A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp star α Circini^*

¹ Department of Physics and Astronomy, Uppsala University, Box 515, 751 20 Uppsala, Sweden e-mail: oleg@fysast.uu.se
² Institute of Astronomy, Vienna University, Turkenschanzstrasse 17, 1180 Vienna, Austria
³ Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia

Received: 13 January 2009
Accepted: 15 March 2009

Abstract

Context. Chemically peculiar (CP) stars are unique natural laboratories for the investigation of the microscopic diffusion processes of chemical elements. The element segregation under the influence of gravity and radiation pressure leads to the appearance of strong abundance gradients in the atmospheres of CP stars. Consequently, the atmospheric temperature-pressure structure of these objects could deviate significantly from the atmospheres of normal stars with homogeneous abundances.

Aims. In this study we performed a self-consistent, empirical model atmosphere study of the brightest rapidly oscillating Ap star α Cir. We account for chemical stratification in the model atmosphere calculations and assess the importance of non-uniform vertical element distribution on the model structure, energy distribution and hydrogen line profiles.

Methods. For the chemical stratification analysis we use the ddafit minimization tool in combination with a magnetic spectrum synthesis code. The model atmospheres with inhomogeneous vertical distributions of elements are calculated with the llmodels stellar model atmosphere code.

Results. Based on an iterative procedure of the chemical abundance analysis of 52 ions of 35 elements, stratification modeling of 4 elements (Si, Ca, Cr and Fe) and subsequent re-calculations of the atmospheric structure, we derived a new model atmosphere of α Cir which is consistent with the inferred atmospheric chemistry of the star. We find T_eff = 7500 K, log g = 4.1, and demonstrate that chemical stratification has a noticeable impact on the model structure and modifies the formation of the hydrogen Balmer lines. At the same time, the energy distribution appears to be less sensitive to the presence of large abundance gradients.

Conclusions. Our spectroscopically determined T_eff of α Cir agrees with the fundamental effective temperature of this star. This shows that temperatures inferred in detailed spectroscopic analyses of cool magnetic CP stars are not affected by a large systematic bias.