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S. Vennes, P.A. Thejll, R. Genova Galvan, and J. Dupuis
We present new effective temperature and surface gravity determinations for a sample of 90 hot white dwarfs detected in the Extreme Ultraviolet Explorer (EUVE) all-sky survey. The measurements, based on spectroscopy of the balmer line series obtained at the Michigan-Dartmouth-MIT observatory (MDM), Mount Stromlo Observatory (MSO), Lick Observatory, and Cerro-Tololo-Interamerican Observatory (CTIO), are used to constrain the space density as well as the population age and mass distribution of a sample of 110 EUV-selected DA white dwarfs in the solar neighborhood. We find a mass spectrum narrowly peaked near 0.56 M_solar, indicative of normal C-O core with a thin hydrogen layer, and a significant population of 10 ultra-massive (M >= 1.1 M_solar) white dwarfs; we also find that all objects fall between effective temperatures of ~25,000 and ~75,000 K and are younger than 30 Myr. Using Wood's evolutionary models we determine a DA white dwarf birthrate in the solar neighborhood of (0.7-1.0)E-12 pc^(-3) yr^(-1). Although most objects are on normal C-O cooling tracks, we suggest that a few low-mass white dwarfs and the population of ultra-massive white dwarfs may follow different paths with, respectively, He or, possibly, O-Ne-Mg cores.
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