Spectra obtained with the Hubble Space T elescope Goddard High Resolution Spectrograph are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the di †use interstellar clouds seen along the line of sight to the star 23 Ori. Multiple absorption components are present for each of several distinct types of gas, which are characterized by di †erent relative abundance and depletion patterns and physical conditions. Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas and representing about 92% of the total N(H I), is seen for various neutral and singly ionized species at ]20 km s~1 km s~1. Most typically severely depleted species are less depleted by factors of 2È4, com-

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value ∼ 1/30 solar at z ∼ 3 to a nearly solar value at z = 0. In the absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in quasar spectra are expected to show such a rise in global metallicity. However, it has been difficult to determine whether or not DLAs show this effect, primarily because of the very small number of DLA metallicity measurements at low redshifts. -2 -In an attempt to put tighter constraints on the low-redshift end of the DLA metallicity-redshift relation, we have observed Zn II and Cr II lines in four DLAs at 0.09 < z < 0.52, using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with z < 0.4. In all the three DLAs for which our observations offer meaningful constraints on the metallicity, the data suggest that the metallicities are much lower than the solar value. These results are consistent with recent imaging studies indicating that these DLAs may be associated with dwarf or low surface brightness galaxies.We combine our results with higher redshift data from the literature to estimate the global mean metallicity-redshift relation for DLAs. We find that the global mean metallicity shows at most a slow increase with decreasing redshift. For the redshift range 0.09 < z < 3.90, the slope of the exponential fit to the binned N(H I)-weighted mean Zn metallicity vs. redshift relation is −0.18 ± 0.06 counting Zn limits as detections, −0.22 ± 0.08 counting Zn limits as zeros, and −0.23 ± 0.06 using constraints on metallicity from other elements in cases of Zn limits. The corresponding estimates of the z = 0 intercept of the metallicityredshift relation are −0.74 ± 0.15, −0.75 ± 0.18, and −0.71 ± 0.13, respectively. Roughly similar results are obained if survival analysis or an unbinned N(H I)weighted nonlinear χ 2 approach is used. Thus, the N(H I)-weighted mean metallicity of DLAs does not appear to rise up to solar or near-solar values at z = 0. This weak evolution could be explained by the fact that our absorption-selected sample seems to be dominated by dwarf or low surface brightness galaxies. This suggests that current DLA samples, especially those at low redshifts, could be biased against more enriched galaxies because the latter may cause higher dust obscuration of the background quasars.

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.

We have studied a sample of 809 Mg ii absorption systems with 1.0 ≤zabs≤ 1.86 in the spectra of Sloan Digital Sky Survey quasi‐stellar objects (QSOs), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the intervening absorbers. Normalized, composite spectra were derived, for abundance measurements, for the full sample and several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the subsamples by comparing their geometric mean spectra with those of matching samples of QSOs without absorbers in their spectra. There is clear evidence for the presence of dust in the intervening absorbers. The 2175‐Å feature is not present in the extinction curves, for any of the subsamples. The extinction curves are similar to the Small Magellanic Cloud (SMC) extinction curve with a rising ultraviolet (UV) extinction below 2200 Å. The absorber rest‐frame colour excess, E(B−V), derived from the extinction curves, depends on the absorber properties and ranges from <0.001 to 0.085 for various subsamples. The column densities of Mg ii, Al ii, Si ii, Ca ii, Ti ii, Cr ii, Mn ii, Fe ii, Co ii, Ni ii and Zn ii do not show such a correspondingly large variation. The overall depletions in the high E(B−V) samples are consistent with those found for individual damped Lyman α systems, the depletion pattern being similar to halo clouds in the Galaxy. Assuming an SMC gas‐to‐dust ratio, we find a trend of increasing abundance with decreasing extinction; systems with NH I∼ 1020 cm−2 show solar abundance of Zn. The large velocity spread of strong Mg ii systems seems to be mimicked by weak lines of other elements. The ionization of the absorbers, in general appears to be low: the ratio of the column densities of Al iii to Al ii is always less than 1/2. QSOs with absorbers are, in general, at least three times as likely to have highly reddened spectra as compared to QSOs without any absorption systems in their spectra.

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.