Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...446..812h&link_type=abstract
Astrophysical Journal v.446, p.812
Astronomy and Astrophysics
Astronomy
30
Ism: General, Stars: Atmospheres, Stars: Individual Alphanumeric: G191-B2B, Stars: White Dwarfs, Ultraviolet: Stars
Scientific paper
We present high-resolution (λ/δλ = 3000) observations of far-ultraviolet (900-1170 Å) 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 bH I = 11.0±1.0 km s-1. Features of O I allow us to estimate the temperature and turbulence (ζ) in the gas containing the neutral species; we find 6200 < T < 8700 K and ζ < 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 χH < 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 × 1017 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 × 1018 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, NH II is comparable to or significantly greater than NH I along the line of sight. Our upper limit to NO VI requires that there be no more than two conductive interfaces along the line of sight.
Bowyer Stuart
Hurwitz Margaret M.
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