Short- and long-term evolution of a stellar disc around a massive black hole: the role of the cusp, stellar evolution and binaries

Abstract

We study the dynamical evolution of a stellar disk orbiting a massive black hole. We explore the role of two-body relaxation, mass segregation, stellar evolution and binary heating in affecting the disk evolution, and consider the impact of the nuclear cluster structure and the stellar-disk mass-function. We use analytic arguments and numerical calculations, and apply them to study the evolution of a stellar disk (similar to that observed in the Galactic center; GC), both on the short (few Myr) and longer (100 Myr) evolutionary timescales. We find the dominant processes affecting the disk evolution are two-body relaxation and mass segregation where as binary heating have only a little contribution. Massive stars play a dominant role in kinematically heating low mass stars, and driving them to high eccentricities/inclinations. Multi-mass models with realistic mass-functions for the disk stars show the disk structure to be mass stratified, with the most massive stars residing in thinner structures. Stellar evolution plays an important role in decreasing the number of massive stars with time, thereby leading to slower relaxation, where the remnant compact objects of these stars are excited to higher eccentricities/inclinations. At these later evolutionary stages dynamical heating by the nuclear cluster plays a progressively more important role. We conclude that the high eccentricities and high inclination observed for the majority on the young O-stars in the Galactic Center suggest that the disk stars had been formed with initially high eccentricities, or that collective or secular processes (not explored here) dominate the disk evolution. The latter processes are less likely to produce mass stratification in the disk; detailed study of the mass-dependent kinematic properties of the disk stars could therefore provide a handle on the processes that dominate its evolution. Finally, we find that the disk structure is expected to keep its coherency, and be observed as a relatively thin disk even after 100 Myrs; two-body relaxation is too inefficient for the disk to assimilate into the nuclear cluster on such timescales. It therefore suggests earlier disks now containing only older, lower mass stars might still be observed in the Galactic center, unless destroyed/smeared by other non-two-body relaxation processes.

Spiral arms, warping, and clumps formation in the Galactic center young stellar disk

“…Our simulations confirm the results of previous models in the case of the evolution of an isolated (no NSC) stellar disk (e.g. (Alexander et al 2007;Šubr & Haas 2014;Mikhaloff & Perets 2017) and references therein), showing only a steady disk-height increase over its 6 Myrs evolution, consistent with 2-body relaxation models. Such models produce a smooth (no clumps or asymmetries), aligned (no warping) structure which does not change its orientation.…”

“…Such processes kinematically heat the disk (i.e. increase the velocity dispersion of the disk stars), leading to higher eccentricities and disk thickening (Alexander et al 2007;Mikhaloff & Perets 2017;Naoz et al 2018), but are not expected to produce any of the aforementioned peculiar kinetic features (clumping, warping, asymmetries). The clump has been suggested to result from an IMBH residing in the disk; however, detailed kinematic studies of IRS13 ruled out the existence of such object (Maillard et al 2004;Genzel et al 2010).…”

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

“…Mass segregation is wellunderstood in spherical clusters (e.g. AH09; Bahcall & Wolf 1977;Spitzer 1987) and in axisymmetric disks (Alexander et al 2007;Mikhaloff & Perets 2017), but this is the first time that mass segregation has been explored in an eccentric disk with apsidally aligned orbits. AH09 studied mass segregation in an sphericallysymmetric, isotropic star cluster around an SMBH using Fokker-Planck methods.…”

“…Vertical mass segregation is a similar process to radial mass segregation, by which heavy stars sink to low inclinations while scattering light stars to high inclinations. It has been studied in the context of axisymmetric stellar disks around SMBHs (Alexander et al 2007;Mikhaloff & Perets 2017). They showed that in such systems, the scale height of the disk decreases with stellar mass due to two-body relaxation.…”

“…Since END stars at smaller semimajor axes have larger equilibrium eccentricities, they are more easily torqued to orbits that endanger them to tidal disruption (Madigan et al 2018). Additionally, as in axisymmetric disks (see Alexander et al 2007;Mikhaloff & Perets 2017), we expect vertical mass segregation to occur such that heavy stars sink to lower inclinations than light stars. This paper is organized as follows: In § 2, we present our methods, including setup and initial conditions of our simulations.…”