White Dwarf Mass Loss, Rotation, Individual Masses and the Identification of the White Dwarf Remnants of Upper Main Sequence Stars

Abstract

This review focuses upon current observational evidence of white dwarf mass loss, rotation rates and the identification of evolutionary (progenitor) links between white dwarfs and upper main sequence stars. The latter objective rests upon the determination of white dwarf masses through a number of methods, which are discussed. The conclusions are as follows: (1) There is some evidence in the far ultraviolet to suggest that hot DA white dwarfs lose mass via a weak wind. This mass outflow is manifested by the presence of sharp, shortward-shifted absorption lines in the IUE high resolution spectra of a number of hot DA white dwarfs (Bruhweiler and Kondo 1983). There is at present very weak direct evidence of actual mass loss by hot non-DA white dwarfs, except for strong wind outflow from the lower gravity pulsating central star of K1–16. The DO stars, KPD 0005+5106 and PG 1034+001, may be important exceptions; (2) The general conclusion at present concerning white dwarf rotation rates is that they are slow rotators with established upper limit υ sin i 0.6 M⊙) and therefore no link is indicated between these objects and upper main sequence progenitors; (4) At the present time it is not clear whether massive white dwarf progenitors would be expected to leave DA or non-DA remnants. Here, theory and observation provide conflicting indications; (5) The magnetic degenerates tend to have higher than average white dwarf mass and the majority are therefore expected to be the progeny of young, peculiar A and B stars on the upper main sequence. This conclusion is supported by a number of independent lines of evidence; (6) Higher-than-average mass white dwarfs (and therefore the remnants of progenitors more massive and younger than the typical and most populous old disk stars in the solar neighborhood), have been identified through kinematical properties, through gravitational redshift determinations in wide and common proper motion binaries, through kinematical membership in young moving groups, through the gravitational redshifts of the white dwarf members of the Hyades cluster, and through the spectroscopic identification and analysis of white dwarf members of very young open clusters whose turnoff masses to the red giant branch exceed 5 M⊙; A number of evolutionary implications and future prospects are discussed.