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Preprint C/01

HS 0705+6700: a new eclipsing sdB binary

H. Drechsel (1), U. Heber (1), R. Napiwotzki (1), R. Østensen (2;3), J.­E. Solheim (3), F. Johannessen (3), S. L. Schuh (4), J. Deetjen (4), and S. Zola (5)

(1) Dr. Remeis­Sternwarte Bamberg, Astronomisches Institut der Universität Erlangen­Nürnberg, Sternwartstraße 7, D­96049 Bamberg, Germany
(2) Isaac Newton Group of Telescopes, E­37800 Santa Cruz de La Palma, Canary Islands, Spain
(3) University of Tromsø, Department of Physics, N­9037 Tromsø, Norway
(4) Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
(5) Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30­244 Cracow, Poland

To appear in: Astronomy & Astrophysics

Abstract. We report the discovery of an eclipsing binary - HS 0705+6700 - being an sdB star with a faint companion. From its light curve the orbital period of 8263.87 s, the mass ratio of the system q = 028, the inclination of 84.4° and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection effect. The semi­amplitude of the radial velocity curve K1 = 85.8 km s-1 and a mass function of f(m) = 0.00626 Msol are determined. A spectroscopic analysis of the blue spectra results in Teff = 28800 K, log g = 5.40, and log (nHe/nH) = -2.68. These characteristics are typical for sdB stars, as is its mass of 0.48 Msol. According to its mass (0.13 Msol) and radius (0.19 Rsol), the companion is an M dwarf. The primary is in a core helium burning phase of evolution, and the system must have gone through a common envelope stage when the primary was near the tip of the red giant branch.

Key words: subdwarfs - binaries: eclipsing - binaries: spectroscopic - stars: early­type - stars: fundamental parameters - stars: individual: HS 0705+6700

Paper (673k gzip'ed Postscript including figures)

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