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: (1) close encounters of two single stars, (2) tidal breakup of binary stars by the central black hole, as originally proposed by Hills, and (3) three-body interactions between a star and a binary black hole (BBH). Mechanism 1 expels hypervelocity stars to the solar radius, R-0 = 8 kpc, at a negligible rate, similar to10(-11) yr(-1). Mechanism 2 expels hypervelocity stars at a rate similar to10(-5) (eta/0.1) yr(-1), where eta is the fraction of stars in binaries with semimajor axis a(b)less than or similar to0.3 AU. For solar mass stars, the corresponding number of hypervelocity stars within the solar radius is similar to60(eta/0.1)(a(b)/0.1 AU)(1/2). For mechanism 3, 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, a semimajor axis of 0.5 x 10(-3) pc and a mass ratio of 0.01), the rate of ejection of hypervelocity stars can be as large as similar to10(-4) yr(-1), and the expected number of hypervelocity stars within the solar radius can be as large as similar to10(3). Hypervelocity stars may be detectable by the next generation of large-scale optical surveys.