Diffusion in white dwarf stars

Diffusion appears to be important in modifying the surface composition of white dwarf stars and explaining the extreme purity of their atmospheres. This study attempts to calculate diffusion rates more carefully than has been done previously. Transport coeficients are calculated by evaluating collision cross section integrals including screened potentials. Multicomponent flow equations are used in preference to trace element approximations. Diffusion time scales obtained this way are longer than those which have been found by other authors, but they are still shorter than cooling times for white dwarfs. The trace amounts of metals observed in the spectra of these stars must be transients in the atmosphere destined to sink out of sight. In one example, the diffusion calculation has been coupled to the calculation of stellar structure. Some observational predictions of the calculated diffusion rates are discussed.