Deuterium abundance toward G191-B2B:: Results from the FUSE mission

High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905 1187 A, were obtained with the Far Ultraviolet Spectroscopic Explorer ( FUSE). These data were used in conjunction with existing high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) observations to evaluate the total H I, D I, O I and N I column densities along the line of sight. Previous determinations of N (D I) based upon GHRS and STIS observations were controversial as a result of the saturated strength of the D I Lyalpha line. In the present analysis the column density of D I has been measured using only the unsaturated Lybeta and Lygamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are log N (D I) = 13.40 +/- 0.07, log N(O I) = 14.86 +/- 0.07, and log N(N I) = 13.87 +/- 0.07, quoted with 2 sigma uncertainties. The measurement of the H I column density by pro le fitting of the Lyalpha line has been found to be uncertain. If additional weak, hot interstellar components are added to the three detected clouds along the line of sight, the H I column density can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Lyalpha are excellent. The new estimate of N (H I) toward G191-B2B reads log N(H I) = 18.18 +/- 0.18 (2 sigma), so that the average D/H ratio on the line of sight is D/H = 1.66(-0.6)(+0.9))x 10(-5) (2 sigma).