Carbon-Enrichment in the Atmosphere of the Cool DO White Dwarf MCT 0501-2858

We present the first comprehensive spectroscopic overview of a hot degenerate star with observations of MCT 0501-2858 (=EUVE 0503-289), covering the complete energy distribution from 70 to 6700 Angstroms . Optical spectroscopy reveals a helium-rich atmosphere with dominant He II and weak He I line absorption characteristic of the class of cool DO white dwarfs defined by Wesemael, Green, & Liebert (1985, ApJS, 58, 379), but also shows the C IV/He II blend near 4650--4690 Angstroms characteristic of ultra-hot PG 1159 stars like MCT 0130-1937 with effective temperatures > 10(5) K. However, Extreme Ultraviolet Explorer (EUVE) photometric measurements have already imposed a much lower temperature of T_eff = 60-70,000 K (Vennes et al. 1994, ApJ, 421, L35). MCT 0501-2858 is very peculiar and has spectroscopic properties intermediate to the cool DO white dwarfs and their hot PG 1159 progenitors. New extreme ultraviolet (EUV) spectroscopy obtained with EUVE reveals a bright continuum overlaid by rich He II, C III and C IV line spectra --truncated below ~ 230 Angstroms by He II photoionization. The spectrum is almost extinct near 750 Angstroms due to interstellar attenuation; we derive a neutral hydrogen column density of 6-10 times 10(18) cm(-2) , consistent with earlier photometric estimates. Using new Opacity Project radiative cross-sections and our comprehensive optical/EUV C IV line absorption spectrum--in particular the lines emerging from 2s, 2p, 3s, 5p, 5d--we determine a carbon abundance 3 to 10 times larger than the solar abundance, while other heavy elements are only marginally present. This extraordinary carbon abundance is reminiscent of the peculiar C-rich atmosphere of hot PG 1159 stars and may be the result of on-going chemical separation in photospheric layers. This work has been supported by NASA grant NAG5-2405.