The Effect of Star–Disk Interactions on Highly Eccentric Stellar Orbits in Active Galactic Nuclei: A Disk Loss Cone and Implications for Stellar Tidal Disruption Events

Abstract

Galactic center black holes appear to be nearly universally surrounded by dense stellar clusters. When these black holes go through an active accretion phase, the multiple components of the accretion disk, stellar cluster, and black hole system all coexist. We analyze the effect of drag forces on highly eccentric stellar orbits incurred as stars puncture through the disk plane. Disk crossings dissipate orbital energy, drawing eccentric stars into more circular orbits. For high surface density disks, such as those found around black holes accreting near the Eddington mass accretion limit, the magnitude of this energy dissipation can be larger than the mean scatterings that stars receive by two body relaxation. One implication of this is the presence of a disk "loss cone" for highly eccentric stellar orbits where the dissipation from disk interaction outweighs scatter via two body relaxation. The disk loss cone is larger than the tidal disruption loss cone for near-Eddington black hole accretion rates. Stellar orbits within the disk loss cone are lost from the overall cluster as stellar orbits are circularized and stars are potentially ablated by their high-velocity impacts with the disk. We find, however, that the presence of the disk loss cone has a minimal effect on the overall rate of stellar tidal disruptions. Stars are still efficiently fed to the black hole from more-distant stellar orbits that receive large-enough per orbit scatter to jump over the disk loss cone and end up tidally disrupted.

Starfall: A heavy rain of stars in 'turning on' AGN

“…Likewise, about half of the NSC orbits with semi-major axis less than the disk radius will cross the disk retrograde. Over the AGN disk lifetime, the population of embedded objects within AGN disks grows due to the orbital grind-down and capture of NSC orbiters (Artymowicz et al 1993;Fabj et al 2020;MacLeod & Lin 2020) as well as in situ star formation (Goodman & Tan 2004;Dittmann & Miller 2020). The population of embedded objects in the disk is the result of a competition between population loss due to mergers, scatterings out of the disk, and extreme mass ratio inspirals (EMRIs) onto the SMBH on the one hand, and disk capture, star formation, and stellar evolution on the other.…”

Stellar Evolution in AGN Disks

“…This interaction results in energy and momentum loss which, over many passages, can bring a star into a circular orbit corotating with the disk (Syer et al 1991;Artymowicz et al 1993). This trapping process relies on hydrodynamical drag as well as the excitation of resonant density waves and bending waves, and can be an efficient mechanism for r 10 pc (Artymowicz et al 1993;Just et al 2012;Kennedy et al 2016;MacLeod & Lin 2020;Fabj et al 2020). Since the stellar density is expected to be high in these regions (n * ∼ 10 6 pc −3 ) a large number of stars can potentially be trapped during periods of AGN activity.…”

Aligning Nuclear Cluster Orbits with an Active Galactic Nucleus Accretion Disk

“…In the second phase, i decreases until the orbiter has been captured by the disk. MacLeod & Lin (2020), used a disk model based on that of Rauch (1995), who find that at i ≤ 30 • , orbit circularization takes ≤ 0.1Myr, which is less than the lowest value we assume for the fiducial AGN lifetime. Rauch (1995) find the circularization time can become significant (> 0.1Myr) at i ≥ 30 • .…”