Diffusion in White Dwarfs: New Results and Comparative Study

The calculations yielding the present estimates of heavy element diffusion time scales in white dwarfs explicitly take into account screening effects in the evaluation of the transport properties of white dwarf plasmas. It is noted that, contrary to the findings of most previous studies, the diffusion time scales of various elements do not differ by large amounts, and do not depend in any simple way on the masses of the diffusing ions. The time scales obtained are much smaller than evolutionary time scales, confirming the need for mechanisms that compete against settling in order to account for the metal traces in the spectra of several cool white dwarfs. An explicit explanation for the large differences that have been found in the diffusion time scales estimated by various investigators is given.