The Peculiar Atmospheric Composition of the DO White Dwarf MCT 0501-2858

MCT 0501-2858 is a helium rich DO white dwarf (T=70,000K, log g =7.0) discovered in the Montreal-Cambridge-Tololo (MCT) survey of blue southern hemisphere objects (Vennes et al. 1994, ApJL, 421, L35). DO white dwarfs are much rarer than hydrogen rich DA types and may represent an earlier stage in the evolution of these objects. Additional study of this object with the Voyager probe and EUVE satellite revealed that it is located at an unusually low ISM column density (Barstow et al. 1994, MNRAS, 267, 653; Vennes et al. 1994). We report identification of the strongest spectral lines in the high-resolution IUE (1150-1950 Angstroms) spectrum of this object and the results of LTE model atmosphere fitting to the elemental abundances. The ultraviolet spectrum of this object is characterized by weak, low ionization ISM lines and strong photospheric lines of C III lambda 1247, C IV lambda lambda 1230, 1548, 1550, N IV lambda 1718, N V lambda lambda 1239, 1243, O IV lambda lambda 1338, 1343, O V lambda 1371, and Si IV lambda lambda 1393, 1402. We derive abundances with respect to helium by number of 3 x 10(-3) , 6 x 10(-5) , 3 x 10(-3) , and 6 x 10(-5) , for C, N, O, and Si, respectiveley. We find an excess of C and O (C/He=0.5 solar O/He = .3 solar) relative to all other elements. N and Si are present at .03 solar and .12 solar respectiveley. MCT 0501-2858 is lacking in Fe and an upper limit has been obtained for the abundances of both Fe and Ni of 1 x 10(-6) . Given the high C and 0 abundances, one would expect similar amounts of Fe and Ni, but they are not apparent in the spectra, contrary to what is observed in DA white dwarfs of similar temperature (Holberg et al. 1994, ApJL, 425, L105). We present a detailed comparison of the abundances derived from synthetic FUV and EUV models to the IUE and EUVE data (Vennes et al. 1994, BAAS, 26, 868). Higher resolution spectra are hoped for with HST to refine the Ni and Fe abundances. This work has been supported by NASA grant NAG5-2405.