Letter to the Editor

On the sdOB primary of the post common-envelope binary AA Doradus (LB 3459)^⋆,⋆⋆

Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
e-mail: rauch@astro.uni-tuebingen.de

Received: 14 March 2011
Accepted: 6 June 2011

Abstract

Context. AA Dor is an eclipsing, post common-envelope binary with an sdOB-type primary and a low-mass secondary. Eleven years ago, an NLTE spectral analysis showed a discrepancy in the surface gravity that was derived by radial-velocity and light-curve analysis, log g = 5.21 ± 0.1 (cm/s²) and log g = 5.53 ± 0.03, respectively.

Aims. We aim to determine both the effective temperature and surface gravity of AA Dor precisely from high-resolution, high-S/N observations taken during the occultation of the secondary.

Methods. We calculated an extended grid of metal-line blanketed, state-of-the-art, non-LTE model atmospheres in the parameter range of the primary of AA Dor. Synthetic spectra calculated from this grid were compared to optical observations.

Results. We verify from our former analyses and determine a higher log g = 5.46 ± 0.05. The main reason are new Stark-broadening tables that were used for calculating of the theoretical Balmer-line profiles.

Conclusions. Our result for the surface gravity agrees with the value from light-curve analysis within the error limits, thereby solving the so-called gravity problem in AA Dor.