Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany
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Preprint 03/00

NLTE spectral analysis of the sdOB primary of the eclipsing binary system LB 3459 (AA Dor)

T. Rauch

Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

2000, Astronomy & Astrophysics, 356, 665

Abstract. We present a spectral analysis of the sdOB primary star of the binary system LB 3459 based on high-resolution high-S/N optical and UV spectra. The metal abundances are determined by means of state-of-the-art NLTE model atmospheres.
We determined T_eff=42kK and log(g)=5.2 within very small error limits. The He (1/125 solar), C (1/265), N (1/33), O (1/12), and Si (1/5) abundances appear strongly depleted while that of Fe and Ni are roughly solar and Mg is strongly enriched by a factor of 6.
The spectroscopic distance to LB 3459 is d=396pc. The mass of the primary component of LB 3459 is 0.330M_sun derived from comparisons with theoretical models for sdO stars in the log(T_eff) - log(g) plane. The mass of the secondary is then 0.066M_sun derived from the mass function. There remains some disagreement between the radius derived from log(g) and the above mass, and that derived from analysis of the radial-velocity curve and the eclipse curves. LB 3459 is a close binary system which had experienced a common envelope (CE) phase during its evolution. It fits in the "low mass case B" scenario of Iben & Livio (1993) and the secondary is a brown dwarf.
The spectroscopically determined rotational velocity of the primary is v_rot=34+-10km/sec. Thus even bound rotation (v_rot=45.7km/sec) cannot be ruled out.

Key words: stars: abundances - stars: binaries: eclipsing - stars: evolution - stars: fundamental parameters - stars: individual: LB 3459 - stars: low-mass, brown dwarfs

Paper (383k gzip'ed Postscript including figures)

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