The Complete Spectral Energy Distribution and the Atmospheric Properties of the Helium-Rich White Dwarf MCT 0501-2858

Abstract

We analyze high-dispersion ultraviolet spectroscopic observations of the peculiar helium-rich white dwarf MCT 0501-2858. Ultraviolet spectra obtained during the 1996 November ORFEUS-SPAS II mission show the complete He II (n=2) line series as well as narrow heavy-element lines; the spectrum constrains the effective temperature (68,600 +/- 1800 K) and surface gravity (log g=7.20+/-0.07), imposing a low mass (0.4 M_solar) and short cooling age (10⁵ yr) for the star. A study of the complete spectral energy distribution, based on EUVE, ORFEUS, and IUE spectra, shows that carbon is the second most abundant element [log (C/He) = -1.9] and that lower abundances are measured for other elements [log (N/He) = -4.0, log (O/He) = -3.3, log (Si/He) = -4.6, and log (P/He) = -6.2]. The new atmospheric parameters (temperature, gravity, and chemical composition) suggest a possible evolutionary filiation with the class of PG 1159 stars.

Based on the development and utilization of ORFEUS (Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometers), a collaboration of the Institute for Astronomy and Astrophysics at the University of Tübingen, the Space Astrophysics Group of the University of California at Berkeley, and the Landessternwarte Heidelberg.