The Mass Distribution of White Dwarfs: An Unwavering Obsession

We discuss some of our current knowledge of the mass distribution of DA and non-DA stars using various methods for measuring white dwarf masses including spectroscopic, trigonometric parallax, and gravitational redshift measurements, with a particular emphasis on the problems encountered at the low end of the cooling sequence where energy transport by convection becomes important. 1. An Unwavering Obsession The unwavering obsession to which the title refers applies only to the first author since the other co-authors are still too young to be obsessed by such a thing as the mass distribution of white dwarf stars. As early as 1976, it was suggested that below Teff ∼ 12, 000 K, convective mixing between the thin superficial hydrogen layer and the more massive underlying helium layer could turn a hydrogen-rich star into a helium-rich star, provided the mass of the hydrogen layer is small enough (a modern value yields MH ≃ 10 −6 M⊙). Furthermore, the effective temperature at which this mixing occurs is a function of the mass of the hydrogen layer: for thicker hydrogen layers, the mixing occurs at lower effective temperatures. Since the process of convective mixing is still poorly understood, the exact ratio of helium to hydrogen after the mixing occurs remains unknown. In particular, it is possible that instead of turning a DA star into a featureless helium-rich DC star, convective mixing may simply enrich the hydrogen-rich atmosphere with large quantities of helium, leading to a mixed hydrogen and helium atmospheric composition. Such a hypothesis is difficult to test, however, since helium becomes spectroscopically invisible below Teff ∼ 12, 000 K, and its presence can only be inferred through indirect methods. Such a method has been proposed by Liebert & Wehrse (1983) who showed that the atmospheric helium abundance could be determined from a detailed examination of the high Balmer lines, since the presence of helium increases the photospheric pressure, and thus produces a quenching of the upper levels of the hydrogen atom which, in turn, affects the line profiles. This method has been put forward on a more quantitative basis by Bergeron et al. (1990) who analyzed 37 cool DA stars using the spectroscopic method of fitting high Balmer