Abstract
We present a detailed spectroscopic analysis of the hot DA white dwarf G191-B2B, using the best signal to noise, high resolution near and far UV spectrum obtained to date. This is constructed from co-added \textit{HST} STIS E140H, E230H, and \textit{FUSE} observations, covering the spectral ranges of 1150-3145\AA\, and 910-1185\AA\, respectively. With the aid of recently published atomic data, we have been able to identify previously undetected absorption features down to equivalent widths of only a few m\AA. In total, 976 absorption features have been detected to $3\sigma$ confidence or greater, with 947 of these lines now possessing an identification, the majority of which are attributed to Fe and Ni transitions. In our survey, we have also potentially identified an additional source of circumstellar material originating from Si {\sc iii}. While we confirm the presence of Ge detected by \citet{vennes05a}, we do not detect any other species. Furthermore, we have calculated updated abundances for C, N, O, Si, P, S, Fe, and Ni, while also calculating, for the first time, an NLTE abundance for Al, deriving Al {\sc iii}/H=$1.60_{-0.08}^{+0.07}\times{10}^{-7}$. Our analysis constitutes what is the most complete spectroscopic survey of any white dwarf. All observed absorption features in the \textit{FUSE} spectrum have now been identified, and relatively few remain elusive in the STIS spectrum.
Highlights
The archetype H rich white dwarf, G191-B2B (WD 0501+527), has long been used both as a photometric standard due to its apparent brightness, and as a spectroscopic ”gold standard” when analysing other hot DA stars
We examined all of the available E140H and E230H datasets (39 and 77 respectively) from the Mikulski Archive for Space Telescopes4 (MAST) website in order to check for discontinuities and errors in the observations
We found that 32 E140H and 66 E230H observations were free of such problems, and were included in the final coadded product, with total exposure times of 53318 and 77743s respectively
Summary
The archetype H rich white dwarf, G191-B2B (WD 0501+527), has long been used both as a photometric standard due to its apparent brightness, and as a spectroscopic ”gold standard” when analysing other hot DA stars. Pioneered by Holberg et al (1985), a grid of theoretical spectra with differing values of Teff and log g can be used to fit either the H Lyman or Balmer absorption profiles to that of an observed spectrum, as such profiles are very sensitive to changes in these parameters Such a method, appears to have limitations for white dwarfs whose Teff >40,000K. This disagreement is present in other white dwarfs in the sample of Barstow et al (2003b) It should be noted, that Chayer et al (1994) reported significant differences in predicted atmospheric abundances of Fe and Ni dependent on the number of transitions included in their calculations. The potential significance of including additional opacities into model atmosphere calculations is discussed, providing a tentative solution to the LymanBalmer line problem This data set is a unique and invaluable record of the archetype white dwarf G191B2B. It will serve as a template for studies of other hot white dwarfs, as a test bed for model atmosphere calculations, and for the improvement of atomic databases
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