3D kinematics of white dwarfs from the SPY project

Abstract

We present kinematics of a sample of 107 DA white dwarfs from the SPY project (ESO SN Ia Progenitor surveY) and discuss kinematic criteria for a distinction of thin disk, thick disk, and halo populations. This is the first homogeneous sample of white dwarfs for which 3D space motions have been determined. Since the percentage of old stars among white dwarfs is higher than among main-sequence stars, they are presumably valuable tools in studies of old populations such as the halo and the thick disk. Studies of white dwarf kinematics can help to determine the fraction of the total mass of our Galaxy contained in the form of thick disk and halo white dwarfs, an issue which is still under discussion. Radial velocities and spectroscopic distances obtained by the SPY project are combined with our measurements of proper motions to derive 3D space motions. Galactic orbits and further kinematic parameters are computed. We calculate individual errors of kinematic parameters by means of a Monte Carlo error propagation code. Our kinematic criteria for assigning population membership are deduced from a sample of F and G stars taken from the literature for which chemical criteria can be used to distinguish between thin disk, thick disk and halo. Candidates for thick disk and halo members are selected in a first step from the classical U–V-velocity diagram. Our final assignment of population membership is based on orbits and position in the Jz-eccentricity diagram. We find four halo and twelve thick disk white dwarfs. We also present a systematic study of the e ects of ignoring the radial velocity in kinematic investigations.