High‐Velocity White Dwarfs: Thick Disk, Not Dark Matter

Abstract

We present an alternative interpretation of the nature of the extremely cool, high-velocity white dwarfs identified by Oppenheimer et al (2001) in a high-latitude astrometric survey. We argue that the velocity distribution of the majority of the sample is more consistent with the high-velocity tail of a rotating population, probably the thick disk, rather than a pressure-supported halo system. Indeed, the observed numbers are well matched by predictions based on the kinematics of a complete sample of nearby M dwarfs. Analysing only stars showing retrograde motion gives a local density close to that expected for white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of the white dwarfs need be assigned to the dark-matter, heavy halo. However, luminosity functions derived from observations of these stars can set important constraints on the age of the oldest stars in the Galactic Disk.