A source of high-velocity white dwarfs

Abstract

We investigate whether the recently-observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disk of the Galaxy. In particular we consider binaries where the primary is sufficiently massive to explode as a type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems which are broken up, a large number of the secondaries receive kick velocities ~100 - 200 km/s and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birthrate of these stars over the entire Galaxy to those seen locally with high velocities relative to the LSR. We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disk may make a significant contribution to the observed population of high-velocity white dwarfs.