Ultraviolet spectroscopy of the DBV white dwarf star GD 358

We report on analysis of UV spectra of the DBV GD 358 obtained with the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST). We confirm the detection of He II (1640 Angstrom and C II (1335 Angstrom reported by Sion et al. in 1988. The He II feature indicates a temperature of 27,000 +/- 1000 K, somewhat higher than previous estimates. We derive a carbon abundance -5.75 < log [n(C)/n(He)] < -5.55. In addition, we detect, for the first time, hydrogen in the spectrum. Whether this hydrogen is either photospheric or interstellar, the inferred photospheric upper limit is log [n(H)/n(He)] < -5, translating into a mass of <5 x 10(-18)M*. We discuss the implications of these findings on GD 358's asteroseismologically determined structure and the impact on theories of white dwarf formation and evolution.