High-resolution spectroscopy of G191-B2B in the extreme-ultraviolet
journal contributionposted on 24.10.2012, 09:04 by R.G. Cruddace, M.P. Kowalski, D. Yentis, C.M. Brown, H. Gursky, Martin A. Barstow, N.P. Bannister, G.W. Fraser, J.E. Spragg, J.S. Lapington, J.A. Tandy, B. Sanderson, J.L. Culhane, T.W. Barbee, J.F. Kordas, W. Goldstein, G.G. Fritz
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 low-density, ionized He component is clearly present along the line of sight, which if completely interstellar implies a He ionization 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 C IV absorption-line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data that includes a significant abundance of photospheric He. The 99% 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.