The kinematics of Galactic disc white dwarfs in Gaia DR2

Abstract

We present an analysis of the kinematics of Galactic disc white dwarf stars in the Solar neighbourhood using data from Gaia Data Release 2. Selection of white dwarfs based on parallax provides the first large, kinematically unbiased sample of Solar neighbourhood white dwarfs to date. Various classical properties of the Solar neighbourhood kinematics have been detected for the first time in the WD population. The disc white dwarf population exhibits a correlation between absolute magnitude and mean age, which we exploit to obtain an independent estimate of the Solar motion with respect to the Local Standard of Rest. This is found to be $(U,V,W)_{\odot} = (9.5\pm1.2, 7.5\pm1.2, 8.2\pm1.2)$ kms$^{-1}$. The $UW$ components agree with studies based on main sequence stars, however the $V$ component differs and may be affected by systematics arising from metallicity gradients in the disc. The velocity ellipsoid is shown to vary strongly with magnitude, and exhibits a significant vertex deviation in the $UV$ plane of around 15 degrees, due to the non-axisymmetric Galactic potential. The results of this study provide an important input to proper motion surveys for white dwarfs, which require knowledge of the velocity distribution in order to correct for missing low velocity stars that are culled from the sample to reduce subdwarf contamination.