Discovery of a New White Dwarf in a Binary System (EUVE 0720-317) in the Extreme Ultraviolet Explorer Survey and Implications for the Late Stages of Stellar Evolution.

S. Vennes, J.R. Thorstensen

The Astrophysical Journal Letters, 433, p.29-32, 1994

Abstract

A new pre-cataclysmic binary is identified in th Extreme Ultraviolet Explorer (EUVE) all-sky survey. The bright source EUVE 0720-317 shows a hot hydrogen-rich white dwarf optical continuum with overlying narrow Balmer-line emission. Using high signal-to-noise spectroscopy in the 4100-6700 A range we identify a late-type companion and find a 1.3 d periodic modulation in the emission-line velocities and strengths. We determine the components' spectral types (DAO and dM0-2), orbital velocities (gamma = 15 +/- 12 km/s). A first estimate of the white dwarf gravitational redshift, gamma_g = 45 +/- 20 km/s, and theoretical mass-radius relationships imply R_DAO = 0.010-0.016 R_solar and M_DAO = 0.55-0.90 M_solar. The orbital inclination is therefore i >= 52 deg, consistent with the large amplitude variations found in H_alpha equivalent widths that imply i >= 42 deg. We show that the discovery of new close WD+MS binary systems in EUV sky surveys has important implications for theory of common-envelope evolution, in particular for the predicted close binary birthrate and orbital and stellar parameters.

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