ORFEUS OBSERVATIONS OF G191-B2B - NEUTRAL AND IONIZED-GAS IN THE LOCAL INTERSTELLAR-MEDIUM

We present high-resolution (lambda/delta lambda = 3000) observations of far-ultraviolet (900-1170 Angstrom) absorption lines in the spectrum of the hot DA white dwarf G191-B2B, collected with the Berkeley EUV/FUV spectrometer in the ORFEUS telescope aboard the space platform Astro-SPAS during the mission of space shuttle Discovery flown in 1993 September. Lines of H I, C II, C* II, C III, N I, N II, N III, and O I are detected, and useful upper limits are set on C I and O VI. The apparent position of the H I ''Lyman edge,'' actually the convergence of many lines approaching the H I ionization potential, determines a narrow range of allowed values for the broadening parameter b for H I; equivalent widths of discrete H I features in combination with the column density of H I determined from Extreme Ultraviolet Explorer observations (Dupuis et al. 1995) confirm this range. We find b(HI) = 11.0 +/- 1.0 km s(-1). Features of O I allow us to estimate the temperature and turbulence (xi) in the gas containing the neutral species; we find 6200 < T < 8700 K and xi < 4.3 km s(-1). If the temperature and turbulence are similar to those found by Linsky et al. (1993) for the nearby line of sight to Capella, the column densities of the neutral species O I and N I are consistent with the column of H I and cosmic elemental abundances, suggesting similar ionization fractions for all three elements in the local cloud, and low depletion. The equivalent widths of the C II and N II features are significantly larger than would be expected from the local cloud gas unless hydrogen in the local cloud is very highly ionized. We rule out this possibility on the grounds that the resulting high electron density would overproduce the population of excited-state C* II, whose column density is constrained by our data, and requires chi(H) < 0.75 in the local cloud. We conclude that one or more additional clouds exist along the line of sight. The column density of hydrogen nuclei in gas beyond the local cloud could be as low as 4 x 10(17) cm(-2), if the local cloud ionization is at its upper limit and the region is well separated in velocity from the local cloud, or as high as 2 x 10(18) cm(-2) adopting Doppler b-values for carbon and nitrogen that reconcile their column densities with cosmic abundances in each case. Regardless of how the ionized gas is distributed between the local cloud and other regions, N-H II is comparable to or significantly greater than N-H I along the line of sight. Our upper limit to N-O VI requires that there be no more than two conductive interfaces along the line of sight.