Ejection of hypervelocity stars by the (binary) black hole(s) in the Galactic center
Abstract
Description
We study three processes that eject hypervelocity (>10^3 km/s) 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) three-body 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. Mechanism (ii) expels hypervelocity stars at a rate ~ 10^{-5}(η/0.1)/yr, where ηis the fraction of stars in binaries with semimajor axis a_b<~0.3 AU. For solar-mass stars, the corresponding number of hypervelocity stars within the solar radius R_0=8 kpc is ~60(η/0.1)(a_b/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 0.5x10^{-3} pc and mass ratio of 0.01), the rate of ejecting hypervelocity stars can be as large as ~10^{-4}/yr 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.
23 pages, including 2 figures; accepted for publication in ApJ
23 pages, including 2 figures; accepted for publication in ApJ