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

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 = M₂/M₁ 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 towards the binary orbital plane. We calculate the ejection rates as the binary SMBHs inspiral, and find that they eject stars with velocities v_∞ > 1000 km s⁻¹ at rates of ∼ 4 × 10⁻² − 2 × 10⁻¹ yr⁻¹ for q = 1 (∼ 10⁻⁴ − 10⁻³ yr⁻¹ for q = 0.01) over their lifetimes, and can emit a burst of HVSs with v_∞ > 3000 km s⁻¹ 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 ∼ 100 HVS velocity samples with v_∞ 200 km s⁻¹ are needed to confidently distinguish between a binary and single SMBH origin.