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Issue A&A Volume 430, Number 1, January IV 2005 Page(s) 223 - 243 Section Stellar structure and evolution DOI 10.1051/0004-6361:20040232

A&A 430, 223-243 (2005)
DOI: 10.1051/0004-6361:20040232

Hot subdwarfs from the ESO Supernova Ia Progenitor Survey

I. Atmospheric parameters and cool companions of sdB stars

T. Lisker^{1, 2}, U. Heber¹, R. Napiwotzki^{1, 3}, N. Christlieb⁴, Z. Han⁵, D. Homeier⁶ and D. Reimers<sup>4

1</sup>  Dr.-Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
    e-mail: lisker@sternwarte.uni-erlangen.de
²  Institute of Astronomy, ETH Zürich, Department of Physics, HPF D8, ETH Hönggerberg, 8093 Zürich, Switzerland
³  Department of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁴  Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵  The Yunnan Observatory, Academia Sinica, Kunming 650011, PR China
⁶  Department of Physics and Astronomy, University of Georgia, Athens, GA 30602-2451, USA

(Received 9 February 2004 / Accepted 27 August 2004)

Abstract
We present the analysis of a high-resolution, high-quality sample of optical spectra for 76 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey (SPY, Napiwotzki et al. 2001). Effective temperature, surface gravity, and photospheric helium abundance are determined simultaneously by fitting the profiles of hydrogen and helium lines using synthetic spectra calculated from LTE and NLTE model atmospheres. We perform a detailed comparison of our measurements with theoretical calculations, both for single star evolution and for binary population synthesis models of close binary evolution. The luminosity evolution given by the standard EHB evolutionary tracks from Dorman et al. (1993) shows an overall agreement in shape with our observations, although a constant offset in luminosity exists. The various simulation sets for binary formation channels of sdB stars calculated by Han et al. (2003) are compared individually to our data for testing our current understanding of sdB formation processes and the physical effects involved. The best-matching sets manage to reproduce the observed sdB distribution in the temperature-gravity-plane well. However, they do not match the observed cumulative luminosity function, indicating that theoretical improvement is necessary. We also investigate composite-spectrum objects showing clear signatures of a cool companion with optical and infrared photometry. These stars have cool main sequence companions of spectral types F to K. Typical helium abundances of composite and non-composite sdB stars do not differ.

Key words: binaries: spectroscopic -- stars: abundances -- stars: atmospheres -- stars: fundamental parameters -- stars: horizontal-branch -- subdwarfs

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