Surface chemistry of stars. I. Diffusion of heavy ions in white dwarf envelopes

We have examined the diffusion separation of heavy elements in the envelopes of white dwarfs. Even when there is a deep convective zone, gravitational sedimentation is an efficient process. The convective mixing enhances the settling process. The surface abundances are determined by a balance between accreting interstellar matter and this diffusive separation. The abundances of trace elements are extremely low, and bear no relation to the primordial abundances