Modelling ApBp star atmospheres with stratified abundances consistent with atomic diffusion

Abstract

Elemental abundances have been found – both empirically and theoretically – to be stratified in the magnetic atmospheres of many chemically peculiar (CP) stars, but these findings are rarely if ever taken into account for example in (Zeeman) Doppler mapping, resulting in a disturbing problem of self-consistency. Such a lack of self-consistency is also present in many studies of atomic diffusion which have usually been based on vertically homogeneous atmospheric models. In this paper we go one step further in our modelling of diffusion in magnetic CP star atmospheres and discuss self-consistent equilibrium stratifications of elements, in particular of Fe. In an iterative approach, the atmospheric structure is adjusted to the latest stratification profiles of the various elements until zero particle flux throughout the atmosphere is achieved. It can be shown that the resulting stratifications are different from those calculated with vertically homogeneous atmospheres – mainly on account of the change in temperature structure – and that the unphysical behaviour resulting from the latter atmospheres may be avoided. An important finding concerns the mutual interaction between the various elements, leading to stratification profiles that differ substantially depending on whether e.g. Fe alone is allowed to diffuse or whether other elements may diffuse too. This presentation ends with a discussion of these interactions in the context of time-dependent diffusion calculations.