Model atmospheres of chemically peculiar stars

Self-consistent empirical stratified model of HD 24712

¹ Institute of Astronomy, Vienna University, Turkenschanzstrasse 17, 1180 Vienna, Austria e-mail: denis@jan.astro.univie.ac.at
² Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia
³ Department of Astronomy and Space Physics, Uppsala University, Box 515, 751 20 Uppsala, Sweden

Received: 7 January 2009
Accepted: 19 March 2009

Abstract

Context. High-resolution spectra of some chemically peculiar stars clearly demonstrate the presence of strong abundance gradients in their atmospheres. However, these inhomogeneities are usually ignored in the standard scheme of model atmosphere calculations, breaking the consistency between model structure and spectroscopically derived abundance pattern.

Aims. In this paper we present the first empirical self-consistent stellar atmosphere model of the roAp star HD 24712  with stratification of chemical elements included, and which is derived directly from the observed profiles of spectral lines without time-consuming simulations of physical mechanisms responsible for these anomalies.

Methods. We used the LLmodels  stellar model atmosphere code and DDAFIT  minimization tool for analysis of chemical element stratification and construction of a self-consistent atmospheric model. Empirical determination of Pr and Nd stratification in the atmosphere of HD 24712  is based on NLTE line formation for Pr ii/iii  and Nd ii/iii with the use of the DETAIL code.

Results. Based on an iterative procedure of stratification analysis and subsequent re-calculation of model atmosphere structure, we constructed a self-consistent model of HD 24712, i.e. the model whose temperature-pressure structure is consistent with the results of the stratification analysis. It is shown that stratification of chemical elements leads to considerable changes in model structure compared to the non-stratified homogeneous case. We find that accumulation of rare earth elements (REE) allows for the inverse temperature gradient to be present in the upper atmosphere of the star with a maximum temperature increase of about 600 K.