Gravitational interactions of stars with supermassive black hole binaries - II. Hypervelocity stars

TL;DR

This paper investigates how binary supermassive black holes eject hypervelocity stars, analyzing their velocities, directions, and ejection rates, and compares these signatures to those from single SMBHs to distinguish their origins.

Contribution

It provides a detailed analysis of hypervelocity star ejections by binary SMBHs, including velocity distributions, ejection probabilities, and rates, highlighting differences from single SMBH mechanisms.

Findings

Ejection probability increases with SMBH mass ratio and separation.

Fastest stars are ejected toward the binary orbital plane.

Binary SMBHs can produce bursts of hypervelocity stars with velocities over 3000 km/s.

Abstract

Supermassive black holes (SMBHs) in galactic nuclei can eject hypervelocity stars (HVSs). Using restricted three-body integrations, we study the properties of stars ejected by circular, binary SMBHs as a function of their mass ratios $q = M_2 / M_1$ and separations $a$, focusing on the stellar velocity and angular distributions. We find that the ejection probability is an increasing function of $q$ and $a$, and that the mean ejected velocity scales with $q$ and $a$ similar to previous work but with modified scaling constants. Binary SMBHs tend to eject their fastest stars toward the binary orbital plane. We calculate the ejection rates as the binary SMBHs inspiral, and find that they eject stars with velocities $v_\infty > 1000$ km/s at rates of $\sim 4 \times 10^{-2} - 2 \times 10^{-1}$ yr$^{-1}$ for $q = 1$ ($\sim 10^{-4} - 10^{-3}$ yr$^{-1}$ for $q = 0.01$) over their lifetimes, and can emit a burst of HVSs with $v_\infty > 3000$ km/s as they coalesce. We integrate the stellar distributions over the binary SMBH inspiral and compare them to those produced by the "Hills mechanism" (in which a single SMBH ejects a star after tidally separating a binary star system), and find that $N \sim 100$ HVS velocity samples with $v_\infty \gtrsim 200$ km/s are needed to confidently distinguish between a binary and single SMBH origin.