Putting atomic diffusion theory of magnetic ApBp stars to the test: evaluation of the predictions of time-dependent diffusion models

TL;DR

This study critically tests advanced atomic diffusion models for magnetic ApBp stars by comparing their predictions with observations, revealing significant discrepancies in element abundances and questioning the models' accuracy in explaining surface chemical distributions.

Contribution

It provides the first direct comparison between sophisticated time-dependent diffusion models and empirical surface maps, highlighting their limitations in matching observed element distributions.

Findings

Mean Fe and Cr abundances are grossly underestimated by models.

Doppler imaging can accurately reconstruct horizontal abundance maps.

Current diffusion models poorly predict observed chemical spot patterns.

Abstract

A series of recent theoretical atomic diffusion studies has address the challenging problem of predicting inhomogeneous vertical and horizontal chemical element distributions in the atmospheres of magnetic ApBp stars. Here we critically assess the most sophisticated of such diffusion models - based on a time-dependent treatment of the atomic diffusion in a magnetised stellar atmosphere - by direct comparison with observations as well by testing the widely used surface mapping tools with the spectral line profiles predicted by this theory. We show that the mean abundances of Fe and Cr are grossly underestimated by the time-dependent theoretical diffusion model, with discrepancies reaching a factor of 1000 for Cr. We also demonstrate that Doppler imaging inversion codes, based either on modelling of individual metal lines or line-averaged profiles simulated according to theoretical three-dimensional abundance distribution, are able to reconstruct correct horizontal chemical spot maps despite ignoring the vertical abundance variation. These numerical experiments justify a direct comparison of the empirical two-dimensional Doppler maps with theoretical diffusion calculations. This comparison is generally unfavourable for the current diffusion theory, as very few chemical elements are observed to form overabundance rings in the horizontal field regions as predicted by the theory and there are numerous examples of element accumulations in the vicinity of radial field zones, which cannot be explained by diffusion calculations.