Diffusion time scales in white dwarfs.

The time scales of a few representative elements diffusing in hydrogen- and helium-rich white-dwarf envelopes are presented. The dependences on depth, the stellar mass, and effective temperature are investigated. It is shown that diffusion processes are sufficiently rapid that the observed monoelemental character of white-dwarf spectra can be explained. Even deep convective mixing, present in the surface layers of cooler white dwarfs, cannot bring back to the photosphere heavy elements that have diffused downward. The present calculations lead to estimates of diffusion time scales that are so short that no trace element should be seen in the spectra of white dwarfs. Mechanisms such as accretion competing with diffusion may have to be invoked to explain the small but measurable abundances of heavy elements in the spectra of these objects. For the vast majority of white dwarfs these conclusions are not affected by our having neglected radiative forces or by inaccuracies in the transport coefficients. However, improvements in the estimates of transport coefficients in dense plasmas are needed.