AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 71. Between the Stars II: The ISM, Galactic and Extragalactic
Display, Wednesday, June 2, 1999, 10:00am-6:30pm, Southwest Exhibit Hall

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[71.16] The D/H Ratio in Interstellar Gas towards G191-B2B from STIS Echelle Observations

M. S. Sahu (NASA/GSFC \& NOAO), W. B. Landsman (Raytheon STX), F. C. Bruhweiler (CUA), T. R. Gull, C. A. Bowers (NASA/GSFC), D. Lindler, K. Feggans (ACC), M. A. Barstow (U Leicester), I. Hubeny (NASA/GSFC \& NOAO), J. B. Holberg (U Arizona)

We present STIS echelle observations of interstellar D~{\sc i} and H~{\sc i} Ly{\alpha} and N~{\sc i} (1199.5, 1200.2 and 1200.7Å), C~{\sc ii} 1334.5Å, C\ast~{\sc ii} 1335.7Å, O~{\sc i} 1302Å, Si~{\sc ii} (1190, 1193, 1260, 1304 and 1526Å), Si~{\sc iii} 1206.5Å, Al~{\sc ii} 1670.8Å, S~{\sc ii} 1259.5Å\ and Fe~{\sc ii} 1608.5Å\ in the line of sight to the nearby (69~pc) hot, white dwarf (WD) G191-B2B. Compared to the GHRS study of G191-B2B by Vidal-Madjar et al. 1998 (VM98), the STIS E140H spectra have a higher velocity resolution (3 km s-1), better S/N (between 20 to 50) and broader wavelength coverage (1150 to 1700 Å). We use the Barstow & Hubeny stratified non-LTE model atmosphere calculations which include the effects of line-blanketing from more than 9\times106 atomic transitions (mainly Ni and Fe), both to determine the NLTE shape of the stellar Ly\alpha profile and to estimate the contamination of the interstellar lines by WD photospheric lines. The interstellar N~{\sc i} 1200.7Å, Si~{\sc ii} 1193 & 1304Å\ and Fe~{\sc ii} lines show no contamination by WD photospheric lines and are given more weight in our analysis. VM98 reported three components while we detect only two velocity components in all the interstellar species observed: one at ~ 8.5 km s-1 and one at ~ 19.3 km s-1 which we identify as the LIC component. Using the NLTE stellar Ly{\alpha} profile and a total column density of N(H~{\sc i}) ~ 2 \times 1018 cm-2 for both components (consistent with EUVE observations), we derive confidence contours. We find the D/H ratio with 2\sigma confidence limits to lie within 1.77±0.2\times10-5. This value is consistent with the value of (D/H)LIC = 1.6±0.1\times10-5 determined towards Capella (Linsky et al. 1995). The STIS data provide no evidence for local or cloud-to-cloud variation in the D/H ratio as suggested by VM98. Re-analysis of the GHRS data and comparison to the STIS data is in progress.

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