M.J. Stift and G. Alecian

Time-dependent atomic diffusion in magnetic ApBp stars

Figure 1. Time-dependent diffusion of Fe compared to the equi- librium stratification for a model with T_eff = 10 000 K, log g = 4.0 and a magnetic field of 10 kG strength, inclined by 60° with re- spect to the surface normal. The various curves are labelled with the time in years elapsed since the assumed onset of diffusion. The dash-dotted line gives the equilibrium stratification - the radiative acceleration of Fe is equal to 4.0 throughout the atmosphere.

Abstract

Numerical modelling of surface abundance distributions in ApBp star atmospheres constitutes a challenging astrophysical problem. This article is intended to deepen our understanding of how atomic diffusion affects the atmospheric structure of magnetic ApBp stars, and in particular how time-dependent calculations may be compared to the alternative method of estimating equilibrium stratifications. Our numerical calculations - with the stellar atmosphere adjusted self-consistently to the abundance profiles - show that final stationary solutions of the time-dependent diffusion problem (constant particle flux throughout the stellar atmosphere) are seemingly at variance with equilibrium stratifications (zero particle flux). In this work we will provide some understanding of the origin of these differences and try to elucidate the as yet little explored behaviour of time-dependent atomic diffusion. To this purpose, we assess the influence of the boundary condition at the bottom of the atmosphere, we investigate how the stratifications depend on magnetic field angle and strength, and we have a look at possible interactions between different chemical elements. Based on a grid of atmospheric models and stratifications reflecting dipolar magnetic geometries, we also present predicted line profiles for different oblique rotator models. Finally, we shortly discuss the consequences of our findings for the interpretation of abundance maps of magnetic ApBp stars.

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Last Revised: 2016 January 18th