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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 Teff=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 0.330Msun derived from comparisons with theoretical models for sdO stars in the log(Teff) - log(g) plane. The mass of the secondary is then 0.066Msun 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 vrot=34+-10km/sec. Thus even bound rotation (vrot=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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