White dwarfs in common proper motion binary systems: Mass distribution andkinematics

We present the mass distribution, gravitational redshifts, radial velocitie s, and space motions of white dwarf stars in common proper motion binary sy stems. The mass distribution we derive for the 41 DA white dwarfs in this s tudy has a mean of 0.68 +/- 0.04 M.. This distribution has a slightly highe r mean and larger dispersion than most previous white dwarf studies. We hyp othesize that this is due to a higher fraction of cool (average T-eff simil ar to 10,000 K), hence old, white dwarfs in our sample. Our results indicat e 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., w hich skews the mean toward a higher mass. Both the mean and individual whit e dwarf masses we report here are in better agreement with those determined from model atmosphere spectroscopic fits to line profiles than with most p revious gravitational redshift studies of cool white dwarfs. Our results in dicate that measurement biases and weak geocoronal emission lines in the ob served spectra may have affected previous gravitational redshift measuremen ts. These have been minimized in our study. We present measurements for som e previously unobserved white dwarfs, as well as independent new measuremen ts for some that have been reported in the literature. A list of complete s pace motions for 50 wide binary white dwarfs is presented, derived from rad ial velocity measurements of their nondegenerate companions. We find that t he UVW space motions and dispersions of the common proper motion binaries t hat contain white dwarf components are consistent with those of old, metal- poor disk stars.