A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs. DA Stars
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We measure apparent velocities (v(app)) of the H alpha and H beta Balmer line cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Assuming these WDs are nearby and comoving, we correct our velocities to the local standard of rest so that the remaining stellar motions are random. By averaging over the sample, we are left with the mean gravitational redshift, < v(g)>: we find < v(g)> = < v(app)> = 32.57 +/- 1.17 km s(-1). Using the mass-radius relation from evolutionary models, this translates to a mean mass of 0.647(-0.014)(+0.013)M(circle dot). We interpret this as the mean mass for all DAs. Our results are in agreement with previous gravitational redshift studies but are significantly higher than all previous spectroscopic determinations except the recent findings of Tremblay & Bergeron. Since the gravitational redshift method is independent of surface gravity from atmosphere models, we investigate the mean mass of DAs with spectroscopic T(eff) both above and below 12,000 K; fits to line profiles give a rapid increase in the mean mass with decreasing T(eff). Our results are consistent with no significant change in mean mass: < M >(hot) = 0.640 +/- 0.014 M(circle dot) and < M >(cool) = 0.686(-0.039)(+0.035)M(circle dot).