Gravitational interactions of stars with supermassive black hole binaries. I. Tidal disruption events

Abstract

Stars approaching supermassive black holes (SMBHs) in the centers of galaxies can be torn apart by strong tidal forces. We study the physics of tidal disruption by a binary SMBH as a function of the binary mass ratio $q = M_2 / M_1$ and separation $a$, exploring a large set of points in the parameter range $q \in [0.01, 1]$ and $a/r_{t1} \in [10, 1000]$. We simulate encounters in which field stars approach the binary from the loss cone on parabolic, low angular momentum orbits. We present the rate of disruption and the orbital properties of the disrupted stars, and examine the fallback dynamics of the post-disruption debris in the "frozen-in" approximation. We conclude by calculating the time-dependent disruption rate over the lifetime of the binary. Throughout, we use a primary mass $M_1 = 10^6 M_\odot$ as our central example. We find that the tidal disruption rate is a factor of $\sim 2 - 7$ times larger than the rate for an isolated BH, and is independent of $q$ for $q \gtrsim 0.2$. In the "frozen-in" model, disruptions from close, nearly equal mass binaries can produce intense tidal fallbacks: for binaries with $q \gtrsim 0.2$ and $a/r_{t1} \sim 100$, roughly $\sim 18 - 40 \%$ of disruptions will have short rise times ($t_\textrm{rise} \sim 1 - 10$ d) and highly super-Eddington peak return rates ($\dot{M}_\textrm{peak} / \dot{M}_\textrm{Edd} \sim 2 \times 10^2 - 3 \times 10^3$).