EJECTION OF HYPER VELOCITY STARS BY THE (BINARY) BLACK HOLE IN THE GALACTIC CENTER

摘要

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, ～10~(-11) yr~(-1). Mechanism 2 expels hypervelocity stars at a rate ～10~(-5)(η/0.1) yr~(-1), where η is the fraction of stars in binaries with semi-major axis a_b approx< 0.3 AU. For solar mass stars, the corresponding number of hypervelocity stars within the solar radius is ～60(η/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 ～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.