the distant universe and underscore the wealth of complementary information from high-resolution observations with both alma and jwst. | jwst and alma discern the assembly of structural and obscured components in a high-redshift starburst galaxy |
low mass x-ray binaries (lmxbs) are systems in which a compact object accretes from a binary companion star via an accretion disk. the x-ray properties of lmxbs show strong variability over timescales ranging from milliseconds to decades, much of which is tied to the extreme environment of the inner accretion disk, hence an understanding of this behaviour is key to understanding how matter behaves in such an environment. grs 1915+105 and mxb 1730-335 are two lmxbs which show particularly unusual variability. grs 1915+105 shows a large number of distinct classes of second-to-minute scale variability, consisting of repeated patterns of dips and flares. mxb 1730 shows type ii x-ray bursts; minute-scale increases in x-ray intensity with a sudden onset and a slow decay. more recently two new objects, igr j17091-3624 and gro j1744-28 have been shown to display similar behaviours. in this thesis i present a new framework with which to classify variability in igr j17091. i perform a comparison study between this source and grs 1915. in grs 1915, hard x-rays lag soft x-rays in all variability classes; in igr j17091, i find that the sign of this lag varies between variability classes. additionally, while grs 1915+105 accretes at close to its eddington limit, i find that igr j17091-3624 accretes at only ~5-33% of its eddington limit. i also perform a study of variability in gro j1744 and find that it is more complex than in mxb 1730, consisting of at least 4 separate phenomena which may have separate physical origins. one of these phenomena, `structured bursting', consists of patterns of flares and dips similar to those seen in grs 1915 and igr j17091. i compare these types of variability and discuss the possibility of a physical link. i also present the alternative hypothesis that structured bursting is caused my 'hiccup accretion' similar to that seen in systems approaching the propeller regime. | recurrent instability in lmxb accretion disks: how strange is grs 1915+105? |
magnetized plasma columns and extended magnetic structures with both foot points anchored to a surface layer are an important building block of astrophysical dissipation models. current loops shining in x-rays during the growth of plasma instabilities are observed in the corona of the sun and are expected to exist in highly magnetized neutron star magnetospheres and accretion disk coronae. for varying twist and system sizes, we investigate the stability of line-tied force-free flux tubes and the dissipation of twist energy during instabilities using linear analysis and time-dependent force-free electrodynamics simulations. kink modes ($m=1$) and efficient magnetic energy dissipation develop for plasma safety factors $q\lesssim 1$, where $q$ is the inverse of the number of magnetic field line windings per column length. higher order fluting modes ($m>1$) can distort equilibrium flux tubes for $q>1$ but induce significantly less dissipation. in our analysis, the characteristic pitch $p_0$ of flux tube field lines determines the growth rate ($\propto p_0^3$) and minimum wavelength of the kink instability ($\propto p_0^{-1}$). we use these scalings to determine a minimum flux tube length for the growth of the kink instability for any given $p_0$. by drawing analogies to idealized magnetar magnetospheres with varying regimes of boundary shearing rates, we discuss the expected impact of the pitch-dependent growth rates for magnetospheric dissipation in magnetar conditions. | safety first: stability and dissipation of line-tied force-free flux tubes in magnetized coronae |
protoplanetary discs (ppds) have been widely observed around young stars and are supposed to be the birth cradle of planets. these objects are cold, dense and magnetised. among them stand the so-called transition discs (tds) characterised by a dust cavity in their inner regions, whose diameter varies from a few au to a few hundreds au. the formation of such cavities remains unexplained. moreover, such cavities are not only detected in the dust radial density profiles, but they are also observed in the gas radial density profiles. a striking observation, challenging the intuition, states that in spite of their diminished surface density profile, ppds accrete merely as much as full ppds. their accretion rates are similar to one measured in standard ppds (\dot{m}ppd∼ 10-7 m⊙ yr-1). such an observation suggests a fast inward motion of matter inside the cavity. a possible explanation for these high accretion rates is the presence of magnetised winds that would allow matter to fall onto the star at high radial velocity. the aim of my work is to address this observational discrepancy using magnetohydrodynamic (mhd) winds to account for the accretion in tds. with my contribution, i showed with global numerical simulations how wind-emitting cavity-hosting discs could support a fast inner accretion. i present the results of 2.5 d and 3d simulations modelling tds with non-ideal magnetic winds. i focus on mass accretion through the cavity and on the 3 d stability of this cavity against hydro and magnetohydrodynamic instabilities. | magnetised winds in transition discs |
context. stellar winds of massive stars are known to be driven by line absorption of uv photons, a mechanism that is prone to instabilities, causing the wind to be clumpy. the clumpy structure hampers wind mass-loss estimates, limiting our understanding of massive star evolution. the wind structure also impacts accretion in high-mass x-ray binary (hmxb) systems.aims: we aim to analyse the wavelength-dependent variability of x-ray absorption in the wind to study its structure. such an approach is possible in hmxbs, where the compact object serves as an x-ray backlight. we probe different parts of the wind by analysing data taken at superior and inferior conjunctions.methods: we applied excess variance spectroscopy to study the wavelength-dependent soft (2-14 å) x-ray variability of the hmxb cygnus x-1 in the hard spectral state. excess variance spectroscopy quantifies the variability of an object above the statistical noise as a function of wavelength, which allows us to study the variability of individual spectral lines. this technique was applied to high-resolution gratings spectra provided by chandra, accounting for various systematic effects. the frequency dependence is investigated by changing the time binning.results: the strong orbital phase dependence we observe in the excess variance is consistent with column-density variations predicted by a simple model for a clumpy wind. we identify spikes of increased variability with spectral features found by previous spectroscopic analyses of the same data set, most notably from silicon in over-dense clumps in the wind. in the silicon line region, the variability power is redistributed towards lower frequencies, hinting at increased line variability in large clumps. in prospect of the microcalorimetry missions that are scheduled to launch within the next decade, excess variance spectra present a promising approach to constraining the wind structure, especially if accompanied by models that consider changing ionisation. | stellar-wind variability in cygnus x-1 from high-resolution excess variance spectroscopy with chandra |
analytical studies of black hole accretion usually presumes the stability of the stationary transonic configuration. various authors in the past several decades demonstrated the validity of such an assumption for inviscid hydrodynamic flow. inviscid approximation is a reasonable approach for low angular momentum advection dominated flow in connection to certain supermassive black holes at the centres of the galaxies (including our own) fed from a number of stellar donors. introduction of a weak viscosity, as a first order linear correction involving the viscosity parameter, however, may sometimes provide a more detail understanding of the observed black hole spectra. the transonic behaviour of the stationary solutions have been studied for the aforementioned quasi-viscous accretion for all possible geometric configurations of axisymmetric flow. for a sufficiently low range of the viscosity parameter, transonic solutions containing one or three critical points have been found for allowed ranges in the astrophysical parameters under the post-newtonian pseudo-schwarzschild scheme. with the introduction of such viscosity parameter, the only feasible critical points are of saddle and spiral types in contrary to the inviscid case where centre type points were formed instead of spiral ones. introduction of linear perturbations on stationary flow solutions and their time evolution in both standing and radially propagating wave forms have been examined (completely analytically) in detail. our analysis shows that similar kind of secular instability exists in all the considered disk models at large distance in the asymptotic limit, however, the model itself is valid only within a certain length scale and the disks sustain within that length scale only for at least a considerable time scale. | transonic behaviour and stability analysis of quasi-viscous black hole accretion |
during the rosat and xmm-newton sky surveys a very small number of giant (factor >100), soft, x-ray flares have been seen in galactic nuclei. their very rarity shows that they are not produced by a common agn process. while they all tend to show similar decay curves, the rise and plateau sections of the flare vary considerably between sources. flares which rise and decay on a timescale of weeks to months fit easily into the category of tidal disruption events (tde) where the black hole is temporarily fueled by stellar debris. but what about events, such as ngc 3599 and gsn 069 which have a peak flux lasting for years or ic 3599 where the flare repeated after 20 years ? the behavior of these three galaxies is similar to that of black hole binaries such as grs 1915+105, where the flare is commonly attributed to an accretion disc instability, leading to an emptying and refilling of the inner disc. however, recent numerical simulations indicate that tde occurring in galaxies with m_{bh}<6×10{6} solar masses, may also have light curves which take years to reach peak flux. we explore both of these possibilities in detail. | massive, long-duration, soft x-ray flares from galactic nuclei |
system parameters are re-determined: $m_1=0.86\pm0.18m\odot$, $m_2=0.103\pm0.022m\odot$, $a=1.508\pm 0.100\times 10^{10}$cm, and $i=69\pm3^{\circ}$. the secondary component is a semi-degenerate helium star loosing mass at a rate $\dot m=4.93\pm 1.65\times10^{-9}m\odot/yr$. the accretion disk is sufficiently hot to avoid thermal instability. the orbital light curve recovered from observations made in 1962 shows minimum shifted to phase $\phi=0.50$, corresponding to $o-c=0.0060$d. together with mimima observed in 1992-99 this implies that the orbital period is increasing at a rate $dp/dt\approx 8.5 \times 10^{-13}$ consistent with predictions involving the emission of gravitational waves. | am cvn -- system parameters and gravitational waves |
although during the last decade new observations and new theoretical results have brought better understanding of the physics of accretion onto compact objects, many old and several new questions and problems await answers and solutions. i show how the disc thermal-viscous instability model applied to both cataclysmic variable stars and x-ray binary transients compels us to conclude that assuming the existence in these systems of a flat accretion disc extending down to the accretor's surface or to the last stable orbit and fed with matter at its outer edge is too simple and inadequate a description of these objects. it is also clear that, in most cases, these discs cannot driven by (anomalous) viscosity only. the origin of the superhumps observed in cataclysmic variables and x-ray binaries is, contrary to the common opinion, still unknown. in accreting magnetic white dwarf systems outbursts not of the dwarf-nova type can be due to the magnetic gating instability and/or thermonuclear micronova explosions. although the "typical" lightcvurves of x-ray transients can be described by analytical formulae (but their decay phase is not exponential), observations show that in many cases the light variations in these systems are much more complex. an elementary argument shows the impossibility of magnetars in pulsing ultraluminous x-ray systems, but we still do not have a complete, self-consistent description of supercritical accretion onto magnetized neutron stars and the resulting (necessarily beamed) emission. although it is (almost) universally believed that active galactic nuclei contain accretion discs of the same type as those observed in binary systems, the evidence supporting this alleged truth is slim and the structure of accretion flows onto supermassive black holes is still to be determined. | problems in the astrophysics of accretion onto compact celestial bodies |
studies over the last two decades have revealed that the comoving star formation rate (sfr) and the black hole accretion rate densities have been steeply declining since z~2. tracing the evolution of the cold molecular gas which fuels star formation and black hole accretion in galaxies at intermediate redshift (0.5<z<1) is therefore essential to obtain a coherent picture connecting high-z galaxies with their present-day descendants. we present co j=2-1 and 3-2 line observations in the quadruply-lensed quasar rxj1131-1231 at z~0.65 obtained using the plateau de bure interferometer (pdbi) and the combined array for research in millimeter-wave astronomy (carma), making this the first resolved co study at intermediate redshift. we perform dynamical lens modeling of the co emission in the visibility-plane using our code uvmcmcfit (github.com/astro313/uvmcmcfit), finding that the asymmetry in its double-horned line profile is a result of differential lensing, with a magnification factor varying from ∼3 to ∼9 across different kinematic components. we recover an intrinsically symmetric line profile and a source-plane velocity gradient that suggest the presence of an extended, ~6kpc radius gas disk with a dynamical mass of ∼8×1010m⊙, a gas mass of ~1.5×1010m⊙, and a gas mass fraction of ∼19% in rxj1131-1231. the modest gas fraction is consistent with the observed trend of decreasing molecular gas content in star-forming galaxies since z∼2. based on our spectral energy distribution (sed) modeling, we find a lensing-corrected stellar mass of ∼3×1010m⊙ and a sfr of ~120 m⊙ yr-1, a rate comparable to those of local mergers and high-z disk galaxies. the co source size, gas depletion timescale and star formation efficiency of rxj1131-1231 suggest that its star formation is driven by global gravitational instabilities rather than merger interactions. we also find a black hole-to-bulge mass ratio of >0.27%, which is higher than those of local galaxies, suggesting that its black hole mass is largely in place while its stellar bulge is still assembling. our results thus support the emerging picture that quasars grow faster and/or earlier than their host galaxies at earlier epochs. | gas dynamical imaging and dust properties of the strongly-lensed quasar host galaxy rxj1131-1231 at z~0.65 |
we show a relation of the pre- and post-nova activity of ct ser (nova 1948) and v446 her (nova 1960), known to have remarkably similar orbital periods p orb. we use both photographic and ccd photometric observations. post-nova ct ser shows a nova-like high-state activity with only a slight decrease of absolute magnitude m opt. in the interpretation, its m opt is above the region of the thermal-viscous instability (tvi) regime, so the disk is ionized out to its outer rim, similarly to before its classical nova (cn) outburst. on the contrary, v446 her started its cn outburst from the tvi regime. extensive changes in activity type in years after cn outburst were observed in v446 her. the features consistent with the dwarf-nova outbursts appeared at most three decades after the end of the cn outburst. it suggests that its disk transitioned into the tvi region in that it was before its cn outburst. the pre-nova and post-nova states of activity did not change significantly for a given system, except for several decades after the cn outburst. even pre-novae and post-novae with mutually similar p orb, hence secondary star (donor) spectral types and dimensions and disk radii, can show largely discrepant m opt and disk states in the similar epochs (several decades) surrounding the cn outburst. | relation between long-term activity and luminosity of the pre- and post-novae ct ser and v446 her |
x-ray flux and pulse period fluctuations in an accretion-powered pulsar convey important information about the disk-magnetosphere interaction. it is shown that simultaneous flux and period measurements can be analysed with a kalman filter based on the standard magnetocentrifugal accretion torque to generate accurate time-dependent estimates of three hidden state variables, which fluctuate stochastically and cannot be measured directly: the mass accretion rate, the maxwell stress at the disk-magnetosphere boundary, and the radiative efficiency of accretion onto the stellar surface. the inferred fluctuation statistics carry implications for the physics of hydromagnetic instabilities at the disk-magnetosphere boundary and searches for continuous gravitational radiation from low-mass x-ray binaries. | tracking hidden magnetospheric fluctuations in accretion-powered pulsars with a kalman filter |
the neutron star (ns) in a high mass x-ray binary (hmxb) commonly accretes from the stellar wind of its companion. observations suggest that the accretion flow around some sources (e.g. oao 1657-415) should be highly structured, which may indicate the existence of accretion disks. the ns accretion flow in a hmxb can be simplified as a bondi-hoyle-lyttleton (bhl) accretion flow with imposed transverse upstream gradient. we use 2d axisymmetric and 3d simulations to investigate the problem of bhl accretion with and without upstream gradient, focusing on the regime of high (upstream) mach number, small accretor size and weak upstream gradient, which is relevant to many observed hmxb systems but has not been explored with simulations before. when there is no upstream gradient, we find the accretion flow to be always stable. however, when the upstream gradient is small but nonzero, the flow is significantly more prone to instability than previously expected. for small accretor size, the unstable flow is highly turbulent, reducing accretion rate and preventing disk formation. the instability we observe is different from the "flip-flop" instability of 2d planar bhl accretion, and the physical origin of the instability is discussed. we also investigate the contribution of orbital effects not captured in the bhl model (e.g. coriolis force), and find that they tend to increase stability. comparing our results with previous studies, we discuss the stability of the flow and the possibility of disk formation in different regimes of the parameter space. in general, a highly structured flow can develop when the upstream gradient is sufficiently large, but forming an accretion disk is difficult. | wind accretion in high mass x-ray binaries: stability and disk formation |
dwarf novae are interacting binaries in which outbursts are caused by thermal instability in the accretion disk around the white dwarf [1]. the sub-class su uma type is characterized by two types of outbursts; during a brighter and longer outburst oscillations with periods slightly exceeding the orbital (superhumps) are detected. the gap in the statistical distribution of dwarf novae according to their orbital periods is distinguished between 2 and 3 hours [2] with a reduced number of dwarf novae in this interval. during the superoutburst of asassn-19fy in 2020, based on photometry carried out by crimean observatory telescopes (1.25-m telescope azt-11 and 0.38-m telescope k-380), 1-m telescope on sanglokh observatory and aavso data, the mean period of positive superhumps was 0.09278 days. this defines the system as a long-period dwarf nova in the period gap. the transition between stages b and c of the superhumps evolution [3] was identified. it occurred on the fast extinction of the superoutburst.the superhump period increased at a rate p˙ = 13.2 × 10−4 at stage b. two rebrightenings occurred at stage c. by the superhump period asassn-19fy belongs to the su uma type dwarf nova with the highest mass ratio. however, its other characteristics (rebrightenings and long return to a quiescence) are inherent in the wz sge type dwarf nova, which have the smallest mass ratio. this peculiarities is probably due to the weakness of the tidal effect in accretion disk in both long-period su uma type and sort-period wz sge type stars. | asassn-19fy: the unusual dwarf nova in the period gap |
v1363 cyg is a cataclysmic variable (cv) and a post-nova. our analysis of its long-term optical activity used the archival data from the aavso database and literature. we showed that the accretion disk of v1363 cyg is exposed to the thermal-viscous instability (tvi) for at least part of the time. the time fraction spent in the high state or the outbursts dramatically changed on the timescale of decades. the highly variable brightness of v1363 cyg displayed several episodes of a strong brightening (bumps in the light curve) from a cool disk in the tvi zone. this can be interpreted to mean their vastly discrepant decay rates show that only some of these bumps can be attributed to the dwarf nova outbursts without strong irradiation of the disk by the hot white dwarf. the bailey relation of the decay rate, if ascribed to a dn outburst of v1363 cyg, speaks in favor of its orbital period porb being very long for a cv, about 20-40 h. a dominant cycle length of about 435 d was present in the brightness changes all the time, even when the disk was well inside the tvi zone. we interpret it as modulation of the companion's mass outflow by differential rotation of the active region(s). | activity of the post-nova v1363 cygni on long timescales |
a variety of astrophysical objects can launch jets: x-ray binaries, active galactic nuclei, gamma-ray bursts, and dwarf novae. jetted astrophysical systems typically possess an accretion disk, with the jet-launching compact object at the center. the radiative powers emitted by the jets and their accretion disks are highly correlated. this correlation implies that accretion and ejection processes are dynamically linked. the magnetic field could provide this link because it is essential for both accretion and ejection. indeed, the magnetic field drives accretion through the magneto rotational instability (mri), launches, and accelerates the jets. however, the structure and origin of the magnetic fields are not well understood. namely, it is a puzzle how the accretion disk manages to transport the magnetic fields from large distances to the compact object. surprisingly this transport appears to be robust and effective across a wide range of central compact object types, e.g., black holes and neutron stars. using first-principles 3d general relativistic magnetohydrodynamic (grmhd) simulations, i investigate the nature of magnetic field transport in both neutron star and black hole accretion systems. i find that the direction of the magnetic transport changes sign between these two qualitatively different systems. i will first detail how the accretion disk advects the magnetic flux inwards towards the black hole. i will show how the magnetic flux transport emerges from the balance between advection and diffusion. i will report, on the surprising result, that while near a black hole the advection brings the magnetic flux inward, and diffusion expels it outward, the situation is the opposite near neutron stars! more specifically, near a neutron star, the diffusion is responsible for bringing the magnetic flux in, and the advection pushes it outwards. these results provide important insights into the robustness of the jet launching mechanism. | magnetic field transport in accretion disk: black holes vs neutron stars |
we discuss different scenarios to explain multiple >1.5 magnitude flares in agn detected with gaia such as accretion disk instabilities, variable absorption in the line of sight, tidal disruption events, or a stellar-mass black hole — neutron star merger in the agn disc. interestingly, the rise and decay times of ~300 and ~900 days are similar for at least three events. the transients are spatially consistent with the nuclei of agns in all cases. we present the detailed analysis of the properties of one of the sources, with multi-wavelength x-ray, optical, nir and ir photometry and optical spectroscopy: during its outburst the spectrum of the source shows a strong blue continuum that fades over time and broad balmer lines with complex, multi-peaked line profiles, in contrast to its archival spectrum where the same lines showed a much simpler morphology. the timescales associated with these events and their integrated energy output are difficult to explain within the framework of known agn variability. | peculiar agn outbursts detected by gaia |
from planets and newborn stars to the evolution of entire galaxies, many astrophysical objects grow and evolve by accumulating mass through a disc. for these objects to grow, matter must both lose angular momentum to flow inward, and avoid being removed from the system via different types of outflows. however, despite decades of research, our understanding of the detailed physics governing angular momentum (and mass) transport in, and the physical mechanisms that drive outflowing matter from, such discs remains fragmented due to the limits of theoretical work and missing observational constraints. recurring outbursts associated with matter flowing onto compact stellar remnants (black holes, neutron stars and white dwarfs) in compact binaries provide strong test beds for constraining key aspects of this poorly understood accretion process. in this talk i will review: (i) our current understanding of compact binary outbursts according to the disc-instability model, (ii) how the fundamental predictions of this model can be tested using observations, and (iiii) the use of observed multi-wavelength spectra and light-curves, taken during these bright outbursts, as powerful diagnostics to probe the mechanisms driving mass inflow and outflow in these binary systems. | binaries, accretion, and outbursts |
we consider the gravitational potential of a rotating star with non-uniform density to derive the orbital and epicyclic frequencies of the particles orbiting the star. we assume that the star is composed of concentric spheroids of constant density, with a global power-law distribution of density inside the star. at the lowest order approximation, we recover the known result for the maclaurin spheroid that the maximum in the radial epicyclic frequency occurs at $r=\sqrt{2}ae$, for eccentricities ${\ge} 1/\sqrt{2}$. we find that the nature of these characteristic frequencies differs based on the geometry of the rotating star. for an oblate spheroid, the orbits resemble retrograde kerr orbits and the location of the radial epicyclic maximum approaches the stellar surface as the density variation inside the star becomes steeper. on the contrary, orbits around a prolate spheroid resemble prograde kerr orbits, but the marginally stable orbit does not exist for prolate-shaped stars. the orbital frequency is larger (smaller) than the keplerian value for an oblate (prolate) star with the equality attained as e → 0 or r → ∞. the radial profiles of the angular velocity and the angular momentum allow for a stable accreting disc around any nature of oblate/prolate spheroid. | epicyclic frequencies of spheroidal stars with non-uniform density |
my dissertation focuses on the effect of magnetic fields on disk and core evolution during star-formation. we investigate the fragmentation scales of gravitational instability of a rotationally-supported self-gravitating protostellar disk using linear perturbation analysis in the presence of two nonideal magnetohydrodynamic (mhd) effects: ohmic dissipation and ambipolar diffusion. our results show that molecular clouds exhibit a preferred lengthscale for collapse that depends on mass-to-flux ratio, magnetic diffusivities, and the toomre-q parameter. in addition, the influence of the magnetic field on the preferred mass for collapse leads to a modified threshold for the fragmentation mass, as opposed to a jeans mass, that might lead to giant planet formation in the early embedded phase. furthermore, we apply the nonideal mhd threshold for fragmentation scales to fit the data of prestellar core lifetimes and as well as the number of enclosed cores formed in a clump, as found with the observations of herschel and submillimeter array (sma), respectively. our results show that the trend found in the observed lifetime and fragmentation mass cannot be explained in a purely hydrodynamic scenario. our best-fit model exhibits b ~ n0.43, which signifies a means to indirectly infer the effect of the ambipolar diffusion on mildly supercritical dense regions of molecular clouds. we also develop a semi-analytic formalism of episodic mass accretion (therefore episodic luminosity) from a disk to star, which provides a good match to the observed luminosity distribution of protostars. in contrast, neither a constant nor a time-dependent but smoothly varying mass accretion rate is able to do so. our analytic work provides insight into global mhd simulations of protoplanetary disks that we carry out using the feosad numerical code. our numerical results demonstrate the long-term evolution of disks, including the formation and evolution of clumps, and especially the episodic nature of accretion, which might explain the origin of observed knots in the molecular jet outflows. | the role of nonideal magnetohydrodynamic effects, gravitational instability, and episodic accretion in star-formation |
the thermal instability of accretion disks is widely used to explain the activity of cataclysmic variables, but its manifestation in gas and dust disks in young stars has been studied in less detail. a semi-analytical stationary model is presented for calculating the equatorial temperature of a gas and dust disk around a young star. the model considers the opacity caused by dust and gas, as well as the evaporation of dust at temperatures above 1000 k. using this model, the distributions of the equatorial temperature of the gas and dust disk are calculated under various assumptions on the source of opacity and the presence of dust. it is shown that when all the above processes are considered, the thermal balance equation in the region $r < 1$ au has multiple temperature solutions. thus, the conditions for thermal instability are satisfied in this region. as an illustration of the possible influence of instability on the nature of accretion in a protoplanetary disk, we consider a viscous disk model with $α $-parametrization of turbulent viscosity. it is shown that in such a model a non-stationary mode of disk evolution is realized with alternating phases of accumulation of matter in the inner disk and phases of its rapid accretion onto the star, which leads to a burst character of accretion. the results obtained indicate the need to take this instability into account when modeling the evolution of protoplanetary disks. | effect of dust evaporation and thermal instability on temperature distribution in a protoplanetary disk |
spherical flows are a classic problem in astrophysics which are typically studied from a global perspective. however, much like with accretion discs, there are likely many instabilities and small scale phenomena which would be easier to study from a local perspective. for this purpose, we develop a local model for a spherically contracting/expanding gas cloud, in the spirit of the shearing box, β-plane, and expanding box models which have had extensive use in studies of accretion discs, planets, and stellar winds, respectively. the local model consists of a, spatially homogeneous, periodic box with a time varying aspect ratio, along with a scale factor (analogous to that in frw/newtonian cosmology) relating the box coordinates to the physical coordinates of the global problem. we derive a number of symmetries and conservation laws exhibited by the local model. some of these reflect symmetries of the periodic box, modified by the time dependant geometry, while others are local analogues for symmetries of the global problem. the energy, density, and vorticity in the box also generically increase(/decrease) as a consequence of the collapse(/expansion). we derive a number of non-linear solutions, including a local analogue of uniform density zonal flows, which grow as a consequence of angular momentum conservation. our model is closely related to the accelerated expanding box model of tenerani & velli and is an extension of the isotropic model considered by robertson & goldreich. | a local model for the spherical collapse/expansion problem |
the formation of giant planets has traditionally been divided into two pathways: core accretion and gravitational instability. however, in recent years, gravitational instability has become less favored, primarily due to the scarcity of observations of fragmented protoplanetary disks around young stars and the low occurrence rate of massive planets on very wide orbits. in this study, we present a sphere/irdis polarized light observation of the young outbursting object v960 mon. the image reveals a vast structure of intricately shaped scattered light with several spiral arms. this finding motivated a reanalysis of archival atacama large millimeter/submillimeter array 1.3 mm data acquired just two years after the onset of the outburst of v960 mon. in these data, we discover several clumps of continuum emission aligned along a spiral arm that coincides with the scattered light structure. we interpret the localized emission as fragments formed from a spiral arm under gravitational collapse. estimating the mass of solids within these clumps to be of several earth masses, we suggest this observation to be the first evidence of gravitational instability occurring on planetary scales. this study discusses the significance of this finding for planet formation and its potential connection with the outbursting state of v960 mon. | spirals and clumps in v960 mon: signs of planet formation via gravitational instability around an fu ori star? |
we continue studying convection as a possible factor of episodic accretion in protoplanetary disks. within the model of a viscous disk, the accretion history is analyzed at different rates and regions of matter inflow from the envelope onto the disk. it is shown that the burst-like regime occurs in a wide range of parameters. the long-term evolution of the disk is modeled, including the decreasing-with-time matter inflow from the envelope. it is demonstrated that the disk becomes convectively unstable and maintains burst-like accretion onto the star for several million years. the general conclusion of the study is that convection can serve as one of the mechanisms of episodic accretion in protostellar disks, but this conclusion needs to be verified using more consistent hydrodynamic models. | long-term evolution of convectively unstable disk |
while giant planet occurrence rates increase with stellar mass, occurrence rates of close-in super-earths decrease. this is in contradiction to the expectation that the total mass of the planets in a system scale with the protoplanetary disc mass and hence the stellar mass. since the snow line plays an important role in the planet formation process, we examine differences in the temperature structure of protoplanetary gas discs around stars of different mass. protoplanetary discs likely contain a dead zone at the mid-plane that is sufficiently cold and dense for the magneto-rotational instability to be suppressed. as material builds up, the outer parts of the dead zone may be heated by self-gravity. the temperature in the disc can be below the snow line temperature far from the star and in the inner parts of a dead zone. the inner icy region has a larger radial extent around smaller mass stars. the increased mass of solid icy material may allow for the in situ formation of larger and more numerous planets close to a low-mass star. super-earths that form in the inner icy region may have a composition that includes a significant fraction of volatiles. | formation of super-earths in icy dead zones around low-mass stars |
the disc instability mechanism (dim) is widely accepted to account for the transient behaviour of dwarf novae (dne), which experience short outbursts separated by long quiescence. the duty cycle (the ratio between the outburst duration and the recurrence time) determines the amount of accreted mass by the white dwarf (wds) during outbursts, thus playing an important role in the long-term binary evolution. employing the code of modules for experiments in stellar astrophysics, we systemically investigate the influence of the duty cycles on the evolution of dne and the mass growth of accreting carbon-oxygen (co) wds. our calculations show that, while the dim can considerably influence the accretion process, efficient wd-mass growth requires a particular range of the duty cycle. for wds with the initial masses of 0.6, 0.7, and 1.1 m⊙, these duty cycles are 0.006$\, \le$d$\, \le$0.007, d = 0.005, and, d = 0.003 and the accumulated mass of the wds can reach 0.1, 0.13, and 0.21 m⊙, respectively. in all of our simulations, no co wds can grow their masses to the explosion mass of type ia supernovae of about 1.38 m⊙. because of a much short time-scale of the outburst state, the final donor-star masses and orbital periods are insensitive to the duty cycles. therefore, we propose that the dim in dne could alleviate the wd mass problem to some extent. | white dwarf mass growth in cataclysmic variables: roles of dwarf novae |
we report the detection of the disk/torus, outflow, and inflow structures traced by h2o masers toward a high-mass young stellar object w51 north during its h2o maser outburst stage using the karl g. jansky very large array (vla). it is found that the disk has a radius of ~4000 au and an inclination angle with respect to the sky plane of ~60° by combining the vla and the atacama large millimeter/submillimeter array data. additionally, a peculiar flow perpendicular to the sio bipolar outflow is detected in the h2o maser, sio, and hc3n lines, which is newly-identified as an infalling streamer rather than an old outflow from this source, as reported in previous studies. combining the vla map and the tianma radio telescope monitoring of the h2o masers suggests that the origin of the luminosity outburst of h2o masers during 2020 january-april is likely related to the energy release from the collision between the infalling streamer and the disk. this may provide an additional mechanism to account for the luminosity outburst or episodic accretion beyond disk fragmentation caused by gravitational instability. | luminosity outburst energized by the collision between the infalling streamer and disk in w51 north |
in this paper, we investigate the astrophysical processes of stellar-mass black holes (smbhs) embedded in advection-dominated accretion flows (adafs) of supermassive black holes (smbhs) in low-luminosity active galactic nuclei. the smbh is undergoing bondi accretion at a rate lower than the smbh. outflows from the smbh-adaf dynamically interact with their surroundings and form a cavity inside the smbh-adaf, thereby quenching the accretion onto the smbh. rejuvenation of the bondi accretion is rapidly done by turbulence. these processes give rise to quasi-periodic episodes of smbh activities and create flickerings from relativistic jets developed by the blandford-znajek mechanism if the smbh is maximally rotating. accumulating successive smbh-outflows trigger a viscous instability of the smbh-adaf, leading to a flare following a series of flickerings. recently, the similarity of near-infrared flare's orbits has been found by gravity/vlti astrometric observations of sgr a∗: their loci during the last 4 yr consist of a ring in agreement with the well-determined smbh mass. we apply the present model to sgr a*, which shows quasi-periodic flickerings. an smbh of ~40m ⊙ is preferred orbiting around the central smbh of sgr a* from fitting radio to x-ray continuum. such an extreme mass ratio inspiraling provides an excellent laboratory for lisa/taiji/tianqin detection of mhz gravitational waves with strains of ~10-17, as well as their polarization. | accretion-modified stars in accretion disks of active galactic nuclei: the low-luminosity cases and an application to sgr a* |
as the number of planetary mass objects (pmos, $\lessapprox$13 m$_{\rm{jupitr}}$) at wider separation ($\gtrapprox$10 au) grows, there is emerging evidence that they form differently from their higher-mass brown-dwarf (bd) counterparts. namely, pmos' atmospheres are enriched by metals which is usually interpreted as a sign of solid accretion. this points to the formation channel through core accretion. however, there has hitherto been no quantitative analysis at population level to investigate the amount and timing of solid accretion. here, we analyze a sample of five directly-imaged exoplanets with measured stellar and planetary chemical abundances (51 eri b, $\beta$ pic b, hip 65426 b, and hr 8799 c and e). we show that these pmos accrete large amount of solids regardless of formation channels. on average $\gtrapprox$100 m$_\oplus$ solids (ranging from 98.6 to 845.2 m$_\oplus$ for individual systems) are accreted to enrich planet atmospheres if forming via core accretion whereas the solid accretion needs to be $\gtrapprox$20 m$_\oplus$ (ranging from 22.4 to 782.3 m$_\oplus$) if forming via gravitational instability. the result implies that the solid accretion process has to happen at an early stage ($<$1 myr) when large amount of solids are available in young protoplanetary disks. | early accretion of large amount of solids for directly-imaged exoplanets |
in current theories of planet formation, close-orbiting planets as massive as neptune are expected to be very rare around low-mass stars. we report the discovery of a neptune-mass planet orbiting the `ultracool' star lhs 3154, which is nine times less massive than the sun. the planet's orbital period is 3.7 days and its minimum mass is 13.2 earth masses, giving it the largest known planet-to-star mass ratio among short-period planets ($<$\,100 days) orbiting ultracool stars. both the core accretion and gravitational instability theories for planet formation struggle to account for this system. in the core-accretion scenario, in particular, the dust mass of the protoplanetary disk would need to be an order of magnitude higher than typically seen in protoplanetary disk observations of ultracool stars. | an extreme test case for planet formation: a close-in neptune orbiting an ultracool star |
in this paper, we try to study the fragmentation conditions of self-gravitational accretion disk in dead zone. this region is located between 1 and 5 au from the star. there is a high bump density at the outer boundary of dead zone. a bump surface density leads to the production of instability and vortices. vortices can produce the spiral waves and affect the fragmentation conditions in the system. so, in this paper, we assume a bump density perturbation and obtain the jeans mass, time scale of fragmentation and critical cooling time for fragmentation. results show that the critical cooling time has larger values around the density peak. this value is dependent on amplitude and width of bump density (βcrit ∼ 14 for f = 1.3). therefore, systems with long cooling time can be fragment. also, the results show that in some areas near the peak, the cooling time can be less than the critical cooling time, which depends on the width and peak of the bump(βc <βcrit). also the results show that in some disks, the critical cooling time is less than the cooling time of system in thermal equilibrium state. it is found that in the presence of bump density perturbation, the critical value of β is dependent on the radius and it does not have a same value in all radii. other result obtained from this study is that the jeans mass decreases near the peak of density and therefore the probability of gravitational instability increases in this area. the minimum jeans mass is obtained 0.005msun ∼ 5mjup for disk with low accretion rate. | the critical cooling time for the fragmentation of accretion disks in the presence of bump density perturbation |
my dissertation focuses on the effect of magnetic fields on disk and core evolution during star-formation. we investigate the fragmentation scales of gravitational instability of a rotationally-supported self-gravitating protostellar disk using linear perturbation analysis in the presence of two nonideal magnetohydrodynamic (mhd) effects: ohmic dissipation and ambipolar diffusion. our results show that molecular clouds exhibit a preferred lengthscale for collapse that depends on mass-to-flux ratio, magnetic diffusivities, and the toomre-q parameter. in addition, the influence of the magnetic field on the preferred mass for collapse leads to a modified threshold for the fragmentation mass, as opposed to a jeans mass, that might lead to giant planet formation in the early embedded phase. furthermore, we apply the nonideal mhd threshold for fragmentation scales to fit the data of prestellar core lifetimes and as well as the number of enclosed cores formed in a clump, as found with the observations of herschel and sma, respectively. our results show that the trend found in the observed lifetime and fragmentation mass cannot be explained in a purely hydrodynamic scenario. our best-fit model exhibits b ∝ n0.43, which provides a means to indirectly infer the effect of the ambipolar diffusion on mildly supercritical dense regions of molecular clouds. we also develop a semi-analytic formalism of episodic mass accretion (therefore episodic luminosity) from disk to star, which provides a good match to the observed luminosity distribution of ysos, whereas neither a constant nor a time-dependent but smoothly varying mass accretion rate is able to do so. our analytic work provides insight into global mhd simulations of protoplanetary disks that we carry out using the feosad code, and i will also introduce some numerical results that demonstrate the long-term evolution of disks, including the formation and evolution of clumps and the episodic nature of accretion. | mapping the role of the magnetic fields in the partially ionized environments of magnetized interstellar clouds and disks |
young stellar objects in their pre-main sequence phase are characterized by irregular changes in brightness, generally attributed to an increase of the mass accretion rate due to various kind of instabilities occurring in the circumstellar disk. in the era of large surveys aimed to monitor the sky, we present a pipeline to detect irregular bursts, in particular exors-like (ex lupi type eruptive variables), in the light curves. the procedure follows a heuristic approach and is tested against the light curves already collected for a few objects presently recognized as bona fide or candidate exors. | automatic detection of accretion bursts in young stellar objects: a new algorithm for long-term sky surveys |
thirty years ago it was observed that for many stars the emitted energy spectrum shows an extra bump in the infrared part. this infrared excess indicates a large gaseous disk encompassing the star. such accretion disks are regions of planet formation. understanding the mechanisms that can result in an outward angular momentum transport is the central problem of planet formation, particularly in the theory of accretion disks. when a planet forms in a disk, angular momentum has to be carried away from the planet otherwise its rotation speed would be far too large. only turbulence can achieve such a large angular momentum transport. accretion disks can be turbulent even in the absence of a magnetic field. however, it is still an open question whether purely hydrodynamic instabilities are efficient enough for the momentum transport. this question can be addressed by particularly designed laboratory experiments and numerical simulations in an taylor-couette (tc) setup. it has been shown that classical turbulent tc flows are not efficient enough. however, adding axial stratification opens a route to a new instability. this stratorotational instability (sri) has attracted attention in recent years. we show preliminary experimental and numerical results that highlight nonlinear aspects of the flow. | stratorotational instability in a thermally stratified taylor-couette flow |
this paper develops a unified linear theory of cross field plasma instabilities, including the farley-buneman, electron thermal, and ion thermal instabilities, in spatially uniform collisional plasmas with partially unmagnetized multi-species ions. collisional plasma instabilities in weakly ionized, highly dissipative, weakly magnetized plasmas play an important role in the lower earth's ionosphere and may be of importance in other planetary ionospheres, stellar atmospheres, cometary tails, molecular clouds, accretion disks, etc. in the earth's ionosphere, these collisional plasma instabilities cause intense electron heating. in the solar chromosphere, they can do the same-an effect originally suggested from spectroscopic observations and modeling. based on a simplified 5-moment multi-fluid model, the theoretical analysis presented in this paper produces the linear dispersion relation for the combined thermal farley-buneman instability with an important long-wavelength limit analyzed in detail. this limit provides an easy interpretation of different instability drivers and wave dissipation. this analysis of instability, combined with simulations, will enable us to better understand plasma waves and turbulence in these commonly occurring collisional space plasmas. | unified fluid theory of the collisional thermal farley-buneman instability including magnetized multi-species ions |
mediterranean ecosystems such as those found in california, central chile, southern europe, and southwest australia host numerous, diverse, fire-adapted micro-ecosystems. these micro-ecosystems are as diverse as mountainous conifer to desert-like chaparral communities. over the last few centuries, human intervention, invasive species, and climate warming have drastically affected the composition and health of mediterranean ecosystems on almost every continent. increased fuel load from fire suppression policies and the continued range expansion of non-native insects and plants, some driven by long-term drought, produced the deadliest wildfire season on record in 2018. as a consequence of these fires, a large number of structures are destroyed, releasing household chemicals into the environment as uncontrolled toxins. the mobilization of these materials can lead to health risks and disruption in both human and natural systems. this article identifies drivers that led to a structural weakening of the mosaic of fire-adapted ecosystems in california, and subsequently increased the risk of destructive and explosive wildfires throughout the state. under a new climate regime, managing the impacts on systems moving out-of-phase with natural processes may protect lives and ensure the stability of ecosystem services. | invasive species, extreme fire risk, and toxin release under a changing climate |
we present experiments on finite liquid volumes, hereafter referred to as blobs, of variable densities impacting an interface between two immiscible liquids at high reynolds and weber numbers. such processes occurred on a massive scale during the giant impacts that formed terrestrial planets and satellites, including the earth and the moon. we find that the fall distance of the blob controls an abrupt transition in coalescence regime and in the amount of mixing with the lower liquid. this transition coincides with a brief and global breakup of the impacting blob into drops. for small fall distances, the large-scale flow following impact behaves as a turbulent fountain: a mixture of immiscible liquids penetrates in the lower liquid, collapses and spreads along the immiscible interface. we derive an experimental scaling relation for turbulent mixing of the impacting blob with the lower liquid as a function of a richardson number. | experiments on the impact and turbulent coalescence of a blob at a liquid-liquid interface |
the earth and space science partnership (essp) continues to collaborate with pennsylvania teachers to research how grades 4-9 students learn selected topics in earth and space science. we have previously conducted semi-structured interviews with students across a wider range of grade levels (4 - 16) to facilitate the development of an empirical "learning progression." this learning progression will reveal the pathways students follow as they progress from novice to expert understanding of the astronomical phenomena presented in an instructional sequence that includes study of the solar system. during the initial round of interviews, we also included questions to determine student ideas about what experimental techniques astronomers use in their work as they study these phenomena. we found that because students rarely engaged directly in observational astronomy, they often relied on cultural experiences or classroom experiences from other scientific disciplines to construct an answer. for example, many believed that the only way for astronomers to study a planet is to directly sample it and return that material to earth. because the questions in our original interview protocol did not allow us to fully sample student ideas in this particular area, we have designed a new study to answer the question "what are students' ideas about the scientific practices used by astronomers in their study of the properties of objects that make up the solar system?" in this poster, we will present preliminary findings of our analysis of a new round of interviews that uses a newly designed, open-ended interview protocol to query students from two different 9th grade earth and space science classes from a large pennsylvania high school. we will also present our ideas for how instruction can support student understanding of these ideas in astronomy, and how other, similar studies in other scientific domains may allow us to more generally study how students reason about scientific practices across disciplines.we gratefully acknowledge support from the nsf msp program award due#0962792. | have astronomers been to neptune? results of a study of high school students' ideas about how astronomers study the solar system |
the spin, or normalized angular momentum λ, of dark matter halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables such as stellar masses or sizes. there is currently no way to infer the λ parameter of individual halos hosting observed galaxies. here, we present a first attempt to measure λ starting from the dynamically distinct disks and stellar halos identified in high-resolution cosmological simulations with the galactic structure finder (gsf). in a subsample of nihao galaxies analyzed with gsf, we find tight correlations between the total angular momentum of the dark matter halos, jh, and the azimuthal angular momentum, jz, of the dynamical distinct stellar components of the form: log(jh) = α + β⋅log(jz). the stellar halos have the tightest relation with α = 9.50 ± 0.42 and β = 0.46 ± 0.04. the other tight relation is with the disks, for which α = 6.15 ± 0.92 and β = 0.68 ± 0.07. while the angular momentum is difficult to estimate for stellar halos, there are various studies that calculated jz for disks. in application to the observations, we used gaia dr2 and apogee data to generate a combined kinematics-abundance space, where the galaxy's thin and thick stellar disks stars can be neatly separated and their rotational velocity profiles, vϕ(r), can be computed. for both disks, vϕ(r) decreases with radius with ∼2 km s−1 kpc−1 for r ≳ 5 kpc, resulting in velocities of vϕ,thin = 221.2 ± 0.8 km s−1 and vϕ,thick = 188 ± 3.4 km s−1 at the solar radius. we use our derived vϕ,thin(r) and vϕ,thick(r) together with the mass model for the galaxy of cautun et al. (2020, mnras, 494, 4291) to compute the angular momentum for the two disks: jz, thin = (3.26 ± 0.43)×1013 and jz, thick = (1.20 ± 0.30)×1013 m⊙ kpc km s−1, where the dark halo is assumed to follow a contracted nfw profile. adopting the correlation found in simulations, the total angular momentum of the galaxy's dark halo is estimated to be jh = 2.69−0.32+0.37 1015 m⊙ kpc km s−1 and the spin estimate is λmw = 0.061−0.016+0.022, which translates into a probability of 21% using the universal log normal distribution function of λ. if the galaxy's dark halo is assumed to follow a nfw profile instead, the spin becomes λmw = 0.088−0.020+0.024, making the milky way a more extreme outlier (with a probability of only 0.2%). | a first estimate of the milky way dark matter halo spin |
the large magellanic cloud (lmc) will induce a dynamical friction (df) wake on infall to the milky way (mw). the mw's stellar halo will respond to the gravity of the lmc and the dark matter (dm) wake, forming a stellar counterpart to the dm wake. this provides a novel opportunity to constrain the properties of the dm particle. we present a suite of high-resolution, windtunnel-style simulations of the lmc's df wake that compare the structure, kinematics, and stellar tracer response of the dm wake in cold dm (cdm), with and without self-gravity, versus fuzzy dm (fdm) with ma= 10-23 ev. we conclude that the self-gravity of the dm wake cannot be ignored. its inclusion raises the wake's density by ~10%, and holds the wake together over larger distances (~50 kpc) than if self-gravity is ignored. the dm wake's mass is comparable to the lmc's infall mass, meaning the dm wake is a significant perturber to the dynamics of mw halo tracers. an fdm wake is more granular in structure and is ~20% dynamically colder than a cdm wake, but with comparable density. the granularity of an fdm wake increases the stars' kinematic response at the percent level compared to cdm, providing a possible avenue of distinguishing a cdm versus fdm wake. this underscores the need for kinematic measurements of stars in the stellar halo at distances of 70-100 kpc. | structure, kinematics, and observability of the large magellanic cloud's dynamical friction wake in cold versus fuzzy dark matter |
although a variety of techniques have been employed for determining the milky way dark matter halo mass distribution, the range of allowed masses spans both light and heavy values. knowing the precise mass of our galaxy is important for placing the milky way in a cosmological λcdm context. we show that hypervelocity stars (hvss) ejected from the center of the milky way galaxy can be used to constrain the mass of its dark matter halo. we use the asymmetry in the radial velocity distribution of halo stars due to escaping hvss, which depends on the halo potential (escape speed) as long as the round trip orbital time is shorter than the stellar lifetime, to discriminate between different models for the milky way gravitational potential. adopting a characteristic hvs travel time of 330 myr, which corresponds to the average mass of main sequence hvss, we find that current data favors a mass for the milky way in the range (1.2 - 1.9) ×1012m⊙ . | constraining the milky way mass with hypervelocity stars |
the reflex motion and distortion of the milky way (mw) halo caused by the infall of a massive large magellanic cloud (lmc) has been demonstrated to result in an excess of orbital poles of dark matter halo particles toward the lmc orbital pole. this was suggested to help explain the observed preference of mw satellite galaxies to coorbit along the vast polar structure (vpos). we test this idea by correcting the positions and velocities of the mw satellites for the galactocentric-distance-dependent shifts inferred from a lmc-infall simulation. while this should substantially reduce the observed clustering of orbital poles if it were mainly caused by the lmc, we instead find that the strong clustering remains preserved. we confirm the initial study's main result with our simulation of an mw-lmc-like interaction, and use it to identify two reasons why this scenario is unable to explain the vpos: (1) the orbital pole density enhancement in our simulation is very mild (~10% within 50-250 kpc) compared to the observed enhancement (~220%-300%), and (2) it is very sensitive to the specific angular momenta (am) of the simulation particles, with the higher-am particles being affected the least. particles in simulated dark matter halos tend to follow more radial orbits (lower am), so their orbital poles are more easily affected by small offsets in position and velocity caused by a lmc infall than objects with more tangential velocity (higher am), such as the observed dwarf galaxies surrounding the mw. the origin of the vpos thus remains unexplained. | on the effect of the large magellanic cloud on the orbital poles of milky way satellite galaxies |
recent studies have suggested that the milky way (mw)'s dark matter (dm) halo may be significantly tilted with respect to its central stellar disk, a feature that might be linked to its formation history. in this work, we demonstrate a method of constraining the orientation of the minor axis of the dm halo using the angle and frequency variables. this method is complementary to other traditional techniques, such as orbit fitting. we first test the method using a simulated tidal stream evolving in a realistic environment inside an mw-mass host from the fire cosmological simulation, showing that the theoretical description of a stream in the action-angle-frequency formalism still holds for a realistic dwarf galaxy stream in a cosmological potential. utilizing the slopes of the line in angle and frequency space, we show that the correct rotation frame yields a minimal slope difference, allowing us to put a constraint on the minor axis location. finally, we apply this method to the sagittarius stream's leading arm. we report that the mw's dm halo is oblate with the flattening parameter in the potential $q\sim0.7-0.9$ and the minor axis pointing toward $(\ell,b) = (42^{o},48^{o})$. our constraint on the minor axis location is weak and disagrees with the estimates from other works; we argue that the inconsistency can be attributed in part to the observational uncertainties and in part to the influence of the large magellanic cloud. | constraining the tilt of the milky way's dark matter halo with the sagittarius stream |
galactic rotation curves are often considered the first robust evidence for the existence of dark matter. however, even in the presence of a dark matter halo, other galactic-scale observations, such as the baryonic tully-fisher relation and the radial acceleration relation, remain challenging to explain. this has motivated long-distance, infrared modifications to gravity as an alternative to the dark matter hypothesis as well as various dark matter theories with similar phenomenology. in general, the standard lore has been that any model that reduces to the phenomenology of modified newtonian dynamics (mond) on galactic scales explains essentially all galaxy-scale observables. we present a framework to test precisely this statement using local milky way observables, including the vertical acceleration field, the rotation curve, the baryonic surface density, and the stellar disk profile. we focus on models that predict scalar amplifications of gravity, i.e., models that increase the magnitude but do not change the direction of the gravitational acceleration. we find that models of this type are disfavored relative to a simple dark matter halo model because the milky way data requires a substantial amplification of the radial acceleration with little amplification of the vertical acceleration. we conclude that models which result in a mond-like force struggle to simultaneously explain both the rotational velocity and vertical motion of nearby stars in the milky way. | testing dark matter and modifications to gravity using local milky way observables |
stellar streams result from the tidal disruption of satellites and star clusters as they orbit a host galaxy, and can be very sensitive probes of the gravitational potential of the host system. we select and study narrow stellar streams formed in a milky-way-like dark matter halo of the aquarius suite of cosmological simulations, to determine if these streams can be used to constrain the present day characteristic parameters of the halo’s gravitational potential. we find that orbits integrated in both spherical and triaxial static navarro-frenk-white potentials reproduce the locations and kinematics of the various streams reasonably well. to quantify this further, we determine the best-fit potential parameters by maximizing the amount of clustering of the stream stars in the space of their actions. we show that using our set of aquarius streams, we recover a mass profile that is consistent with the spherically averaged dark matter profile of the host halo, although we ignored both triaxiality and time evolution in the fit. this gives us confidence that such methods can be applied to the many streams that will be discovered by the gaia mission to determine the gravitational potential of our galaxy. | modeling the gravitational potential of a cosmological dark matter halo with stellar streams |
observational studies of nearby galaxies have demonstrated correlations between the mass of the central supermassive black holes (bhs) and properties of the host galaxies, notably the stellar bulge mass or central stellar velocity dispersion. motivated by these correlations, the theoretical paradigm has emerged in which bhs and bulges coevolve. however, this picture was challenged by observational and theoretical studies, which hinted that the fundamental connection may be between bhs and dark matter halos, and not necessarily with their host galaxies. based on a study of 3130 elliptical galaxies selected from the sloan digital and rosat all sky surveys we demonstrate that the central stellar velocity dispersion exhibits a significantly tighter correlation with the total gravitating mass, traced by the x-ray luminosity of the hot gas, than with the stellar mass. this hints that the central stellar velocity dispersion, and hence the central gravitational potential, may be the fundamental property of elliptical galaxies that is most tightly connected to the larger-scale dark matter halo. furthermore, using the central stellar velocity dispersion as a surrogate for the bh mass, we find that in elliptical galaxies the inferred bh mass and inferred total gravitating mass within the virial radius (or within five effective radii) can be expressed as mbh \propto m_tot1.6^{+0.6-0.4} (or mbh \propto m5r_{eff}1.8^{+0.7-0.6}). these results are consistent with a picture in which the bh mass is directly set by the central stellar velocity dispersion, which, in turn, is determined by the total gravitating mass of the system. | connecting dark matter halos with the galaxy center and the supermassive black hole |
we consider the possibility that the milky way's dark matter halo possesses a non-vanishing equation of state. consequently, we evaluate the contribution due to the speed of sound, assuming that the dark matter content of the galaxy behaves like a fluid with pressure. in particular, we model the dark matter distribution via an exponential sphere profile in the galactic core, and inner parts of the galaxy whereas we compare the exponential sphere with three widely used profiles for the halo, i.e. the einasto, burkert and isothermal profile. for the galactic core, we also compare the effects due to a dark matter distribution without black hole with the case of a supermassive black hole in vacuum and show that present observations are unable to distinguish them. finally we investigate the expected experimental signature provided by gravitational lensing due to the presence of dark matter in the core. | effects of non-vanishing dark matter pressure in the milky way galaxy |
hypervelocity stars (hvss) travel from the galactic centre across the dark matter halo of the milky way, where they are observed with velocities in excess of the galactic escape speed. because of their quasi-radial trajectories, they represent a unique probe of the still poorly constrained dark matter component of the galactic potential. in this paper, we present a new method to produce such constraints. our likelihood is based on the local hvs density obtained by back-propagating the observed phase space position and quantifies the ejection probability along the orbit. to showcase our method, we apply it to simulated gaia samples of 200 stars in three realistic galactic potentials with dark matter components parametrized by spheroidal nfw profiles. we find that individual hvss exhibit a degeneracy in the scale mass-scale radius plane (ms - rs) and are able to measure only the combination α = m_s/r_s^2. likewise, a degeneracy is also present between α and the spheroidal axis-ratio q. in the absence of observational errors, we show the whole sample can nail down both parameters with sub-per cent precision (about 1{{ per cent}} and 0.1{{ per cent}} for α and q, respectively) with no systematic bias. this remarkable power to constrain deviations from a symmetric halo is a consequence of the galactocentric origin of hvss. to compare our results with other probes, we break the degeneracy in the scale parameters and impose a mass-concentration relation. the result is a competitive precision on the virial mass m200 of about 10{{ per cent}}. | on measuring the galactic dark matter halo with hypervelocity stars |
the dark matter halos that surround milky way-like galaxies in cosmological simulations are, to first order, triaxial. nearly 30 yr ago it was predicted that such triaxial dark matter halos should exhibit steady figure rotation or tumbling motions for durations of several gigayears. the angular frequency of figure rotation predicted by cosmological simulations is described by a log-normal distribution of pattern speed ωp with a median value 0.15 h km s-1 kpc-1 (∼0.15 h rad gyr-1 ∼ 9° h gyr-1) and a width of 0.83h km s-1 kpc-1. these pattern speeds are so small that they have generally been considered both unimportant and undetectable. in this work we show that even extremely slow figure rotation can significantly alter the structure of extended stellar streams produced by the tidal disruption of satellites in the milky way halo. we simulate the behavior of a sagittarius-like polar tidal stream in triaxial dark matter halos with different shapes, when the halos are rotated about the three principal axes. for pattern speeds typical of cosmological halos, we demonstrate, for the first time, that a sagittarius-like tidal stream would be altered to a degree that is detectable even with current observations. this discovery will potentially allow for a future measurement of figure rotation of the milky way's dark matter halo, perhaps enabling the first evidence of this relatively unexplored prediction of cold dark matter. | detecting the figure rotation of dark matter halos with tidal streams |
the density of dark matter near the sun, ρdm, ⊙, is important for experiments hunting for dark matter particles in the laboratory, and for constraining the local shape of the milky way's dark matter halo. estimates to date have typically assumed that the milky way's stellar disc is axisymmetric and in a steady-state. yet the milky way disc is neither, exhibiting prominent spiral arms and a bar, and vertical and radial oscillations. we assess the impact of these assumptions on determinations of ρdm, ⊙ by applying a free-form, steady-state, jeans method to two different n-body simulations of milky way-like galaxies. in one, the galaxy has experienced an ancient major merger, similar to the hypothesized gaia-sausage-enceladus; in the other, the galaxy is perturbed more recently by the repeated passage and slow merger of a sagittarius-like dwarf galaxy. we assess the impact of each of the terms in the jeans-poisson equations on our ability to correctly extract ρdm, ⊙ from the simulated data. we find that common approximations employed in the literature - axisymmetry and a locally flat rotation curve - can lead to significant systematic errors of up to a factor ~1.5 in the recovered surface mass density ~2 kpc above the disc plane, implying a fractional error on ρdm, ⊙ of the order of unity. however, once we add in the tilt term and the rotation curve term in our models, we obtain an unbiased estimate of ρdm, ⊙, consistent with the true value within our 95 per cent confidence intervals for realistic 20 per cent uncertainties on the baryonic surface density of the disc. other terms - the axial tilt, 2nd poisson and time-dependent terms - contribute less than 10 per cent to ρdm, ⊙ (given current data) and can be safely neglected for now. in the future, as more data become available, these terms will need to be included in the analysis. | estimating the local dark matter density in a non-axisymmetric wobbling disc |
aims: we present a new strong lensing mass reconstruction of the bullet cluster (1e 0657-56) at z = 0.296, based on wfc3 and acs hst imaging and vlt/fors2 spectroscopy. the strong lensing constraints underwent substantial revision compared to previously published analysis, there are now 14 (six new and eight previously known) multiply-imaged systems, of which three have spectroscopically confirmed redshifts (including one newly measured from this work).methods: the reconstructed mass distribution explicitly included the combination of three mass components: (i) the intra-cluster gas mass derived from x-ray observation; (ii) the cluster galaxies modeled by their fundamental plane scaling relations and (iii) dark matter.results: the model that includes the intra-cluster gas is the one with the best bayesian evidence. this model has a total rms value of 0.158″ between the predicted and measured image positions for the 14 multiple images considered. the proximity of the total rms to resolution of hst/wfc3 and acs (0.07-0.15''fwhm) demonstrates the excellent precision of our mass model. the derived mass model confirms the spatial offset between the x-ray gas and dark matter peaks. the fraction of the galaxy halos mass to total mass is found to be fs = 11 ± 5% for a total mass of 2.5 ± 0.1 × 1014m⊙ within a 250 kpc radial aperture. | the bullet cluster at its best: weighing stars, gas, and dark matter |
how do galaxy properties (such as stellar mass, luminosity, star formation rate, and morphology) and their evolution depend on the mass of their host dark matter halo? using the galaxy and mass assembly group catalogue, we address this question by exploring the dependence on host halo mass of the luminosity function (lf) and stellar mass function (smf) for grouped galaxies subdivided by colour, morphology, and central/satellite. we find that spheroidal galaxies in particular dominate the bright and massive ends of the lf and smf, respectively. more massive haloes host more massive and more luminous central galaxies. the satellites lf and smf, respectively, show a systematic brightening of characteristic magnitude, and increase in characteristic mass, with increasing halo mass. in contrast to some previous results, the faint-end and low-mass slopes show little systematic dependence on halo mass. semi-analytic models and simulations show similar or enhanced dependence of central mass and luminosity on halo mass. faint and low-mass simulated satellite galaxies are remarkably independent of halo mass, but the most massive satellites are more common in more massive groups. in the first investigation of low-redshift lf and smf evolution in group environments, we find that the red/blue ratio of galaxies in groups has increased since redshift z ≈ 0.3 relative to the field population. this observation strongly suggests that quenching of star formation in galaxies as they are accreted into galaxy groups is a significant and ongoing process. | galaxy and mass assembly: luminosity and stellar mass functions in gama groups |
using a dark matter simulation we show how halo bias is determined by local density and not by halo mass. this is not totally surprising as, according to the peak-background split model, local matter density (bar δ) is the property that constrains bias at large scales. massive haloes have a high clustering because they reside in high density regions. small haloes can be found in a wide range of environments which differentially determine their clustering amplitudes. this contradicts the assumption made by standard halo occupation distribution (hod) models that bias and occupation of haloes is determined solely by their mass. we show that the bias of central galaxies from semi-analytic models of galaxy formation as a function of luminosity and colour is therefore not correctly predicted by the standard hod model. using bar δ (of matter or galaxies) instead of halo mass, the hod model correctly predicts galaxy bias. these results indicate the need to include information about local density and not only mass in order to correctly apply hod analysis in these galaxy samples. this new model can be readily applied to observations and has the advantage that, in contrast with the dark matter halo mass, the galaxy density can be directly observed. | what determines large scale galaxy clustering: halo mass or local density? |
as the milky way and its satellite system become more entrenched in near field cosmology efforts, the need for an accurate mass estimate of the milky way's dark matter halo is increasingly critical. with the second and early third data releases of stellar proper motions from gaia, several groups calculated full 6d phase-space information for the population of milky way satellite galaxies. utilizing these data in comparison to subhalo properties drawn from the phat elvis simulations, we constrain the milky way dark matter halo mass to be ~1-1.2 × 1012 m⊙. we find that the kinematics of subhaloes drawn from more- or less-massive hosts (i.e. >1.2 × 1012 m⊙ or <1012 m⊙) are inconsistent, at the 3σ confidence level, with the observed velocities of the milky way satellites. the preferred host halo mass for the milky way is largely insensitive to the exclusion of systems associated with the large magellanic cloud, changes in galaxy formation thresholds, and variations in observational completeness. as more milky way satellites are discovered, their velocities (radial, tangential, and total) plus galactocentric distances will provide further insight into the mass of the milky way dark matter halo. | sizing from the smallest scales: the mass of the milky way |
the timing argument provides a lower limit on the mass of the milky way. using a sample of 32 stars at r > 60 kpc drawn from the h3 spectroscopic survey and mock catalogs created from published numerical simulations, we find that m200 > 0.91 × 1012 m⊙ with 90% confidence. we recommend using this limit to refine the allowed prior mass range in more complex and sophisticated statistical treatments of milky way dynamics. the use of such a prior would have significantly reduced many previously published uncertainty ranges. our analysis suggests that the most likely value of m200 is ≈1.5 × 1012 m⊙, but establishing this as the milky way mass requires a larger sample of outer halo stars and a more complete analysis of the inner halo stars in h3. the imminent growth in the sample of outer halo stars due to ongoing and planned surveys will make this possible. | a lower limit on the mass of our galaxy from the h3 survey |
employing the spherical collapse formalism, we investigate the linear evolution of the matter overdensity for energy-momentum-squared gravity (emsg), which in practical phenomenological terms one may imagine as an extension of the λ cdm model of cosmology. the underlying model, while still having a cosmological constant, is a nonlinear material extension of the general theory of relativity and includes correction terms that are dominant in the high-energy regime, the early universe. considering the friedmann-robertson-walker background in the presence of a cosmological constant, we find the effects of the modifications arising from emsg on the growth of perturbations at the early stages of the universe. considering both possible negative and positive values of the model parameter of emsg, we discuss its role in the evolution of the matter density contrast and growth function in the level of linear perturbations. while emsg leaves imprints distinguishable from λ cdm , we find that the negative range of the emsg model parameter is not well behaved, indicating an anomaly in the parameter space of the model. in this regard, for the evaluation of the galaxy cluster number count in the framework of emsg, we equivalently provide an analysis of the number count of the gravitationally collapsed objects (or the dark matter halos). we show that the galaxy cluster number count decreases compared to the λ cdm model. in agreement with the hierarchical model of structure formation, in emsg cosmology the more massive structures are less abundant, meaning that they form at later times. | evolution of spherical overdensities in energy-momentum-squared gravity |
the dark matter halo masses of galaxies can be estimated from their stellar masses via abundance matching (am). for both the milky way and m31, the am mass is higher than the mass inferred from kinematics. the higher am masses exacerbate the missing satellite problem. the difference is especially pronounced for m31, for which ${m}_{200}^{\mathrm{am}}\gtrsim {10}^{13}\,{m}_{\odot }$ but ${m}_{200}^{\mathrm{kin}}\lt 2\times {10}^{12}\,{m}_{\odot }$ . this is more than expected from scatter in the am relation, and may suggest the need for separate am relations for early and late type galaxies. | dark matter halo masses from abundance matching and kinematics: tensions for the milky way and m31 |
the gaia map of the milky way reveals a pair of triangular features at nearly symmetric locations on opposite sides of the galactic center. in this paper we explore the implications of these features assuming they are manifestations of a caustic ring in the dark matter distribution of the milky way halo. the existence of a series of such rings is predicted by the caustic ring model. the model's phase-space distribution is that acquired by a rethermalizing bose-einstein condensate of axions or axion-like particles. we show that dust is gravitationally entrained by cold axion flows and propose this as an explanation for the sharpness of the triangular features. the locations of the features imply that we on earth are much closer to the fifth caustic ring than thought on the basis of pre-gaia observations. most likely we are inside its tricusp cross-section. in that case the dark matter density on earth is dominated by four cold flows, termed big, little, up and down. if we are outside the tricusp cross-section the dark matter density on earth is dominated by two cold flows, big and little. we use the triangular features in the gaia map, and a matching feature in the iras map, to estimate the velocity vectors and densities of the four locally dominant flows. | implications of triangular features in the gaia skymap for the caustic ring model of the milky way halo |
we use the halo occupation distribution (hod) framework to characterize the predictions from two independent galaxy formation models for the galactic content of dark matter haloes and its evolution with redshift. our galaxy samples correspond to a range of fixed number densities defined by stellar mass and span 0 ≤ z ≤ 3. we find remarkable similarities between the model predictions. differences arise at low galaxy number densities which are sensitive to the treatment of heating of the hot halo by active galactic nuclei. the evolution of the form of the hod can be described in a relatively simple way, and we model each hod parameter using its value at z = 0 and an additional evolutionary parameter. in particular, we find that the ratio between the characteristic halo masses for hosting central and satellite galaxies can serve as a sensitive diagnostic for galaxy evolution models. our results can be used to test and develop empirical studies of galaxy evolution, and can facilitate the construction of mock galaxy catalogues for future surveys. | the evolution of the galaxy content of dark matter haloes |
aims: it is well known that the presence of baryons affects the dark matter host haloes. exploring the galaxy assembly history together with the dark matter haloes properties through time can provide a way to measure these effects.methods: we have studied the properties of four milky way mass dark matter haloes from the aquarius project during their assembly history, between z = 0 - 4. in this work, we used a published sph run and the dark matter only counterpart as case studies. to asses the robustness of our findings, we compared them with one of the haloes run using a moving-mesh technique and different sub-grid scheme.results: our results show that the cosmic evolution of the dark matter halo profiles depends on the assembly history of the baryons. we find that the dark matter profiles do not significantly change with time, hence they become stable, when the fraction of baryons accumulated in the central regions reaches 80 per cent of its present mass within the virial radius. furthermore, the mass accretion history shows that the haloes that assembled earlier are those that contain a larger amount of baryonic mass aforetime, which in turn allows the dark matter halo profiles to reach a stable configuration earlier. for the sph haloes, we find that the specific angular momentum of the dark matter particles within the five per cent of the virial radius at z = 0, remains approximately constant from the time at which 60 per cent of the stellar mass is gathered. we have explored different theoretical and empirical models for the contraction of the haloes through redshift. a model to better describe the contraction of the haloes through redshift evolution must depend on the stellar mass content in the inner regions. | dark matter response to galaxy assembly history |
understanding the links between the activity of supermassive black holes (smbhs) at the centres of galaxies and their host dark matter haloes is a key question in modern astrophysics. the final data release of the sdss-iv eboss provides the largest contemporary spectroscopic sample of galaxies and quasi-stellar objects (qsos). using this sample and covering the redshift interval z = 0.7-1.1, we have measured the clustering properties of the eboss qsos, emission-line galaxies (elgs), and luminous red galaxies (lrgs). we have also measured the fraction of qsos as a function of the overdensity defined by the galaxy population. using these measurements, we investigate how qsos populate and sample the galaxy population, and how the host dark-matter haloes of qsos sample the underlying halo distribution. we find that the probability of a galaxy hosting a qso is independent of the host dark matter halo mass of the galaxy. we also find that about 60 per cent of eboss qsos are hosted by lrgs and about 20-40 per cent of qsos are hosted by satellite galaxies. we find a slight preference for qsos to populate satellite galaxies over central galaxies. this is connected to the host halo mass distribution of different types of galaxies. based on our analysis, qsos should be hosted by a very broad distribution of haloes, and their occurrence should be modulated only by the efficiency of galaxy formation processes. | quasars at intermediate redshift are not special; but they are often satellites |
we propose a versatile and accurate method to estimate the halo mass and concentration from the kinematics of satellite galaxies. we construct the 6d phase-space distribution function of satellites from a cosmological simulation based on the similarity of internal dynamics for different halos. within the bayesian statistical framework, not only can we efficiently infer the halo mass and concentration, we can also treat various observational effects, including the selection function, incomplete data, and measurement errors, in a rigorous and straightforward manner. through tests with mock samples, we show that our method is valid and accurate, as well as more precise than pure steady-state methods. it can constrain the halo mass to within ∼20% using only 20 tracers and has a small intrinsic uncertainty of ∼10%. in addition to the clear application to the milky way and similar galaxies, our method can be extended to galaxy groups or clusters. | a versatile and accurate method for halo mass determination from phase-space distribution of satellite galaxies |
recent analyses of the pal 5 and gd-1 tidal streams suggest that the inner dark matter halo of the milky way is close to spherical, in tension with predictions from collisionless n-body simulations of cosmological structure formation. we use the eris simulation to test whether the combination of dissipative physics and hierarchical structure formation can produce milky way-like galaxies whose dark matter halos match the tidal stream constraints from the gd-1 and pal 5 clusters. we use a dynamical model of the simulated eris galaxy to generate many realizations of the gd-1 and pal 5 tidal streams, marginalize over observational uncertainties in the cluster galactocentric positions and velocities, and compare with the observational constraints. we find that the total density and potential of eris contributed by baryons and dark matter satisfies constraints from the existing milky way stellar stream data, as the baryons both round and redistribute the dark matter during the dissipative formation of the galaxy, and provide a centrally concentrated mass distribution that rounds the inner potential. the eris dark matter halo or a spherical navarro-frenk-white dark matter work comparably well in modeling the stream data. in contrast, the equivalent dark matter-only erisdark simulation produces a prolate halo that cannot reproduce the observed stream data. the ongoing gaia mission will provide decisive tests of the consistency between {{λ }}{cdm} and milky way streams, and should distinguish between models like eris and more spherical halos. | around the way: testing λcdm with milky way stellar stream constraints |
we measure for the first time the outermost edges of the milky way (mw) halo in terms of the depletion and turnaround radii. the inner depletion radius, rid, identified at the location of maximum infall velocity, separates a growing halo from the draining environment, while the turnaround radius, rta, marks the outermost edge of infalling material toward the halo, both of which are located well outside the virial radius. using the motions of nearby dwarf galaxies within 3 mpc, we obtain a marginal detection of the infall zone around the mw with a maximum velocity of ${v}_{\inf ,\max }=-{46}_{-39}^{+24}\,\mathrm{km}\ {{\rm{s}}}^{-1}$ . this enables us to measure rid = 559 ± 107 kpc and rta = 839 ± 121 kpc. the measured depletion radius is about 1.5 times the mw virial radius (r200m) measured from internal dynamics. compared with halos in the cosmological simulation illustris tng100, the factor 1.5 is consistent with that of halos with similar masses and dynamical environments to the mw but slightly smaller than typical values of local group analogs, potentially indicating the unique evolution history of the mw. these measurements of halo edges directly quantify the ongoing evolution of the mw outer halo and provide constraints on the current dynamical state of the mw that are independent from internal dynamics. | the outermost edges of the milky way halo from galaxy kinematics |
important tracers for the dark matter halo of the galaxy are hypervelocity stars (hvss), which are faster than the local escape velocity of the galaxy and their slower counterparts, the high-velocity stars in the galactic halo. such hvss are believed to be ejected from the galactic centre (gc) through tidal disruption of a binary by the super-massive black hole (hills mechanism). the hyper-muchfuss survey aims at finding high-velocity potentially unbound hot subdwarf stars. we present the spectroscopic and kinematical analyses of a he-sdo as well as three candidates among the sdb stars using optical keck/esi and vlt (x-shooter, fors) spectroscopy. proper motions are determined by combining positions from early-epoch photographic plates with those derived from modern digital sky surveys. the galactic rest frame velocities range from 203 km s-1 to 660 km s-1, indicating that most likely all four stars are gravitationally bound to the galaxy. with teff = 47 000 k and a surface gravity of log g = 5.7, sdss j205030.39-061957.8 (j2050) is a spectroscopic twin of the hypervelocity he-sdo us 708. as for the latter, the gc is excluded as a place of origin based on the kinematic analysis. hence, the hills mechanism can be excluded for j2050. the ejection velocity is much more moderate (385 ± 79 km s-1) than that of us 708 (998 ± 68 km s-1). the binary thermonuclear supernova scenario suggested for us 708 would explain the observed properties of j2050 very well without pushing the model parameters to their extreme limits, as required for us 708. accordingly, the star would be the surviving donor of a type ia supernova. three sdb stars also showed extreme kinematics; one could be a hvs ejected from the gc, whereas the other two could be ejected from the galactic disk through the binary supernova mechanism. alternatively, they might be extreme halo stars. | spectroscopic twin to the hypervelocity sdo star us 708 and three fast sdb stars from the hyper-muchfuss project |
the standard explanation for galaxy spin starts with the tidal-torque theory (ttt), in which an ellipsoidal dark-matter protohalo, which comes to host the galaxy, is torqued up by the tidal gravitational field around it. we discuss a complementary picture, using the relatively familiar velocity field, instead of the tidal field, whose intuitive connection to the surrounding, possibly faraway matter arrangement is more obscure. in this 'spin from primordial inner motions' (spim) concept, implicit in ttt derivations but not previously emphasized, the angular momentum from the gravity-sourced velocity field inside a protohalo largely cancels out, but has some excess from the aspherical outskirts. at first, the net spin scales according to linear theory, a sort of comoving conservation of familiar angular momentum. then, at collapse, it is conserved in physical coordinates. small haloes are then typically subject to secondary exchanges of angular momentum. the ttt is useful for analytic estimates. but a literal interpretation of the ttt is inaccurate in detail, without some implicit concepts about smoothing of the velocity and tidal fields. this could lead to misconceptions, for those first learning about how galaxies come to spin. protohaloes are not perfectly ellipsoidal and do not uniformly torque up, as in a naive interpretation of the ttt; their inner velocity fields retain substantial dispersion. furthermore, quantitatively, given initial conditions and protohalo boundaries, spim is more direct and accurate than the ttt to predict halo spins. we also discuss how spim applies to rotating filaments, and the relation between halo mass and spin, in which the total spin of a halo can be thought of as a sum of random contributions. | halo spin from primordial inner motions |
the local density of dark matter is an important quantity. on the one hand, its value is needed for dark matter direct detection searches. on the other hand, a precise and robust determination of the local dark matter density would help us learn about the shape of the dark matter halo of our galaxy, which plays an important role in dark matter indirect detection searches, as well as in many studies in astrophysics and cosmology. there are different methods available to determine the local dark matter density. among them, it is common to study either the vertical kinematics of a selected group of tracers or the rotation curve of the milky way. recent estimates of the local dark matter density have used the precise observations conducted by the esa/gaia mission. however, in spite of the quality of the data released by gaia’s observations, different analyses of the local dark matter density produce dissimilar results. after a brief review of the most common methods to estimate the local density of dark matter, here we argue about different explanations for the discrepancies in the results of recent analyses. we finish by presenting new approaches that have been proposed in the literature and could help us improve our knowledge of this important quantity. | dark matter local density determination based on recent observations |
we explore constraints on the milky way dark matter halo oblateness using three stellar streams from globular clusters ngc3201, m68, and palomar 5. previous constraints on the gravitational potential from dynamical equilibrium of stellar populations and distant milky way satellites are included. we model the dark halo as axisymmetric with axis ratio $q_\rho ^{\rm h}$ and four additional free parameters of a two power-law density profile. the halo axis ratio, while barely constrained by the ngc3201 stream alone, is required to be close to spherical by the streams of palomar 5 ($q_\rho ^{\rm h}=1.01\pm 0.09$) and m68 ($q_\rho ^{\rm h}=1.14^{+0.21}_{-0.14}$), the latter allowing a more prolate shape. the three streams together are well fitted with a halo axis ratio $q_\rho ^{\rm h}=1.06 \pm 0.06$ and core radius ~20 kpc. our estimate of the halo shape agrees with previous studies using other observational data and is in tension with cosmological simulations, predicting that most spiral galaxies have oblate dark matter haloes with the short axis perpendicular to the disc. we discuss why the impact of the magellanic clouds tide is too small to change our conclusion on the halo axis ratio. we note that dynamical equilibrium of a spherical halo in the oblate disc potential implies an anisotropic dark matter velocity dispersion, larger along the vertical direction than the horizontal ones, which should relate to the assembly history of the milky way. | the oblateness of the milky way dark matter halo from the stellar streams of ngc 3201, m68, and palomar 5 |
we apply noether's theorem to observations of main-sequence stars from the gaia data release 2 archive to probe the matter distribution function of the galaxy. that is, we examine the axial symmetry of stars at vertical heights z, $0.2\leqslant | z| \leqslant 3\,\mathrm{kpc}$ , to probe the quality of the angular momentum lz as an integral of motion. the failure of this symmetry test would speak to a milky way, in both its visible and dark matter, that is not isolated and/or not in steady state. the left-right symmetry-breaking pattern we have observed, north and south, reveals both effects, with a measured deviation from symmetry of typically 0.5%. we show that a prolate form of the gravitational distortion of the milky way by the large magellanic cloud, determined from fits to the orphan stream by erkal et al., is compatible with the size and sign of the axial-symmetry-breaking effects we have discovered in our sample of up to 14.4 million main-sequence stars, speaking to a distortion of an emergent, rather than static, nature. | applying noether's theorem to matter in the milky way: evidence for external perturbations and non-steady-state effects from gaia data release 2 |
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