Improved Results for Equilibrium Abundances from Diffusion Calculations

Abstract

Due to high surface gravities, element segregation by gravitational settling is of great importance in white dwarf atmospheres. In hot white dwarfs, as in other hot stars, radiative levitation can efficiently counteract the down-ward diffusion of heavy elements, and the interplay between these forces governs the atmospheric chemical composition. Incorporating the competition of these processes into stellar atmosphere model calculations provides predictions for the vertical stratification and absolute abundances of metals, and is therefore an important task. An overview of the work done in the field of such diffusion calculations, with a focus on white dwarfs, is given in a recent review (Dreizler and Schuh, 2003), which contains the relevant references to both the articles where these ideas where first mentioned as well as to very recent papers which describe current implementations and their applications. Within these latest approaches, the equilibrium formulation (Chayer et al., 1995) is of special interest since it permits a relatively simple description of the balance between gravitational settling and radiative levitation. It therefore allows a self-consistent solution of the chemical stratification within state-of-the-art, fully line blanketed non-LTE stellar atmospheres with many elements, incorporated via detailed model atoms (Dreizler, 1999).