A Critical Look at the Question of Thick vs Thin Hydrogen and Helium Envelopes in White Dwarfs

Abstract

The question of the possible existence of relatively thin hydrogen and helium envelopes in white dwarfs has certainly enlivened the scientific debate in our community over the last decade. It has led to vigorous exchanges-one of them ending as a long, low-down blues song (!)-as well as some misconceptions and even mild disputes at times, but it has also certainly forced us, collectively, to think seriously about the spectral evolution of white dwarfs. Among the large number of papers devoted to the topic, we single out those with a relatively broad perspective: Sion (1986), Fontaine & Wesemael (1987), Shipman (1989), Koester (1989), Fontaine & Wesemael (1991), Shipman (1993), and Bergeron, Ruiz, & Leggett (1996). The more specialized papers devoted to one aspect or another of the subject are too numerous to mention here, but, for a good sample, we refer the reader to the Proceedings of the various white dwarf meetings held in Frascati (1986), Tucson (1987), Hanover (1988), Toulouse (1990), Leicester (1992), and Kiel (1994). This intense research activity is justified by the fact that the question of the thickness of the outer hydrogen and helium layers is critically important in understanding the weird atmospheric compositions of white dwarfs, in calibrating cooling calculations aimed at estimating the age of the Galactic disk and halo, and in constraining the conditions encountered in pre-white dwarf evolution, which, by all accounts, still constitutes a gray area in the theory of stellar evolution.