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We report interstellar C ii column densities or upper limits determined from weak absorption of the 2325.4029 8 intersystem transition observed in six translucent sight lines (A V k 1) with the Space Telescope Imaging Spectrograph (STIS). The sight lines sample a wide range of interstellar characteristics, including total to selective extinction, R V ¼ 2:6 5:1; average hydrogen density along the sight line, hn(H)i ¼ 3 14 cm À3 ; and fraction of H in molecular form, 0-$40%. Four of the sight lines, those toward HD 37021, HD 37061, HD 147888, and HD 207198, have interstellar gas-phase abundances that are consistent with the diffuse sight line ratio of 161 AE 17 carbon atoms in the gas per million hydrogen nuclei. We note that while it has a gas-phase carbon abundance that is consistent with the other sight lines, a large fraction of the C ii toward HD 37061 is in an excited state. The sight line toward HD 152590 has a measured interstellar gas-phase carbon abundance that is well above the diffuse sight line average; the column density of C in this sight line may be overestimated because of noise structure in the data. Toward HD 27778 we find a 3 abundance upper limit of 108 C atoms in the gas per million H, a substantially enhanced depletion of C as compared to the diffuse sight line value. The interstellar characteristics toward HD 27778 are otherwise not extreme among the sample, except for an unusually large abundance of CO molecules in the gas.

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Using the HST Goddard High Resolution Spectrograph (GHRS), we have obtained high S/N echelle observations of the weak interstellar N I 1160, 1161 A absorption doublet toward the stars Gamma Cas, Lambda Ori, Iota Ori, Kappa Ori, Delta Sco, and Kappa Sco. In combination with a previous GHRS measurement of N I toward Zeta Oph, these new observations yield a mean interstellar gas phase nitrogen abundance (per 10$^6$ H atoms) of 10$^6$ N/H = 75 +/- 4. There are no statistically significant variations in the measured N abundances from sightline to sightline and no evidence of density-dependent depletion from the gas-phase. Since N is not expected to be depleted much into dust grains in these diffuse sightlines, its gas-phase abundance should reflect the total interstellar abundance. Consequently, the GHRS observations imply that the abundance of interstellar nitrogen (gas plus grains) in the local Milky Way is about 80% of the solar system value of 10$^6$ N/H = 93 +/- 16. Although this interstellar abundance deficit is somewhat less than that recently found for oxygen and krypton with GHRS, the solar N abundance and the N I oscillator strengths are too uncertain to definitively rule out either a solar ISM N abundance or a 2/3 solar ISM N abundance similar to that of O and Kr.Comment: 14 pages, LaTeX, 2 Postscript figures; ApJ Letters, in pres

We present an analysis of high-resolution Hubble Space Telescope (HST ) Space Telescope Imaging Spectrograph (STIS) observations of O i k1356 and H i Ly absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f (H 2 ), over 2 orders of magnitude in hn H i, and that extend up to 6.5 kpc in length. Since the majority of these sight lines have also been observed by the Far Ultraviolet Spectroscopic Explorer (FUSE ), we have undertaken the study of gasphase O/ H abundance ratio homogeneity using the current sample and previously published Goddard High Resolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight-line density (hn H i) and a gap between the mean O/ H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with large mean density intervals; it is centered near hn H i ¼ 1:5 cm À3 and is similar to trends evident in gas-phase abundances of other elements. Paths less dense than the central value exhibit a mean O/ H ratio of log 10 (O= H) ¼ À3:41 AE 0:01 (or 390 AE 10 ppm), which is consistent with averages determined for several long low-density paths observed by STIS (André et al. 2003) and short low-density paths observed by FUSE ( Moos et al. 2002). Sight lines of higher mean density exhibit an average O/ H value of log 10 (O= H) ¼ À3:55 AE 0:02 (284 AE 12 ppm). The data points for low-hn H i paths are scattered more widely than those for denser sight lines, because O/ H ratios for such paths shorter than 800 pc are generally about 0.10 dex lower than the values for longer ones. Scenarios that would be consistent with these results include a recent infall of metal-poor gas onto the local Galactic disk and an interstellar environment toward Orion that is conducive to reducing the apparent gas-phase oxygen abundance.