arXiv subjects

Aleksey Generozov

Publications and source records attributed to Aleksey Generozov.

17 recordsLinked to original sources

arXiv2026

Preferential alignment of Class 0, Class I protostellar disks in multiple systems across nine nearby molecular clouds

Protostellar disk orientations in multiple systems provide critical insights into the primary mechanisms that govern the formation of multiple-star systems, their subsequent dynamical evolution, and their impact on planet-forming disks. We present a disk alignment study of 512 Class 0, Class I, and flat-spectrum protostars across nine nearby molecular clouds within 500 pc, utilizing data from the CAMPOS and VANDAM surveys. Our sample includes 74 binaries and 31 high-order multiple systems. We find that multiple systems with projected pair separations up to 6000 au exhibit preferential disk alignment with respect to each other across all evolutionary classes, deviating significantly from the random distribution predicted by turbulent fragmentation models. This suggests that the formation of multiple systems cannot be explained by turbulent fragmentation alone. Disk alignment on scales of a few thousand au is also difficult to explain by disk fragmentation as the dominant origin. We further find that the degree of nearest-neighbor disk alignment in higher-order multiples is comparable to that in binaries. Finally, we identify a significant deficit of flat-spectrum protostellar disks in high-order multiple systems as compared to younger Class 0 and Class I phases. The decline is consistent with rapid dynamical evolution, in which most higher-order systems dissolve by the end of the Class I phase.

astro-ph.SR↗

arXiv2025

Impact of a granular mass distribution on the orbit of S2 in the Galactic center

The orbit of the S2 star around Sagittarius A* provides a unique opportunity to test general relativity and study dynamical processes near a supermassive black hole. Observations have shown that the orbit of S2 is consistent with a Schwarzschild orbit at a 10$\sigma$ confidence level, constraining the amount of extended mass within its orbit to less than 1200 M$_\odot$, under the assumption of a smooth, spherically symmetric mass distribution. In this work we investigate the effects on the S2 orbit of granularity in the mass distribution, assuming it consists of a cluster of equal-mass objects surrounding Sagittarius A*. Using a fast dynamical approach validated by full N-body simulations, we perform a large set of simulations of the motion of S2 with different realizations of the cluster objects distribution. We find that granularity can induce significant deviations from the orbit in case of a smooth potential, causing precession of the orbital plane and a variation of the in-plane precession. Interactions with the cluster objects also induce a sort of "Brownian motion" of Sagittarius A*. Mock data analysis reveals that these effects could produce observable deviations in the trajectory of S2 from a Schwarzschild orbit, especially near apocenter. During the next apocenter passage of S2 in 2026, astrometric residuals in Declination may exceed the astrometric accuracy threshold of GRAVITY of about 30 $\mu as$, as it happens in 35 to 60% of simulations for black holes of 20 to 100 M$_\odot$. This presents a unique opportunity to detect, for the first time, scattering effects on the orbit of S2 caused by stellar-mass black holes, thanks to the remarkable precision achievable with GRAVITY. We also demonstrate that any attempt to constrain the extended mass enclosed within the orbit of S2 must explicitly account for granularity in the stellar-mass black hole population.

astro-ph.GA↗

arXiv2024

The S stars' zone of avoidance in the Galactic center

This paper investigates the origin and orbital evolution of S stars in the Galactic center using models of binary disruption and relaxation processes. We focus on explaining the recently discovered ``zone of avoidance'' in S-star orbital parameters, defined as a region where no S stars are observed with pericenters of $\log(r_p / {\rm AU}) \leq 1.57 + 2.6(1 - e)$ pc. We demonstrate that the observed S-star orbital distributions, including this zone of avoidance and their thermal eccentricity distribution, can be largely explained by the continuous disruption of binaries near the central supermassive black hole, followed by orbital relaxation. Our models consider binaries originating from large scales ($5$--$100$ pc) and incorporate empirical distributions of binary properties. We simulate close encounters between binaries and the black hole, tracking the remnant stars' orbits. The initially highly eccentric orbits of disrupted binary remnants evolve due to nonresonant and resonant relaxation in the Galactic center potential. While our results provide insights into the formation mechanism of S stars, there are limitations, such as uncertainties in the initial binary population and mass function and simplifications in our relaxation models. Despite these caveats, our study demonstrates the power of using S-star distributions to probe the dynamical history and environment of the central parsec of our Galaxy.

astro-ph.GA↗

arXiv2024

Capture of stars into gaseous discs around massive black holes: Alignment, circularization and growth

The majority of massive black holes (MBHs) likely hosted gas discs during their lifetimes. These could either be long-lived active galactic nuclei (AGN) discs, or shorter-lived discs formed following singular gas infall events, as was likely the case in our own Galactic Center. Stars and compact objects in such environments are therefore expected to interact with the gaseous disc as they go through it, and potentially become aligned and fully embedded within it. The interactions of embedded stars with the gas could give rise to a plethora of physical processes affecting the stars, including growth through accretion of gas, migration in the disc, stellar captures, and mergers with other stars. The impact of such processes strongly depends on the population of stars that eventually align with the disc and become embedded in it. Here we make use of analytic tools to analyze the alignment process, accounting for both geometric drag and gas dynamical friction. We find that up to $\sim 50\%$ of main sequence stars and stellar mass black holes in the central 0.1 pc can align with AGN discs in the Galactic Center and similar galactic nuclei. The orbits of aligned stars are typically circularized and are prograde with respect to the AGN disc. Furthermore, alignment and accretion are intimately linked, and the capture of stars by an AGN disc can potentially explain the origin of the young stellar disc in the Galactic Center with a top-heavy mass function, even without the need for a star-formation event.

astro-ph.GA↗

arXiv2023

Binary formation through gas-assisted capture and the implications for stellar, planetary and compact-object evolution

Binary systems are ubiquitous and their formation requires two-body interaction and dissipation. In gaseous media, interactions between two initially unbound objects could result in gas-assisted binary formation, induced by a loss of kinetic energy to the ambient gas medium. Here we derive analytically the criteria for gas-assisted binary capture through gas dynamical friction dissipation. We validate them with few-body simulations and explore this process in different gas-rich environments: gas-embedded star-forming regions (SFR), gas-enriched globular clusters, AGN disks and protoplanetary-disks. We find that gas-assisted binary capture is highly efficient in SFRs, potentially providing a main channel for the formation of binaries. It could also operate under certain conditions in gas-enriched globular clusters. Thin AGN disks could also provide a fertile ground for gas-assisted binary capture and in particular the formation of black-hole/other compact object binaries, the production of gravitational-wave (GW) and other high-energy transients. Large-scale gaseous disks might be too thick to enable gas-assisted binary capture and previous estimates of the production of GW-sources could be overestimated, and sensitive to specific conditions and the structure of the disks. In protoplanetary-disks, while gas-assisted binary capture can produce binary KBOs, dynamical friction by small planetsimals is likely to be more efficient. Overall, we show that gas-assisted binary formation is robust and can contribute significantly to the binary formation rate in many environments. In fact, the gas-assisted binary capture rates are sufficiently high such that they will lead to multicaptures, and the formation of higher multiplicity systems.

astro-ph.GA↗

arXiv2022

Constraints on the origins of hypervelocity stars: velocity distribution, mergers and star-formation history

In recent years surveys have identified several dozen B stars in the Milky Way halo moving faster than the local escape speed. The origin of most of these hypervelocity stars (HVSs) is still poorly constrained. Here we show that the velocity distribution, and in particular the deficiency in >700 km/s HVSs is inconsistent with binary disruptions by the massive black hole (MBH) in the Galactic Centre. This conclusion holds in the full and empty loss cone regime, and for secular instabilities in eccentric disks. Accounting for multiple close encounters between binaries and the MBH, does not qualitatively change the results. Moreover, there is no observed counterpart population in the Galactic Centre that is consistent with the HVSs. The star-formation history could be tuned explain the HVS velocity distribution, but this tuning would produce a mismatch with the observed HVS flight times. Frequent stellar collisions of the binary components due to interactions with the MBH do not significantly impact the velocity distribution in the Galactic halo. Such collisions, however, can leave observable remnants in the Galactic Centre, and potentially explain the origins of G2-like dust clouds.

astro-ph.GA↗

arXiv2021

A Lopsided Outer Solar System

Axisymmetric disks of eccentric orbits in near-Keplerian potentials are unstable to an out-of-plane buckling. Recently, Zderic et al. (2020) showed that an idealized disk saturates to a lopsided mode. Here we show that this apsidal clustering also occurs in a primordial scattered disk in the outer solar system which includes the orbit-averaged gravitational influence of the giant planets. We explain the dynamics using Lynden-Bell (1979)'s mechanism for bar formation in galaxies. We also show surface density and line of sight velocity plots at different times during the instability, highlighting the formation of concentric circles and spiral arms in velocity space.

astro-ph.EP↗

arXiv2021

Mass ratio, the hills mechanism, and the galactic centre S-stars

The Galactic centre contains several young populations within its central parsec: a disk between $\sim$0.05 and 0.5 pc from the centre, and the isotropic S-star cluster extending an order of magnitude further inwards in radius. Recent observations (i.e. spectroscopy and hypervelocity stars) suggest that some S-stars originate in the disk. In particular, the S-stars may be remnants of tidally disrupted disk binaries. However, there is an apparent inconsistency in this scenario: the disk contains massive O and Wolf--Rayet stars while the S-stars are lower mass, B stars. We explore two different explanations for this apparent discrepancy: (i) a built-in bias in binary disruptions, where the primary star remains closer in energy to the centre-of-mass orbit than the secondary and (ii) selective tidal disruption of massive stars within the S-star cluster. The first explanation is plausible. On the other hand, tidal disruptions have not strongly affected the mass distribution of the S-stars over the last several Myr.

astro-ph.SR↗

arXiv2021

A stream of hypervelocity stars from the Galactic Center

Recent observations have found a 1700 km/s star [S5-HVS1] that was ejected from the Galactic Center approximately five million years ago. This star was likely produced by tidal disruption of a binary. In particular, the Galactic Center contains a few million year old stellar disk that could excite binaries to nearly radial orbits via a secular gravitational instability. Such binaries would be disrupted by the central supermassive black hole, and would also explain the observed cluster of B stars ~0.01 pc from the Galactic Center. In this paper we predict S5-HVS1 is part of a larger stream, and use observationally motivated N-body simulations to predict its spatial and velocity distribution.

astro-ph.GA↗

arXiv2020

Galactic merger implications for eccentric nuclear disks: a mechanism for disk alignment

The nucleus of our nearest, large galactic neighbor, M31, contains an eccentric nuclear disk--a disk of stars on eccentric, apsidally-aligned orbits around a supermassive black hole (SMBH). Previous studies of eccentric nuclear disks considered only an isolated disk, and did not study their dynamics under galaxy mergers (particularly a perturbing SMBH). Here, we present the first study of how eccentric disks are affected by a galactic merger. We perform N-body simulations to study the disk under a range of different possible SMBH initial conditions. A second SMBH in the disk always disrupts it, but more distant SMBHs can shut off differential precession and stabilize the disk. This results in a more aligned disk, nearly uniform eccentricity profile, and suppression of tidal disruption events compared to the isolated disk. We also discuss implications of our work for the presence of a secondary SMBH in M31.

astro-ph.GA↗

arXiv2020

The Hills Mechanism and the Galactic Center S-stars

Our Galactic center contains young stars, including the few million year old clockwise disk between 0.05 and 0.5 pc from the Galactic center, and the S-star cluster of B-type stars at a galactocentric distance of ~0.01 pc. Recent observations suggest the S-stars are remnants of tidally disrupted binaries from the clockwise disk. In particular, Koposov et al. 2020 discovered a hypervelocity star that was ejected from the Galactic center 5 Myr ago, with a velocity vector consistent with the disk. We perform a detailed study of this binary disruption scenario. First, we quantify the plausible range of binary semimajor axes in the disk. Dynamical evaporation of such binaries is dominated by other disk stars rather than by the isotropic stellar population. For the expected range of semimajor axes in the disk, binary tidal disruptions can reproduce the observed S-star semimajor axis distribution. Reproducing the observed thermal eccentricity distribution of the S-stars requires an additional relaxation process. The flight time of the Koposov star suggests that this process must be effective within 10 Myr. We consider three possibilities: (i) scalar resonant relaxation from the observed isotropic star cluster, (ii) torques from the clockwise disk, and (iii) an intermediate-mass black hole. We conclude that the first and third mechanisms are fast enough to reproduce the observed S-star eccentricity distribution. Finally, we show that the primary star from an unequal-mass binary would be deposited at larger semimajor axes than the secondary, possibly explaining the dearth of O stars among the S-stars.

astro-ph.GA↗

arXiv2020

Mass Segregation in Eccentric Nuclear Disks: Enhanced Tidal Disruption Event Rates for High Mass Stars

Eccentric nuclear disks (ENDs) are a type of star cluster in which the stars lie on eccentric, apsidally-aligned orbits in a disk around a central supermassive black hole (SMBH). These disks can produce a high rate of tidal disruption events (TDEs) via secular gravitational torques. Previous studies of ENDs have included stars with only one mass. Here, we present the first study of an eccentric nuclear disk with two stellar species. We show that ENDs show radial mass segregation consistent with previous results from other cluster types. Additionally, ENDs show vertical mass segregation by which the heavy stars sink to lower inclinations than light stars. These two effects cause heavy stars to be more susceptible to tidal disruption, which can be seen in the higher fraction of heavy stars that are disrupted compared to light stars.

astro-ph.GA↗

arXiv2018

A Generalised Bondi Accretion Model for the Galactic Centre

We develop an analytic, steady-state model for the gas environment in quiescent galactic nuclei. We assume that the mass is constantly supplied by a spherically symmetric distribution of wind emitting stars, and that gravity is solely due to a central supermassive black hole. We show that at some finite radius, where the Keplerian velocity is comparable to the wind velocity, the bulk velocity vanishes. Matter generated below that radius will be accreted onto the black hole, while matter outside it will escape the system. Under certain conditions, the flow may become supersonic at both domains. We obtain radial profiles of the hydrodynamic variables and verify them using a time-dependent hydrodynamic simulation. We delineate the conditions under which radiative cooling can be neglected, and predict the luminosity and spectrum of the free-free X-ray emission from such a system. We discuss applications of our solution to our own Galactic Centre and other quiescent galactic nuclei.

astro-ph.GA↗

arXiv2017

The Delay Time Distribution of Tidal Disruption Flares

Recent observations suggest that stellar tidal disruption events (TDE) are strongly overrepresented in rare, post-starburst galaxies. Several dynamical mechanisms have been proposed to elevate their TDE rates, ranging from central stellar overdensities to the presence of supermassive black hole (SMBH) binaries. Another such mechanism, introduced here, is a radial velocity anisotropy in the nuclear star cluster produced during the starburst. These, and other, dynamical hypotheses can be disentangled by comparing observations to theoretical predictions for the TDE delay time distribution (DTD). We show that SMBH binaries are a less plausible solution for the post-starburst preference, as they can only reproduce the observed DTD with extensive fine-tuning. The overdensity hypothesis produces a reasonable match to the observed DTD (based on the limited data currently available), provided that the initial stellar density profile created during the starburst, $ρ(r)$, is exceptional in both steepness and normalization. In particular, explaining the post-starburst preference requires $ρ\propto r^{-γ}$ with $γ\gtrsim 2.5$, i.e. much steeper than the classic Bahcall-Wolf equilibrium profile of $γ= 7/4$. For "ultrasteep" density cusps ($γ\ge 9/4$), we show that the TDE rate decays with time measured since the starburst as $\dot{N} \propto t^{-(4γ-9)/(2γ-3)} / \ln t$. Radial anisotropies also represent a promising explanation, provided that initial anisotropy parameters of $β_0 \approx 0.5$ are sustainable against the radial orbit instability. TDE rates in initially anisotropic cusps will decay roughly as $\dot{N} \propto t^{-β_0}$. As the sample of TDEs with well-studied host galaxies grows, the DTD will become a powerful tool for constraining the exceptional dynamical properties of post-starburst galactic nuclei.

astro-ph.GA↗

arXiv2015

Circumnuclear Media of Quiescent Supermassive Black Holes

We calculate steady-state, one-dimensional hydrodynamic profiles of hot gas in slowly accreting ("quiescent") galactic nuclei for a range of central black hole masses $M_{\bullet}$, parametrized gas heating rates, and observationally-motivated stellar density profiles. Mass is supplied to the circumnuclear medium by stellar winds, while energy is injected primarily by stellar winds, supernovae, and black hole feedback. Analytic estimates are derived for the stagnation radius (where the radial velocity of the gas passes through zero) and the large scale gas inflow rate, $\dot{M}$, as a function of $M_{\bullet}$ and the gas heating efficiency, the latter being related to the star-formation history. We assess the conditions under which radiative instabilities develop in the hydrostatic region near the stagnation radius, both in the case of a single burst of star formation and for the average star formation history predicted by cosmological simulations. By combining a sample of measured nuclear X-ray luminosities, $L_x$, of nearby quiescent galactic nuclei with our results for $\dot{M}(M_{\bullet})$ we address whether the nuclei are consistent with accreting in a steady-state, thermally-stable manner for radiative efficiencies predicted for radiatively inefficiency accretion flows. We find thermally-stable accretion cannot explain the short average growth times of low mass black holes in the local Universe, which must instead result from gas being fed in from large radii, due either to gas inflows or thermal instabilities acting on larger, galactic scales. Our results have implications for attempts to constrain the occupation fraction of SMBHs in low mass galaxies using the mean $L_x-M_{\bullet}$ correlation, as well as the predicted diversity of the circumnuclear densities encountered by relativistic outflows from tidal disruption events.

astro-ph.HE↗

arXiv2014

Lyman edges in supermassive black hole binaries

We propose a new spectral signature for supermassive black hole binaries (SMBHBs) with circumbinary gas disks: a sharp drop in flux blueward of the Lyman limit. A prominent edge is produced if the gas dominating the emission in the Lyman continuum region of the spectrum is sufficiently cold (T < 20,000 K) to contain significant neutral hydrogen. Circumbinary disks may be in this regime if the binary torques open a central cavity in the disk and clear most of the hot gas from the inner region, and if any residual UV emission from the individual BHs is either dim or intermittent. We model the vertical structure and spectra of circumbinary disks using the radiative transfer code TLUSTY, and identify the range of BH masses and binary separations producing a Lyman edge. We find that compact supermassive binaries with orbital periods of ~0.1 - 10 yr, whose gravitational waves (GWs) are expected to be detectable by pulsar timing arrays (PTAs), could have prominent Lyman edges. Such strong spectral edge features are not typically present in AGN spectra and could serve as corroborating evidence for the presence of a SMBHB.

astro-ph.HE↗

arXiv2013

Physical Properties of the Inner Shocks in Hot, Tilted Black Hole Accretion Flows

Simulations of hot, pressure supported, tilted black hole accretion flows, in which the angular momentum of the flow is misaligned with the black hole spin axis, can exhibit two non-axisymmetric shock structures in the inner regions of the flow. We analyze the strength and significance of these shock structures in simulations with tilt angles of 10 and 15 degrees. By integrating fluid trajectories in the simulations through the shocks, and tracking the variations of fluid quantities along these trajectories, we show that these shocks are strong, with substantial compression ratios, in contrast to earlier claims. However, they are only moderately relativistic. We also show that the two density enhancements resembling flow streams in their shape are in fact merely post-shock compressions, as fluid trajectories cut across, rather than flow along, them. The dissipation associated with the shocks is a substantial fraction ($\simeq3-12$ percent) of the rest mass energy advected into the hole, and therefore comparable to the dissipation expected from turbulence. The shocks should therefore make order unity changes in the observed properties of black hole accretion flows that are tilted.

astro-ph.HE↗