The Aftermath of Tidal Disruption

Summary

ABSTRACT

The circularization and accretion of the debris created by tidally disrupting a star passing near a supermassive black hole depends on the transverse structure of the debris stream. The transverse structure is modified by crossing points in the stream where the orbits are focussed across the stream center or through the orbital plane, the velocity shear across the stream, self-gravity, recombination, shocks, and shear viscosity. Stream-stream collisions may have a weak effect on the orbits because of Lense-Thirring precession, mismatched geometric cross sections, and the kinematics of collisions.

INTRODUCTION

Tidal disruption of stars passing near supermassive black holes (M ∼ 10⁶ M_⊙) provides a mechanism for fueling low-luminosity active galactic nuclei (AGN). It is ineffective for more massive AGN (M ≳ 10⁸ M^⊙) because the tidal gravity of the black hole is too small to destroy a star before it passes through the event horizon. Other processes in dense central star clusters such as star-star collisions may provide a steady accretion rate (Hills 1975, 1978; Frank 1978; Young, Shields, and Wheeler 1977), but the disruption of a star may lead to an observable flare in the luminosity of the AGN. Lacy, Townes, and Hollenbach (1982) first understood the kinematics of disruption, and the current picture of tidal disruption is reviewed in Rees (1988) and Phinney (1989). For a star of mass M_* and radius R_* passing at pericentric distance R_p from a black hole of mass M, the strength of the encounter can be parametrized by the square root of the ratio of the surface gravity to the tidal gravity η = (M_*R³_p/MR³_*)^½ (Press and Teukolsky 1977).