Ejection of Hypervelocity Stars by the (Binary) Black Hole in the Galactic Center

We study three processes that eject hypervelocity ( 10 3 km s −1) stars from the Galactic center: (i) close encounters of two single stars; (ii) tidal breakup of binary stars by the central black hole, as originally proposed by Hills; and (iii) threebody interactions between a star and a binary black hole (BBH). Mechanism (i) expels hypervelocity stars to the solar radius at a negligible rate, ∼ 10 −11 yr −1. Mechanism (ii) expels hypervelocity stars at a rate ∼ 10 −5 (η/0.1) yr −1, where η is the fraction of stars in binaries with semimajor axis ab � 0.3 AU. For solar-mass stars, the corresponding number of hypervelocity stars within the solar radius R0 = 8 kpc is ∼ 60(η/0.1)(ab/0.1 AU) 1/2. For mechanism (iii), Sgr A ∗ is assumed to be one component of a BBH. We constrain the allowed parameter space (semimajor axis, mass ratio) of the BBH. In the allowed region (for example, semimajor axis of 0.5 × 10 −3 pc and mass ratio of 0.01), the rate of ejecting hypervelocity stars can be as large as ∼ 10 −4 yr −1 and the expected number of hypervelocity stars within the solar radius can be as large as ∼ 10 3. Hypervelocity stars may be detectable by the next generation of large-scale optical surveys. Subject headings: black hole physics – Galaxy: center – stellar dynamics 1.