ar X iv : a st ro - p h / 01 12 25 4 v 1 1 1 D ec 2 00 1 High - Resolution Spectroscopy of G 191 - B 2 B in the Extreme Ultraviolet

We report a high-resolution (R=3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme ultraviolet band 220− 245 Å. A lowdensity ionised He component is clearly present along the line-of-sight, which if completely interstellar implies a He ionisation fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the CIV absorption line doublet in an HST STIS spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data which includes a significant abundance of photospheric He. The 99-percent confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients. Subject headings: white dwarfs—stars: individual(G191B2B)—ISM: general— circumstellar matter E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington DC 20375, U.S.A. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, U.K. Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, U.K. Lawrence Livermore National Laboratory, Livermore, California 97550, U.S.A. Praxis Inc., 2200 Mill Road, Alexandria, Virginia 22314, U.S.A.