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F. Bruhweiler (IACS/CUA), M. Barstow (Univ. Leichester), J. Holberg (Univ. Arizona), M. Sahu (AURA/GSFC)
Examination of high resolution (R ~ 100,000) echelle spectra of the nearby, hot DA white dwarf, G191-B2B, obtained by the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) reveals two velocity components to the C IV resonance doublet at 1548 and 1550Å. One component is clearly photospheric and corresponds to the common velocity of other established photospheric features. The second velocity component has no analogue in any of the absorption from the other highly ionized species, N V, Si IV, and Si III, seen in the spectrum of G191-B2B. The FWHM is roughly 10 km s-1, about half that of the photospheric C IV. However, it appears not to be of interstellar origin, as its velocity is intermediate to the two interstellar velocity components seen in the line-of-sight. Moreover, the absence of Si IV absorption at the same velocity is problematic for the interstellar interpretation. Photoionization modeling for a realistic range of interstellar depletions and conditions further implies a non-interstellar origin for this C IV. The most viable explanation is that the C IV is formed close to the star, in the gravitational well of the white dwarf at a gravitational redshift that is -12 km s-1 with respect to the photosphere. Since the relative abundances cannot be determined in the gas producing the C IV, it is unclear if the ion has been selectively, radiatively ejected from the photosphere. If the C IV arises in the gravitational well of G191-B2B, photoionization calculations suggest that the relative abundances could be near solar composition and still yield undetectable Si IV absorption.