White Dwarf Masses-Gravitational Redshifts Revisited

Abstract

We have used the HIRES spectrograph on the Keck telescope to determine radial velocities and Einstein redshifts for 53 white dwarfs that are either in common proper-motion binaries or members of either the Hyades or Praesepe. Five of the field-star systems have non-DA white dwarf components; three others are triple, rather than binary, systems; and five are probably optical doubles rather than true binaries. We have used Wood's (1995) hydrogen-atmosphere white dwarf evolutionary models to estimate radii, and hence determine masses, for the remaining stars. Comparing our results with masses derived using line-profile fitting, we find excellent agreement for white dwarfs with temperatures in excess of 14 000 K but indications of systematic trends in the residuals at cooler temperatures. Such deviations are predicted at temperatures below ˜12 000 K, where convection leads to a higher helium content in the DA atmosphere, higher pressure, and broader lines, and most of the observed residuals are in the expected sense. The field white dwarf mass distribution peaks at ˜0.58 Msun, with a median mass of 0.581 Msunand at least 70% of the stars having Msun≥0.55 . Most of the Hyades and Praesepe stars have higher masses, Msun≥0.65 Msun, as expected given that the progenitor stars had masses of 2.5-3.0 Msun. The mass range covered in Praesepe (˜0.6-0.9 Msun) is surprisingly large, however, and there is poor agreement with previous determinations of the initial-final mass relation.