White Dwarfs: Contributors and Tracers of the Galactic Dark-Matter Halo

We examine the claim by Oppenheimer et al. (2001) that the local halo density of white dwarfs is an order of magnitude higher than previously thought. As it stands, the observational data support the presence of a kinematically distinct population of halo white dwarfs at the >99% confidence level. A maximum-likelihood analysis gives a radial velocity dispersion of σ h/U = 150^(+80)_(-40) km s^(-1) and an asymmetric drift of v h/α = 176^(+102)_(-80) km s^(-1), for a Schwarzschild velocity distribution function with σU:σV:σW = 1:2/3:1/2. Halo white dwarfs have a local number density of 1.1^(+2.1)_(-0.7) x 10^(-4) pc^(-3), which amounts to 0.8^(+1.6)_(-0.5) per cent of the nominal local dark-matter halo density and is 5.0^(+9.5)_(-3.2) times (90% C.L.) higher and thus only marginally in agreement with previous estimates. We discuss several direct consequences of this white-dwarf population (e.g. microlensing) and postulate a potential mechanism to eject young white dwarfs from the disc to the halo, through the orbital instabilities in triple or multiple stellar systems.