A self-consistent chemically stratified atmosphere model for the roAp star 10 Aquilae^⋆

¹ Department of RadiotherapyMedical University of Vienna, Waehringer Guertel 18−20, 1090 Vienna, Austria
e-mail: nicole.nesvacil@univie.ac.at
² Institute for Astronomy (IfA), University of Vienna, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
³ Institute of Astrophysics, Georg-August University, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 109017 Moscow, Russia
⁵ Department of Physics and Astronomy, Uppsala University, PO Box 516, 75120 Uppsala, Sweden

Received: 31 August 2012
Accepted: 5 December 2012

Abstract

Context. Chemically peculiar A-type (Ap) stars are a subgroup of the CP2 stars that exhibit anomalous overabundances of numerous elements, e.g. Fe, Cr, Sr, and rare earth elements. The pulsating subgroup of Ap stars, the roAp stars, present ideal laboratories to observe and model pulsational signatures, as well as the interplay of the pulsations with strong magnetic fields and vertical abundance gradients.

Aims. Based on high-resolution spectroscopic observations and observed stellar energy distributions, we construct a self-consistent model atmosphere for the roAp star 10 Aquilae (HD 176232). It accounts for modulations of the temperature-pressure structure caused by vertical abundance gradients. We demonstrate that such an analysis can be used to determine precisely the fundamental atmospheric parameters required for pulsation modelling.

Methods. Average abundances were derived for 56 species. For Mg, Si, Ca, Cr, Fe, Co, Sr, Pr, and Nd, vertical stratification profiles were empirically derived using the DDAFit minimisation routine together with the magnetic spectrum synthesis codeSynthmag. Model atmospheres were computed with the LLmodels code, which accounts for the individual abundances and stratification of chemical elements.

Results. For the final model atmosphere, T_eff = 7550 K and log    (g) = 3.8 were adopted. While Mg, Si, Co, and Cr exhibit steep abundance gradients, Ca, Fe, and Sr showed much wider abundance gradients between logτ₅₀₀₀ = −1.5 and 0.5. Elements Mg and Co were found to be the least stratified, while Ca and Sr showed strong depth variations in abundance of up to  ≈ 6 dex.