Issue A&A Volume 505, Number 1, October I 2009 Page(s) 227 - 237 Section Stellar structure and evolution DOI 10.1051/0004-6361/200911789 Published online 16 July 2009

A&A 505, 227-237 (2009)
DOI: 10.1051/0004-6361/200911789

Structure and spectra of irradiated secondaries in close binaries

A model calculation of the pre-cataclysmic variable UU Sagittae

A. C. Wawrzyn^{1, 2}, T. S. Barman², H. M. Günther¹, P. H. Hauschildt¹, and K. M. Exter<sup>3

1</sup>  Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
    e-mail: awawrzyn@hs.uni-hamburg.de
²  Lowell Observatory, Hendricks Center for Planetary Studies, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA
    e-mail: barman@lowell.edu
³  Instituut voor Sterrenkunde, Celestijnenlaan 200 D, 3001 Heverlee (Leuven), Belgium

Received 4 February 2009 / Accepted 24 June 2009

Abstract
Context. The standard stellar model atmosphere ignores the influence of external radiation. This assumption, while sufficient for most stars, fails for many short-period binaries.
Aims. In setting up combined model atmospheres for close binaries, we want to constrain the parameters of both components, especially in the case of a hot primary component strongly influencing its cool secondary companion. This situation can be found after common envelope evolution (CEE). The status of both components today allows one to retrace the CEE itself.
Methods. We used our stellar atmosphere code PHOENIX, which includes the effect of irradiation in its radiation transport equation, to investigate the close binary star UU Sge. We combined our calculated spectra of both components, weighted by their visible size, and adjusted the input parameters until reasonable agreement with observations is reached.
Results. We derive a range of 80 000-85 000 K for the effective temperature of the primary and give a rough estimate for the primary's abundances, particularly the nitrogen enrichment. The heated day-side of the secondary has an apparent “effective” or equilibrium temperature of 24 000-26 000 K, nearly independent of its intrinsic luminosity. It shows an enhancement in nitrogen and carbon.
Conclusions. The evolution of the primary and secondary stars were strongly influenced by the other's presence. Radiation from the primary on the secondary's day-side is still an important factor in understanding the secondary's atmospheric structure.

Key words: radiative transfer -- stars: abundances -- binaries: eclipsing -- binaries: close -- stars: atmospheres -- stars: individual: UU Sge

© ESO 2009

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