EVOLUTION OF THE SURFACE ABUNDANCE OF COOL DA WHITE-DWARF STARS

The problem of the evolution of the surface abundance of cool DA white dwarf stars is considered. Since these stars exhibit only hydrogen lines in their spectra, it has long been assumed that their atmospheres were composed of pure hydrogen. However, we shall demonstrate that helium, which is spectroscopically invisible at these temperatures (T(eff) less than or similar to 12,000 K), is present in large amounts at the surface of these stars. The presence of helium is interpreted as strong evidence for convective mixing between the thin superficial hydrogen layer with the more massive underlying helium layer. In particular, we find that most stars mix at a temperature of T(eff) is similar to 11,500 K, which corresponds to the location of the cool edge of the ZZ Ceti instability strip.