Properties and Evolution of White Dwarf Stars

Abstract

This paper reviews the properties and evolutionary status of white dwarf stars, focusing most closely on those aspects which are likely to be of significance to understanding the ultimate fate of planetary nebulae and their central stars. White dwarf stars show a broad variety of chemical compositions. Broadly speaking, they are divided into the DA stars (with H-rich photospheres) and the non-DA stars (with He-rich photospheres), though there are a fairly large number of subtypes. The mass distribution of white dwarf stars is quite narrow, with a mean value near 0.6 and with extremes at 0.43 and 1.05. Different varieties of trace elements (such as C, N, 0, Si, Ca, and Mg) are quite common. I will review several recent proposals for explaining these abundance patterns. A particularly significant question is whether processes operating while the star cools as a white dwarf can account for their variety, or whether at least part of the white dwarf phenomenology is related to events which took place when the object was a planetary nebula or even earlier.