Ejection of hypervelocity stars by the (binary) black hole(s) in the Galactic center
Abstract
We study three processes that eject hypervelocity (>103 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 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 0.5x10-3 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 ~103. Hypervelocity stars may be detectable by the next generation of large-scale optical surveys.
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