FUSE spectroscopy of the sdOB primary of the post common-envelope binary LB 3459(AA Doradus)^*

¹ 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
² Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 2 August 2008
Accepted: 5 September 2008

Abstract

Context. LB 3459(AA Doradus) is an eclipsing, close, post common-envelope binary (PCEB) consisting of an sdOB primary star and an unseen secondary with an extraordinarily low mass (M₂ ≈ 0.066 ) – formally a brown dwarf. A recent NLTE spectral analysis shows a discrepancy with the surface gravity, which is derived from analyses of radial-velocity and lightcurves.

Aims. We aim at precisely determining of the photospheric parameters of the primary, especially of the surface gravity, and searching for weak metal lines in the far UV.

Methods. We performed a detailed spectral analysis of the far-UV spectrum of LB 3459obtained with FUSE by means of state-of-the-art NLTE model-atmosphere techniques.

Results. A strong contamination of the far-UV spectrum of LB 3459by interstellar line absorption hampers a precise determination of the photospheric properties of its primary star. Its effective temperature (T_eff = 42 kK) was confirmed by the evaluation of new ionization equilibria. For the first time, phosphorus and sulfur have been identified in the spectrum of LB 3459. Their photospheric abundances are solar and 0.01 times solar, respectively. From the C III 1174-1177 Å multiplet, we can measure the rotational velocity v_rot = 35 ± 5 km s^-1 of the primary of LB 3459and confirm that the rotation is bound. From a re-analysis of optical and UV spectra (analogue to Rauch 2000, A&A, 356, 665), we determine a slightly higher surface gravity log g = 5.3 ± 0.1 compared to Rauch (2000, log g = 5.2 ± 0.1).

Conclusions. The rotational velocity of the primary of LB 3459is consistent with a bound rotation. The higher log g reduces the discrepancy in mass determination in comparison to analyses of radial-velocity and lightcurves. However, the problem is not completely solved.