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

TL;DR

This paper investigates how interactions between stars and accretion disks around black holes in galactic centers influence stellar orbits, creating a disk loss cone that affects the rate of stellar tidal disruption events.

Contribution

It introduces the concept of a disk loss cone caused by star-disk interactions and analyzes its impact on stellar orbit evolution and tidal disruption rates.

Findings

Disk interactions circularize highly eccentric stellar orbits.

The disk loss cone is larger than the tidal disruption loss cone at near-Eddington accretion rates.

Overall tidal disruption rates are minimally affected by the disk loss cone.

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.