Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities

Abstract

The disruption of binary stars by the tidal field of the black hole in the Galactic center can produce the hypervelocity stars observed in the halo. We use numerical models to simulate the full spectrum of observable velocities of stars ejected into the halo by this binary disruption process. Our models include a range of parameters for binaries with 3-4 M☉ primaries, consideration of radial orbits of the ejected stars through an approximate mass distribution for the Galaxy, and the impact of stellar lifetimes. We calculate the spectrum of ejection velocities and reproduce previous results for the mean ejection velocity at the Galactic center. The model predicts that the full population of ejected stars includes both the hypervelocity stars with velocities large enough to escape from the Galaxy and a comparable number of ejected, but bound, stars of the same stellar type. The predicted median speeds of the population of ejected stars as a function of distance in the halo are consistent with current observations. Combining the model with the data also shows that interesting constraints on the properties of binaries in the Galactic center and on the mass distribution in the Galaxy can be obtained even with modest samples of ejected stars.