White dwarfs in common proper motion binary systems: Mass distribution and kinematics

We present the mass distribution, gravitational redshifts, radial velocities, and space motions of white dwarf stars in common proper motion binary systems. The mass distribution we derive for the 41 DA white dwarfs in this study has a mean of 0.68 +/- 0.04 M.. This distribution has a slightly higher mean and larger dispersion than most previous white dwarf studies. We hypothesize that this is due to a higher fraction of cool (average T-eff similar to 10,000 K), hence old, white dwarfs in our sample. Our results indicate that samples made up of predominantly cool, old white dwarf stars tend to have a bimodal distribution with a second mass peak at similar to1.0 M., which skews the mean toward a higher mass. Both the mean and individual white dwarf masses we report here are in better agreement with those determined from model atmosphere spectroscopic fits to line profiles than with most previous gravitational redshift studies of cool white dwarfs. Our results indicate that measurement biases and weak geocoronal emission lines in the observed spectra may have affected previous gravitational redshift measurements. These have been minimized in our study. We present measurements for some previously unobserved white dwarfs, as well as independent new measurements for some that have been reported in the literature. A list of complete space motions for 50 wide binary white dwarfs is presented, derived from radial velocity measurements of their nondegenerate companions. We find that the UVW space motions and dispersions of the common proper motion binaries that contain white dwarf components are consistent with those of old, metal-poor disk stars.