Search

We present chemical abundance measurements from high-resolution observations of 5 sub-damped Lyman-alpha absorbers at 1.7 < z < 2.4 observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II Clay telescope. Lines of Zn II, Mg I, Mg II, Al II, Al III, S II, Si II, Si IV, C II, C II*, C IV, Ni II, Mn II and Fe II were detected and column densities were determined. The metallicity of the absorbing gas, inferred from the nearly undepleted element Zn, is in the range of < -0.95 to +0.25 dex for the five absorbers in our sample, with three of the systems being near-solar or super-solar. We also investigate the effect of ionisation on the observed abundances using photoionisation modelling. Combining our data with other sub-DLA and DLA data from the literature, we report the most complete existing determination of the metallicity vs. redshift relation for sub-DLAs and DLAs. We confirm the suggestion from previous investigations that sub-DLAs are, on average, more metal-rich than DLAs and evolve faster. We also discuss relative abundances and abundance ratios in these absorbers. The more metal-rich systems show significant dust depletion levels, as suggested by the ratios [Zn/Cr] and [Zn/Fe]. For the majority of the systems in our sample, the [Mn/Fe] vs. [Zn/H] trend is consistent with that seen previously for lower-redshift sub-DLAs. We also measure the velocity width values for the sub-DLAs in our sample from unsaturated absorption lines of Fe II 2344, 2374, 2600 A, and examine where these systems lie in a plot of metallicity vs. velocity dispersion. Finally, we examine cooling rate vs. H I column density in these sub-DLAs, and compare this with the data from DLAs and the Milky Way ISM. We find that most of the systems in our sample show higher cooling rate values compared to those seen in the DLAs.

We have imaged GM Aur with HST, detected its disk in scattered light at 1400A and 1650A, and compared these with observations at 3300A, 5550A, 1.1 microns, and 1.6 microns. The scattered light increases at shorter wavelengths. The radial surface brightness profile at 3300A shows no evidence of the 24AU radius cavity that has been previously observed in sub-mm observations. Comparison with dust grain opacity models indicates the surface of the entire disk is populated with sub-micron grains. We have compiled an SED from 0.1 microns to 1 mm, and used it to constrain a model of the star+disk system that includes the sub-mm cavity using the Monte Carlo Radiative Transfer code by Barbara Whitney. The best-fit model image indicates that the cavity should be detectable in the F330W bandpass if the cavity has been cleared of both large and small dust grains, but we do not detect it. The lack of an observed cavity can be explained by the presence of sub-microns grains interior to the sub-mm cavity wall. We suggest one explanation for this which could be due to a planet of mass, 40 pages, 11 figures, 4 tables, accepted to ApJ

We discuss the thermal pressures ($n_H T$) in predominantly cold, neutral interstellar gas in the Magellanic Clouds, derived from analyses of the fine-structure excitation of neutral carbon, as seen in high-resolution HST/STIS spectra of seven diverse sight lines in the LMC and SMC. Detailed fits to the line profiles of the absorption from C I, C I*, and C I** yield consistent column densities for the 3--6 C I multiplets detected in each sight line. In the LMC and SMC, $N$(C I$_{\rm tot}$) is consistent with Galactic trends versus $N$(Na I) and $N$(CH), but is slightly lower versus $N$(K I) and $N$(H$_2$). As for $N$(Na I) and $N$(K I), $N$(C I$_{\rm tot}$) is generally significantly lower, for a given $N$(H$_{\rm tot}$), in the LMC and (especially) in the SMC, compared to the local Galactic relationship. For the LMC and SMC components with well determined column densities for C I, C I*, and C I**, the derived thermal pressures are typically factors of a few higher than the values found for most cold, neutral clouds in the Galactic ISM. Such differences are consistent with the predictions of models for clouds in systems (like the LMC and SMC) that are characterized by lower metallicities, lower dust-to-gas ratios, and enhanced radiation fields -- where higher pressures are required for stable cold, neutral clouds. The pressures may be further enhanced by energetic activity (e.g., due to stellar winds, star formation, and/or supernova remnants) in several of the regions probed by these sight lines. Comparisons are made with the C I observed in some quasar absorption-line systems., 61 pages, 13 figures, 10 tables, accepted to ApJ

We present deep ground based imaging of the environments of five QSOs that contain sub-Damped Lyman-alpha systems at z L* galaxies for four of the five fields within 10" of the QSO. In contrast to previous imaging surveys of DLA systems at these redshifts, which indicate a range of morphological types and luminosities for the host galaxies of the systems, the galaxies we detect in these sub-DLA fields appear to be luminous (L>L*). In the case of the absorber towards Q1009-0026 at z=0.8866 we have spectroscopic confirmation that the candidate galaxy is at the redshift of the absorber, at an impact parameter of ~35 kpc with a luminosity of 3 < L/L* < 8 depending on the magnitude of the K-correction. These observations are in concordance with the view that sub-DLAs may be more representative of massive galaxies than DLA systems. The environments of the absorbers span a range of types, from the inner disk of a galaxy, the periphery of a luminous galaxy, and the outskirts of interacting galaxies. The large impact parameters to some of the candidate galaxies suggest that galactic outflows or tidal tails are likely responsible for the material seen in absorption. We find a weak correlation between N(HI) and the impact parameter at the 2 sigma level, which may be expected from the heterogeneous population of galaxies hosting the absorption line systems and random orientation angles. In addition, we detect a possible gravitationally lensed image of the BL-Lac object Q0826-2230.

We have combined the results from our recent observations of Damped and sub-Damped Lyman-alpha systems with the MIKE and UVES spectrographs on the Magellan Clay and VLT Kueyen telescopes with ones from the literature to determine the N(HI)-weighted mean metallicity of these systems based both on Fe, a depleted element in QSO absorbers and the local ISM, and Zn a relatively undepleted element. In each case, the N(HI)-weighted mean metallicity is higher and shows faster evolution in sub-DLAs than the classical DLA systems. Large grids of photoionisation models over the sub-DLA \nhI range with CLOUDY show that the ionisation corrections to the abundances are in general small, however the fraction of ionized H can be up to ~90 per cent. The individual spectra have been shifted to the rest frame of the absorber and averaged together to determine the average properties of these systems at z as well as higher and . We calculate the contribution of sub-DLAs to the metal budget of the Universe, and find that the sub-DLA systems at z

The Damped and sub-Damped Lyman-alpha (DLA and sub-DLA) systems seen in the spectra of QSOs offer a unique way to study the interstellar medium of high redshift galaxies. In this paper we report on new abundance determinations in a sample of 10 new systems, nine of the lesser studied sub-DLAs and one DLA, along the line of sight to seven QSOs from spectra taken with the MIKE spectrograph. Lines of Mg I, Mg II, Al II, Al III, Ca II, Mn II, Fe II, and Zn II were detected. Here, we give the column densities and equivalent widths of the observed absorption lines, as well as the abundances determined for these systems. Zn, a relatively undepleted element in the local interstellar medium (ISM) is detected in one system with a high metallicity of [Zn/H]=+0.27\pm0.18. In one other system, a high abundance based on the more depleted element Fe is seen with [Fe/H]=-0.37\pm0.13, although Zn is not detected. The N(HI)-weighted mean metallicity of these sub-DLA systems based on Fe is =-0.76\pm0.11, nearly ~0.7 dex higher (a factor of 5) than what is seen in DLAs in this redshift range. The relative abundance of [Mn/Fe] is also investigated. A clear trend is visible for these systems as well as systems from the literature, with [Mn/Fe] increasing with increasing metallicity in good agreement with with Milky Way stellar abundances., Comment: 19 pages, 21 figures, Fixed Typos and References, Accepted for publication in MNRAS 12/2008

We have studied a sample of 415 associated (z_ab z_em; relative velocity with respect to QSO 3000km/s), so as to understand their origin. From the analysis of the composite spectra, as well as from the comparison of measured equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher apparent ionization, measured by the strength of the C IV absorption lines compared to the Mg II absorption lines, than the intervening absorbers. The ionization so measured appears to be related to apparent ejection velocity, being lower as the apparent ejection velocity is more and more positive. There is clear evidence, from the composite spectra, for SMC like dust attenuation in these systems; the 2175AA absorption feature is not present. The extinction is almost twice that observed in the similarly selected sample of intervening systems. We reconfirm that QSOs with non-zero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg II systems in QSOs with non-zero FIRST radio flux is 1.7 times that in the QSOs with no detection in the FIRST survey. The associated absorbers in radio-detected QSOs which comprise about 12% of our sample, cause 3 times more reddening than the associated absorbers in radio-undetected QSOs. This excess reddening possibly suggests an intrinsic nature for the associated absorbers in radio-detected QSOs., Accepted for publication in ApJ

Observations of low mean metallicity of damped Lyman-alpha (DLA) quasar absorbers at all redshifts studied appear to contradict the predictions for the global mean interstellar metallicity in galaxies from cosmic chemical evolution models. On the other hand, a number of metal-rich sub-DLA systems have been identified recently, and the fraction of metal-rich sub-DLAs appears to be considerably larger than that of metal-rich DLAs, especially at z < 1.5. In view of this, here we investigate the evolution of metallicity in sub-DLAs. We find that the mean Zn metallicity of the observed sub-DLAs may be higher than that of the observed DLAs, especially at low redshifts, reaching a near-solar level at z

We report interstellar FeII and SiII column densities toward six translucent sight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of SiII] at 2335 Angstroms, and several weak Fe transitions including the first reported detections of the 2234 Angstrom line. We derive an empirical f-value for the FeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their R_V values, which range from 2.6 - 5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred micron) toward the targets. The physical conditions along the sight lines suggest that this relationship may be due to differences in the survival of small particles in some interstellar environments. The chemical composition of the small grains could either resemble dust mantles or be silicate rich., accepted for publication in ApJ

We present arguments based on the measured abundances in individual damped Lyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), and also the average abundances inferred in large samples of QSO absorption line systems, to suggest that the amount of dust in intervening QSO absorbers is small and is not responsible for missing many QSOs in magnitude limited QSO surveys. While we can not totally rule out a bimodal dust distribution with a population of very dusty, metal rich, absorbers which push the background QSOs below the observational threshold of current optical spectroscopic studies, based upon the current samples it appears that the metallicity in QSO absorbers decreases with increase in H I column densities beyond 10^{19} cm^{-2}. Thus the sub-DLA population is more metal rich than the DLAs, a trend which may possibly extend to the non-damped Lyman limit systems (NDLLS). Based on the recently discovered mass-metallicity relation for galaxies, we suggest that most sub-DLAs and possibly NDLLS, are associated with massive spiral/elliptical galaxies while most DLAs are associated with low mass galaxies. The sub-DLA galaxies will then contribute a larger fraction of total mass (stellar and ISM) and therefore metals, to the cosmic budget, specially at low redshifts, as compared to the DLAs.

The structure and properties of the diffuse interstellar medium (ISM) on small scales, sub-au to 1 pc, are poorly understood. We compare interstellar absorption-lines, observed towards a selection of O- and B-type stars at two or more epochs, to search for variations over time caused by the transverse motion of each star combined with changes in the structure in the foreground ISM. Two sets of data were used: 83 VLT- UVES spectra with approximately 6 yr between epochs and 21 McDonald observatory 2.7m telescope echelle spectra with 6 - 20 yr between epochs, over a range of scales from 0 - 360 au. The interstellar absorption-lines observed at the two epochs were subtracted and searched for any residuals due to changes in the foreground ISM. Of the 104 sightlines investigated with typically five or more components in Na I D, possible temporal variation was identified in five UVES spectra (six components), in Ca II, Ca I and/or Na I absorption-lines. The variations detected range from 7\% to a factor of 3.6 in column density. No variation was found in any other interstellar species. Most sightlines show no variation, with 3{\sigma} upper limits to changes of the order 0.1 - 0.3 dex in Ca II and Na I. These variations observed imply that fine-scale structure is present in the ISM, but at the resolution available in this study, is not very common at visible wavelengths. A determination of the electron densities and lower limits to the total number density of a sample of the sightlines implies that there is no striking difference between these parameters in sightlines with, and sightlines without, varying components., Comment: 19 pages, 11 figures, accepted for publication in MNRAS

Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over > 90 % of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame UV spectra are cut-off due to the atmospheric absorption. We present here high-resolution VLT/UVES observations of several elements contained in three DLAs and one sub-DLA with 0.6

Sensitive measurements of the interstellar gas-phase oxygen abundance have revealed a slight oxygen deficiency ($\sim$ 15%) toward stars within 500 pc of the Sun as compared to more distant sightlines. Recent $FUSE$ observations of the interstellar gas-phase nitrogen abundance indicate larger variations, but no trends with distance were reported due to the significant measurement uncertainties for many sightlines. By considering only the highest quality ($\geq$ 5 $\sigma$) N/O abundance measurements, we find an intriguing trend in the interstellar N/O ratio with distance. Toward the seven stars within $\sim$ 500 pc of the Sun, the weighted mean N/O ratio is 0.217 $\pm$ 0.011, while for the six stars further away the weighted mean value (N/O = 0.142 $\pm$ 0.008) is curiously consistent with the current Solar value (N/O = 0.138$^{+0.20}_{-0.18}$). It is difficult to imagine a scenario invoking environmental (e.g., dust depletion, ionization, etc.) variations alone that explains this abundance anomaly. Is the enhanced nitrogen abundance localized to the Solar neighborhood or evidence of a more widespread phenomenon? If it is localized, then recent infall of low metallicity gas in the Solar neighborhood may be the best explanation. Otherwise, the N/O variations may be best explained by large-scale differences in the interstellar mixing processes for AGB stars and Type II supernovae., Comment: accepted for publication in the Astrophysical Journal Letters

We present elemental abundance measurements for 9 damped Ly-alpha systems (DLAs) and 1 sub-DLA at 0.1 < z < 1.5 from recent observations with the Multiple Mirror Telescope. Most of these absorbers are found to be metal-poor, while 2 are found to have ~ 30-50 solar metallicities. Combining our data with other data from the literature, we find that the systems with higher [Zn/H] also have stronger depletion as measured by [Cr/Zn] and [Fe/Zn]. The relationship between the metallicity and H I column density is also investigated. Together with our previous MMT survey (Khare et al. 2004) we have discovered 2 of the 4 known absorbers at z < 1.5 that lie above (although near) the "obscuration threshold". This appears to be a result of selecting absorbers with strong metal lines in our sample. It would be interesting to find other similar systems by observing a larger sample and study how much such systems contribute to the cosmic budget of metals. Finally, an analysis of the N(H I)-weighted mean metallicity vs. redshift for our sample combined with data from the literature supports previous conclusions that the N(H I)-weighted mean global DLA metallicity rises slowly at best and falls short of solar levels by a factor of > 4 even at z=0., 20 pages, 16 figures, accepted for publication in MNRAS

We have studied a sample of 809 Mg II absorption systems with 1.0 < z_abs < 1.86 in the spectra of SDSS 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 sub-samples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the sub-samples 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 A feature is not present in the extinction curves, for any of the sub-samples. The extinction curves are similar to the SMC extinction curve with a rising UV extinction below 2200 A. 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 sub-samples. The column densities of several ions do not show such a correspondingly large variation. The depletion pattern is 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 N_H I \~ 10^{20} 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. 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 interstellar elemental abundance measurements derived from Space Telescope Imaging Spectrograph echelle observations of 47 sight lines extending up to 6.5 kpc through the Galactic disk. These paths probe a variety of interstellar environments, covering ranges of nearly four orders of magnitude in molecular hydrogen fraction f(H_2) and more than two in mean hydrogen sight line density . Coupling the current data with Goddard High Resolution Spectrograph data from 17 additional sight lines and the corresponding Far Ultraviolet Spectroscopic Explorer and Copernicus observations of H_2 absorption features, we explore magnesium, phosphorus, manganese, nickel, copper, and germanium gas-phase abundance variations as a function of : density-dependent depletion is noted for each element, consistent with a smooth transition between two abundance plateaus identified with warm and cold neutral interstellar medium depletion levels. The observed scatter with respect to an analytic description of these transitions implies that total elemental abundances are homogeneous on length scales of hundreds of parsecs, to the limits of abundance measurement uncertainty. The probable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dex deficit in copper (and oxygen) abundances within 800 pc of the Sun. Magnesium dust abundances are shown to scale with the amount of silicon in dust and, in combination with a similar relationship between iron and silicon, these data appear to favor the young F and G star values of Sofia & Meyer (2001; ApJ 554, L221) as an elemental abundance standard for the Galaxy., Comment: 58 pages, including 11 figures and 9 tables, accepted by ApJ

The spectra of the Sloan Digital Sky Survey (SDSS) are being used to construct a catalogue of QSO absorption lines, for use in studies of abundances, relevant radiation fields, number counts as a function of redshift, and other matters, including the evolution of these parameters. The catalogue includes intervening, associated, and BAL absorbers, in order to allow a clearer definition of the relationships between these three classes. We describe the motivation for and the data products of the project to build the SDSS QSO absorption line catalogue.

We report spectroscopic observations with the Multiple Mirror Telescope for 11 damped Lyman-alpha absorbers (DLAs) or strong DLA candidates at 0.1 < z < 1.5, including several absorbers discovered in the Sloan Digital Sky Survey. In particular, we have measured absorption lines of Zn II, Cr II, Ni II, Fe II, Mn II, Ti II, Ca II, and Si II. These measurements have doubled the sample of Zn and Cr measurements at z < 1. The average relative abundance patterns in these objects are very similar to those found for high-redshift DLAs reported in the literature. Our observations suggest that the dust content, as determined by [Cr/Zn], does not show much change with redshift. We also examine the sample for correlation of [Cr/Zn] with estimates of the quasar reddening. Our data suggest that the global mean metallicity of DLAs, as measured by the gas phase abundance of Zn, at best shows a weak evolution with redshift over the range 0.4 < z, aastex52, 47 pages including 11 postscript figures. Accepted for publication in The Astrophysical Journal

We present multi-epoch KPNO Coude Feed observations of interstellar NaI and CaII absorption toward the bright star rho Leo. Comparisons of the NaI profiles observed over a period of 8 years reveal significant temporal variations in the NaI column in at least one component, implying that there is ``structure'' at scales of order of the proper motion (~12 AU). Archival HST Goddard High Resolution Spectrograph observations of the CI fine-structure excitation in the variable component suggest that the density is, 12 pages, 2 figures, accepted for publication in the Astrophysical Journal Letters

The extraordinarily bright quasar PHL 1811 at a redshift z = 0.192 provides an attractive opportunity to use ultraviolet absorption-line spectroscopy to study the properties of gas systems in the local universe. An R = 20,000 far-UV spectrum recorded by FUSE revealed 7 extragalactic absorption systems, one of which is a Lyman limit system at z = 0.08093 accompanied by three systems having redshifts which differ from it by less than 0.008. The abundance of O with respect to Fe in the Lyman limit system is not much different from the solar abundance ratio. Supplementary low resolution spectra recorded by STIS (on HST) at longer wavelengths helped to substantiate our identifications of systems in the FUSE spectrum and suggested the presence of an additional 4 systems that could be detected only through their Ly-alpha features. Spectroscopy at visible wavelengths of 7 galaxies within approximately 2' of PHL 1811 indicated that 2 of them are near the redshift of the quasar and 4 have redshifts within 850 km/s of the extragalactic absorption systems. The Lyman limit system is likely associated with an L* galaxy lying 23" from the sightline. Finally, in addition to prominent features at very low velocities arising from the disk of our Galaxy, the strong resonance transitions of C II and Mg II show evidence for material at v = -200 km/s; the column densities of these two species suggest that 17.7 < log N(H I) < 18.1 if the material has a solar composition., 47 pages, 5 figures, to appear in the June 2003 issue of the Astronomical Journal

We present near-IR (NIR) and far-UV observations of the pre-transitional (gapped) disk in HD 169142 using NASA's Infrared Telescope Facility and Hubble Space Telescope. The combination of our data along with existing data sets into the broadband spectral energy distribution reveals variability of up to 45% between (is) approximately1.5-10 micrometers over a maximum timescale of 10 yr. All observations known to us separate into two distinct states corresponding to a high near-IR state in the pre-2000 epoch and a low state in the post-2000 epoch, indicating activity within the (is) approximately less than 1AU region of the disk. Through analysis of the Pa beta and Br gamma lines in our data we derive a mass accretion rate in 2013 May of M (is) approx. = (1.5-2.7) 10(exp -9) solar mass yr(exp -1).We present a theoretical modeling analysis of the disk in HD 169142 using Monte-Carlo radiative transfer simulation software to explore the conditions and perhaps signs of planetary formation in our collection of 24 yr of observations. We find that shifting the outer edge (r (is) approx. = 0.3AU) of the inner disk by 0.05AU toward the star (in simulation of accretion and/or sculpting by forming planets) successfully reproduces the shift in NIR flux. We establish that the (is) approximately 40-70AU dark ring imaged in the NIR by Quanz et al. and Momose et al. and at 7 mm by Osorio et al. may be reproduced with a 30% scaled density profile throughout the region, strengthening the link to this structure being dynamically cleared by one or more planetary mass bodies.

We have obtained high-resolution Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and Kr I λ1236 absorption in 11 sight lines characterized by high extinction, large H I column densities, and/or long path lengths. Previous Goddard High Resolution Spectrograph (GHRS) measurements of these weak features in seven relatively nearby diffuse clouds have shown no evidence for density-dependent depletion of either oxygen or krypton and have yielded a weighted mean gas-phase abundance ratio of log[N(O)/N(Kr)]GHRS = 5.56 ± 0.04. Our STIS measurements yield a lower weighted mean of log[N(O)/N(Kr)]STIS = 5.48; the difference is due primarily to several translucent sight lines in the STIS data set that diverge from the GHRS value. These translucent cloud sight lines pass near dense, star-forming regions, notably the ρ Oph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas, should not be depleted much into grains, these cases suggest a trend toward the enhanced oxygen depletion predicted for denser ISM clouds.

We present a Chandra X-ray observation that shows, unambiguously for the first time, the presence of a giant diffuse X-ray-emitting corona around the edge-on disk galaxy NGC 4631. This corona, with a characteristic thermal temperature of 2-7\times10^6 K, extends as far as 8 kpc away from the galactic plane. The X-ray morphology resembles the radio halo of the galaxy, indicating a close connection between outflows of hot gas, cosmic rays, and magnetic field from the galactic disk. Enhanced diffuse X-ray emission is apparently enclosed by numerous H\alpha-emitting loops blistered out from the central disk of the galaxy, as is evident in a comparison with our deep Hubble Space Telescope imaging.

Based on recent high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph observations over the 1160-1360 A wavelength region, we have discovered that HD 37903 (the illuminating star of the bright reflection nebula NGC 2023 in Orion) exhibits a rich spectrum of vibrationally excited interstellar H2 absorption. We have identified a total of over 500 H2 lines arising from various rotational levels (as high as J = 13) in all 14 excited vibrational states of the ground electronic state. Prior to this study, the only published UV absorption measurements of vibrationally excited interstellar H2 consisted of two weak lines toward the star ζ Ophiuchi. In terms of abundance, the v = 3 J = 1 H2 column density toward HD 37903 is over 200 times greater than that toward ζ Oph. Overall, the populations of the excited H2 rovibrational states toward HD 37903 exhibit a nonthermal distribution that is consistent with the UV fluorescent excitation of dense (nH ≈ 104 cm-3) molecular gas located about 0.5 pc from the B1.5 V star. We contrast these results with those provided by near-IR H2 emission-line studies of NGC 2023 and discuss opportunities for future UV observations of vibrationally excited H2 toward HD 37903 and other sight lines.

We have used integral field echelle spectroscopy with the DensePak fiber-optic array on the KPNO WIYN telescope to observe the central 27" x 43" of the globular cluster M92 in the Na I D wavelength region at a spatial resolution of 4". Two interstellar Na I absorption components are evident in the spectra at LSR velocities of 0 km/s (Cloud 1) and -19 km/s (Cloud 2). Substantial strength variations in both components are apparent down to scales limited by the fiber-to-fiber separations. The derived Na I column densities differ by a factor of 4 across the Cloud 1 absorption map and by a factor of 7 across the Cloud 2 map. Using distance upper limits of 400 and 800 pc for Cloud 1 and Cloud 2, respectively, the absorption maps indicate structure in the ISM down to scales of 1600 and 3200 AU. The fiber-to-fiber Na I column density differences toward M92 are comparable to those found in a similar study of the ISM toward the globular cluster M15. Overall, the structures in the interstellar components toward M92 have significantly lower column densities than those toward M15. We interpret these low column density structures as small-scale turbulent variations in the gas and compare them to the larger-scale, higher column density variations toward M15, which may be the hallmarks of actual H I structures.

We present high-resolution Hubble Space Telescope/Space Telescope Imaging Spectrograph and multiepoch KPNO coude feed observations of multiple interstellar lines toward the binary system HD 32039/40. Comparisons of the Na I profiles observed over a period of 4.25 yr reveal significant temporal variation in the Na I column in at least one component, implying that there is structure at scales on the order of the proper motion (~15-21 AU). Large differences are also observed among multiple components in the profiles of the neutral species between the binary sight lines (5060 AU separation). In addition, significant differences in the S II profile are seen between the stars—the first optical/UV detection of small-scale variations in a dominant ion. Measurements of the C I fine-structure excitation in the variable components suggest that the densities in these components are ~20-200 cm-3, significantly lower than the densities inferred in past H I 21 cm and Na I studies.

Using the DensePak fiber optic array on the KPNO WIYN telescope, we have obtained high S/N echelle spectra of the Na I D wavelength region toward the central 27" x 43" of the globular cluster M15 at a spatial resolution of 4". The spectra exhibit significant interstellar Na I absorption at LSR velocities of +3 km/s (LISM component) and +68 km/s (IVC component). Both components vary appreciably in strength on these scales. The derived Na I column densities differ by a factor of 4 across the LISM absorption map and by a factor of 16 across the IVC map. Assuming distances of 500 pc and 1500 pc for the LISM and IVC clouds, these maps show evidence of significant ISM structure down to the minimum scales of 2000 AU and 6000 AU probed in these absorbers. The smallest-scale N(Na I) variations observed in the M15 LISM and IVC maps are typically comparable to or higher than the values found at similar scales in previous studies of interstellar Na I structure toward binary stars. The physical implications of the small and larger-scale Na I features observed in the M15 maps are discussed in terms of variations in the H I column density as well as in the Na ionization equilibrium.

We present high resolution (R~170,000) Kitt Peak National Observatory Co'ude Feed telescope observations of the interstellar KI 7698 angstrom line towards 5 multiple star systems with saturated NaI components. We compare the KI absorption line profiles in each of the two (or three) lines of sight in these systems, and find significant differences between the sight-lines in 3 out of the 5 cases. We infer that the small scale structure traced by previous NaI observations is also present in at least some of the components with saturated NaI absorption lines, and thus the small scale structures traced by the neutral species are occurring at some level in clouds of all column densities. We discuss the implications of that conclusion and a potential explanation by density inhomogeneities.

We examine the clustering properties of low-redshift Lya and heavy-element QSO absorption line systems seen in the spectra of 13 QSOs at the Galactic poles. This is the densest sample of D1i separat- ed QSOs observed spectroscopically with the Hubble Space Telescope to date. At the median redshift of the Lya sample (z B 0.7), the QSO lines of sight are separated on transverse scales from about 15 to 200 h~1 Mpc H \ 100 h km s~1 Mpc~1), allowing the three-dimensional clustering of the (q 0 \ 0.5, absorbers to be examined on those scales. The Galactic poles are also regions where relatively deep and wide-—eld galaxy redshift surveys have taken place, so the distributions of galaxies and Lya systems can be compared within the same volume of space. There are 545 total absorption lines detected in the com- plete sample from 13 QSOs. We identify 307 Lya systems, of which 18 contain heavy-element lines. We con—rm the relatively slow redshift number density evolution for Lya systems at There are also z ( 1. —ve likely C IV doublets in our sample, for which the Lya line is not accessible. The main results are summarized as follows: 1. Most, but not all, of the very low-z (z \ 0.2) Lya systems appear to be associated with large-scale structures traced by luminous galaxies, based upon the spectrum of the low-redshift QSO PG 1309)355. 2. On scales of 2.5 to 5 h~1 Mpc, the line-of-sight correlation function shows a positive and signi—cant clustering amplitude, for systems with rest equivalent width, W , exceeding 0.24 m los \ 0.95 ^ 0.28, Ae . Most of this signal can be accounted for by a small number of Lya ii clumps, ˇˇ which typically contain about —ve systems spanning D20 h~1 Mpc. There is qualitative evidence that the strength of the corre- lation function rises with the limiting Lya rest equivalent width. 3. Over half of the heavy-element systems are either in groups or have a nearby ii clump ˇˇ of Lya lines. However, there is only weak evidence for a signi—cant heavy-element/Lya cross-correlation function. 4. There is no clear evidence that the Lya systems are distributed in redshift ii peaks ˇˇ and ii voids ˇˇ as is characteristic for the galaxies. In particular, there is no signi—cant periodicity detected on the scale of 128 h~1 Mpc, which had been detected for the galaxies located in the same direction of the sky. There is marginal evidence for a 41.5 h~1 Mpc ii periodicity ˇˇ in the distribution, which is seen for all equivalent width-limited samples. 5. The three-dimensional Lya radial correlation function shows no signi—cant departures from a random sample at any scale or using any limiting equivalent width. This indicates that if there are sheet- like structures spanning separations on the order of 30 h~1 Mpc or more, they are not clearly traced by Lya systems. 6. The fraction of Lya lines with W ( 0.24 which may be clustered in the same way as the heavy- Ae , element systems detected mostly at higher redshifts, is about 39% ^ 6%. This value is likely to depend on the limiting equivalent width and redshift of the sample. These results are consistent with a picture of the Lya forest at low redshift in which some fraction of the Lya lines arise in close proximity to galaxies and structures traced by galaxies, but in which a substantial fraction of the systems are also more uniformly distributed and are largely unrelated to the distribution of luminous galaxies. The results also appear to be in broad agreement with numerical simulations of the

The UV spectrum of the Red Rectangle and its central source, HD 44179, was examined using the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST). Spectra have been obtained of the 0-0, 0-1, and 0-2 vibronic bands of the CO fourth positive system, the 1657 A C I multiplet, and the 1931 A C I line. All of the lines and bands display both absorption and emission components. The components do not display P Cygni profiles, but are nearly symmetrically reversed and show different spreads of radial velocity. The widths and Doppler shifts of each component provide information about the environment of its origin, as do the rotational and vibrational distributions in CO. The 1657 A multiplet is too complex for analysis, but the 1931 A line can be deconvolved into two components: emission with narrow velocity dispersion, 25 km s-1, and absorption with a broad velocity dispersion, 230 km s-1. The radial velocity of these atomic features is consistent with the velocity of the center of mass of this known spectroscopic binary. For the CO bands, we obtained a best fit from a four-component model, consistent for all three bands. There are three emission components and one absorption component. Two emission components display narrow velocity spreads (29 km s-1), and one shows a rotational temperature of 50 K, the other, of about 3000 K. The third emission component has a very wide velocity spread (600 km s-1) and a rotational temperature of about 100 K. The absorption component shows a wide velocity spread (230 km s-1) and a rotational temperature of about 100 K. The high-temperature emission displayed partially-resolved rotational line structure corresponding to levels of J = 50. The Doppler shifts of these features place the narrow, cold emission with one of the binary components and the narrow, hot emission with the center of mass. We discuss these observations with respect to the accretion disk model of this object. We also compare our UV CO observations with the work of others on the IR CO overtone spectra in young stellar objects.

Plausible ratios of deuterium to hydrogen D/H as a function of metallicity, time, and redshift are investigated. Guided by the heavy element abundance patterns observed locally in Galactic dwarf stars and at large redshift in quasi-stellar object absorption line systems, empirical evolution of the relative abundance ratios Li/D, B/D, N/D, O/D, and F/D for QSO absorption line systems are given for the possible evolutionary patterns in D/H. Shortened abstract., Comment: 30 pages including 8 figures, ApJ in press

Using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope, we have obtained high-resolution ultraviolet spectra of the C I absorption toward two stars behind the Local Leo Cold Cloud (LLCC). At a distance (~20 pc) that places it well inside the Local Bubble, the LLCC is the nearest example of the coldest known (T~20 K) diffuse interstellar clouds. The STIS measurements of the C I fine-structure excitation toward HD 85259 and HD 83023 indicate that the thermal gas pressure of the LLCC is much greater than that of the warm clouds in the Local Bubble. The mean LLCC pressure measured toward these two stars (60,000 cm$^{-3}$ K) implies an H I density of ~3000 cm$^{-3}$ and a cloud thickness of ~200 AU at the 20 K cloud temperature. Such a thin, cold, dense structure could arise at the collision interface between converging flows of warm gas. However, the measured LLCC pressure is appreciably higher than that expected in the colliding cloud interpretation given the velocity and column density constraints on warm clouds in the HD 85259 and HD 83023 sightlines. Additional STIS measurements of the Zn II, Ni II, and Cr II column densities toward HD 85259 indicate that the LLCC has a modest "warm cloud" dust depletion pattern consistent with its low dust-to-gas ratio determined from H I 21 cm and 100 micron observations. In support of the inferred sheet-like geometry for the LLCC, a multi-epoch comparison of the Na I absorption toward a high-proper-motion background star reveals a 40% column density variation indicative of LLCC Na I structure on a scale of ~50 AU., Comment: 40 pages, 14 figures, 2 tables, accepted for publication in the Astrophysical Journal

PDS 144 is a pair of Herbig Ae stars that are separated by 5.35" on the sky. It has previously been shown to have an A2Ve Herbig Ae star viewed at 83\circ inclination as its northern member and an A5Ve Herbig Ae star as its southern member. Direct imagery revealed a disk occulting PDS 144 N - the first edge-on disk observed around a Herbig Ae star. The lack of an obvious disk in direct imagery suggested PDS 144 S might be viewed face-on or not physically associated with PDS 144 N. Multi-epoch Hubble Space Telescope imagery of PDS 144 with a 5 year baseline demonstrates PDS 144 N & S are comoving and have a common proper motion with TYC 6782-878-1. TYC 6782-878-1 has previously been identified as a member of Upper Sco sub-association A at d = 145 \pm 2 pc with an age of 5-10 Myr. Ground-based imagery reveals jets and a string of Herbig-Haro knots extending 13' (possibly further) which are aligned to within 7\circ \pm 6\circ on the sky. By combining proper motion data and the absence of a dark mid-plane with radial velocity data, we measure the inclination of PDS 144 S to be i = 73\circ \pm 7\circ. The radial velocity of the jets from PDS 144 N & S indicates they, and therefore their disks, are misaligned by 25\circ \pm 9\circ. This degree of misalignment is similar to that seen in T Tauri wide binaries., 18 pages, 5 figures, 2 tables

We present a multi-wavelength study of the local Leo cold cloud (LLCC), a very nearby, very cold cloud in the interstellar medium. Through stellar absorption studies we find that the LLCC is between 11.3 pc and 24.3 pc away, making it the closest known cold neutral medium cloud and well within the boundaries of the local cavity. Observations of the cloud in the 21-cm HI line reveal that the LLCC is very cold, with temperatures ranging from 15 K to 30 K, and is best fit with a model composed of two colliding components. The cloud has associated 100 micron thermal dust emission, pointing to a somewhat low dust-to-gas ratio of 48 x 10^-22 MJy sr^-1 cm^2. We find that the LLCC is too far away to be generated by the collision among the nearby complex of local interstellar clouds, but that the small relative velocities indicate that the LLCC is somehow related to these clouds. We use the LLCC to conduct a shadowing experiment in 1/4 keV X-rays, allowing us to differentiate between different possible origins for the observed soft X-ray background. We find that a local hot bubble model alone cannot account for the low-latitude soft X-ray background, but that isotropic emission from solar wind charge exchange does reproduce our data. In a combined local hot bubble and solar wind charge exchange scenario, we rule out emission from a local hot bubble with an 1/4 keV emissivity greater than 1.1 Snowdens / pc at 3 sigma, 4 times lower than previous estimates. This result dramatically changes our perspective on our local interstellar medium., 13 pages, 12 figures. Accepted for publication in the Astrophysical Journal. Vector figure version available at http://www.astro.columbia.edu/~jpeek/

We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLA absorbers in quasar spectra differ intrinsically in metallicity, and whether they could arise in galaxies of different masses. Using the recent measurements of the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirm that sub-DLAs have higher mean metallicities than DLAs, especially at $z \lesssim 2$. We find that the intercept of the metallicity-redshift relation derived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. We also show that, while there is a correlation between the metallicity and the rest equivalent width of Mg II $\lambda 2796$ or Fe II $\lambda 2599$ for DLAs, no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe II selection criteria employed in the discovery of both types of systems at lower redshifts, the difference between metallicities of DLAs and sub-DLAs appears to be real and not an artefact of selection. This conclusion is supported by our simulations of Mg II $\lambda 2796$ and Fe II $\lambda 2599$ lines for a wide range of physical conditions. On examining the velocity spreads of the absorbers, we find that sub-DLAs show somewhat higher mean and median velocity spreads ($\Delta v$), and an excess of systems with $\Delta v > 150$ km s$^{-1}$, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend for sub-DLAs appears to be steeper and closer to the trend for Galactic bulge and thick disk stars, possibly suggesting different stellar populations. The absorber data appear to be consistent with galaxy down-sizing. The data are also consistent with the relative number densities of low-mass and high-mass galaxies. It is thus plausible that sub-DLAs arise in more massive galaxies on average than DLAs., Comment: 27 pages, 5 figures, 4 tables. Accepted for publication in New Astronomy

We present an analysis of Kr I 1236 line measurements from 50 sight lines in the Hubble Space Telescope Space Telescope Imaging Spectrograph and Goddard High Resolution Spectrograph data archives that have sufficiently high resolution and signal-to-noise ratio to permit reliable krypton-to-hydrogen abundance ratio determinations. The distribution of Kr/H ratios in this sample is consistent with a single value for the ISM within 5900 pc of the Sun, log(Kr/H) = -9.02+/-0.02, apart from a rough annulus from between about 600 and 2500 pc distant. The Kr/H ratio toward stars within this annulus is elevated by approximately 0.11 dex, similar to previously noted elevations of O/H and Cu/H gas-phase abundances beyond about 800 pc. A significant drop in the gas-phase N/O ratio in the same region suggests that this is an artifact of nucleosynthetic history. Since the physical scale of the annulus' inner edge is comparable to the radius of the Gould Belt and the outer limit of heliocentric distances where the D/H abundance ratio is highly variable, these phenomena may be related to the Gould Belt's origins., Comment: 32 pages, including 6 figures; accepted by ApJ

We present chemical abundance measurements from medium resolution observations of 8 sub-damped Lyman-alpha absorber and 2 strong Lyman-limit systems at z < 1.5 observed with the MIKE spectrograph on the 6.5m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in z < 1.5 quasar absorption line systems (QSOALS) focusing on sub-DLA systems. These observations increase the sample of Zn measurements in z < 1.5 sub-DLAs by ~50%. Lines of Mg I, Mg II, Al II, Al III, Ca II, Mn II, Fe II, and Zn II were detected and column densities were determined. Zn II, a relatively undepleted element and tracer of the gas phase metallicity is detected in two of these systems, with [Zn/H]=-0.05\pm0.12 and [Zn/H]>+0.86. The latter system is however a weak system with N(H I)=-0.68, higher than typical Fe abundances for DLA systems at these redshifts. This high mean [Fe/H] could be due to less depletion of Fe onto dust grains, or to higher abundances in these systems. We also discuss the relative abundances in these absorbers. The systems with high metallicity show high ratios of [Mn/Fe] and [Zn/Fe], as seen previously in another sub-DLA. These higher values of [Mn/Fe] could be a result of heavy depletion of Fe onto grains, unmixed gas, or an intrinsically non-solar abundance pattern. Based on Cloudy modeling, we do not expect ionisation effects to cause this phenomenon., Comment: 20 pages, 21 figures, accepted for publication to MNRAS

We present chemical abundance measurements from high resolution observations of 7 sub-damped Lyman-alpha absorbers and 1 damped Lyman-alpha system at z, 15 pages, 15 Figures, Accepted by MNRAS, updated references and authors

We report observations of four sub-damped Lyα (sub-DLA) quasar absorbers at obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. We measure the available neutrals or ions of C, N, O, Si, P, S, Ar, Mn, Fe, and/or Ni. Our data have doubled the sub-DLA metallicity samples at and improved constraints on sub-DLA chemical evolution. All four of our sub-DLAs are consistent with near-solar or super-solar metallicities and relatively modest ionization corrections; observations of more lines and detailed modeling will help to verify this. Combining our data with measurements from the literature, we confirm previous suggestions that the N(H i)-weighted mean metallicity of sub-DLAs exceeds that of DLAs at all redshifts studied, even after making ionization corrections for sub-DLAs. The absorber toward PHL 1598 shows significant dust depletion. The absorbers toward PHL 1226 and PKS 0439-433 show the S/P ratio consistent with solar, i.e., they lack a profound odd–even effect. The absorber toward Q0439-433 shows super-solar Mn/Fe. For several sub-DLAs at , [N/S] is below the level expected for secondary N production, suggesting a delay in the release of the secondary N or a tertiary N production mechanism. We constrain the electron density using Si ii* and C ii* absorption. We also report different metallicity versus relations for sub-DLAs and DLAs. For two sub-DLAs with detections of emission lines from the underlying galaxies, our measurements of the absorption-line metallicities are consistent with the emission-line metallicities, suggesting that metallicity gradients are not significant in these galaxies.

The high-latitude Galactic H I cloud toward the extragalactic radio source 3C 225 is characterized by very narrow 21 cm emission and absorption indicative of a very low H I spin temperature of about 20 K. Through high-resolution optical spectroscopy, we report the detection of strong, very narrow Na I absorption corresponding to this cloud toward a number of nearby stars. Assuming that the turbulent H I and Na I motions are similar, we derive a cloud temperature of 20 (+6, -8) K (in complete agreement with the 21 cm results) and a line-of-sight turbulent velocity of 0.37+/-0.08 km/s from a comparison of the H I and Na I absorption linewidths. We also place a firm upper limit of 45 pc on the distance of the cloud, which situates it well inside the Local Bubble in this direction and makes it the nearest-known cold diffuse cloud discovered to date., 11 pages, 3 figures, accepted for publication in ApJ Letters

The metallicity in portions of high-redshift galaxies has been successfully measured thanks to the gas observed in absorption in the spectra of quasars, in the Damped Lyman-alpha systems (DLAs). Surprisingly, the global mean metallicity derived from DLAs is about 1/10th solar at 0, Comment: 5 pages, 1 figure. Accepted for publication in A&A

The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at z < 1.5, an epoch spanning \~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the solar value at low redshifts. These surprising results appear to contradict the near-solar mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies., 6 pages,3 figures, To appear in "Probing Galaxies through Quasar Absorption Lines", Proceedings IAU Colloquium 199, 2005, Eds. P. R. Williams, C. Shu, and B. Menard

We find evidence for dust in the intervening QSO absorbers from the spectra of QSOs in the Sloan Digital Sky Survey Data Release 1. No evidence is found for the 2175 A feature which is present in the Milky Way dust extinction curve. The extinction curve resembles the SMC extinction curve. The observed Delta(g-i) excess for QSOs with strong absorption systems appears to be a result of the reddening due to dust in the intervening absorbers., Poster paper presented at the IAU Colloquium #199 on "Probing Galaxies through Quasar Absorption Lines" held in Shanghai, China from March 14th to 18th, 2005

Recent high spatial and spectral resolution investigations of the diffuse interstellar medium (ISM) have found significant evidence for small-scale variations in the interstellar gas on scales less than or equal to 1 pc. To better understand the nature of small-scale variations in the ISM, we have used the KPNO WIYN Hydra multi-object spectrograph, which has a mapping advantage over the single-axis, single-scale limitations of studies using high proper motion stars and binary stars, to obtain moderate resolution (~12 km/s) interstellar Na I D absorption spectra of 172 stars toward the double open cluster h and Chi Persei. All of the sightlines toward the 150 stars with spectra that reveal absorption from the Perseus spiral arm show different interstellar Na I D absorption profiles in the Perseus arm gas. Additionally, we have utilized the KPNO Coude Feed spectrograph to obtain high-resolution (~3 km/s) interstellar Na I D absorption spectra of 24 of the brighter stars toward h and Chi Per. These spectra reveal an even greater complexity in the interstellar Na I D absorption in the Perseus arm gas and show individual components changing in number, velocity, and strength from sightline to sightline. If each of these individual velocity components represents an isolated cloud, then it would appear that the ISM of the Perseus arm gas consists of many small clouds. Although the absorption profiles vary even on the smallest scales probed by these high-resolution data (~30";~0.35pc), our analysis reveals that some interstellar Na I D absorption components from sightline to sightline are related, implying that the ISM toward h and Chi Per is probably comprised of sheets of gas in which we detect variations due to differences in the local physical conditions of the gas., 27 pages text; 8 figures

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha 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 mean sight line density, and that extend up to 6.5 kpc in length. Consequently, we have undertaken the study of gas-phase 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 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 a density of 1.5 cm^-3 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) = -3.41+/-0.01 (or 390+/-10 ppm), which is consistent with averages determined for several long, low-density paths observed by STIS (Andre 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) = -3.55+/-0.02 (284+/-12 ppm). The datapoints for low-density paths are scattered more widely than those for denser sight lines, due to O/H ratios for paths shorter than 800 pc that are generally about 0.10 dex lower than the values for longer ones., 33 pages, including 8 figures and 4 tables; accepted for publication in ApJ, tentatively in Oct 2004

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value $\sim 1/30$ solar at $z \sim 3$ 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. 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. ...(Please see the paper for the complete abstract)., 56 pages, including 13 figures. Accepted for publication in the Astrophysical Journal

We report interstellar C II column densities or upper limits determined from weak absorption of the 2325.4029 A intersystem transition observed in six translucent sightlines with STIS. The sightlines sample a wide range of interstellar characteristics including total-to-selective extinction, R_{V} = 2.6 - 5.1; average hydrogen density along the sightline, = 3 - 14 cm^{-3}; and fraction of H in molecular form, 0 - 40%. Four of the sightlines, those toward HD 37021, HD 37061, HD 147888 and HD 207198, have interstellar gas-phase abundances that are consistent with the diffuse sightline ratio of 161 +/- 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 sightlines, a large fraction of the C II toward HD 37061 is in an excited state. The sightline toward HD 152590 has a measured interstellar gas-phase carbon abundance that is well above the diffuse sightline average; the column density of C in this sightline may be overestimated due to noise structure in the data. Toward HD 27778 we find a 3 sigma abundance upper limit of, Comment: Accepted for publication in the Astrophysical Journal

We present Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph observations of high-ionization interstellar absorption toward HD 14434 [(l, b) = (135.1, -3.8); d ~ 2.3 kpc], an O5.5 V star in the Perseus OB1 Association. Intermediate-velocity interstellar Si IV and C IV absorption is present at V_LSR = -67 km/s, while low-ionization gas associated with the Perseus arm is detected at ~ -50 km/s. Neither N V nor O VI is detected at V_LSR = -67 km/s; although Al III and Fe III, tracers of warm ionized gas, are seen. The high-ion column densities in the -67 km/s component are log[N(C IV)] = 13.92 +/- 0.02 cm^-2, log[N(Si IV)] = 13.34 +/- 0.02 cm^-2, log[N(N V)] < 12.65 cm^-2, and log[N(O VI)] < 13.73 cm^-2 (3-sigma limits). The observed C IV/Si IV ratio of 3.8 +/- 0.3 in this intermediate-velocity cloud (IVC) is similar to the Galactic average (4.3 +/- 1.9). Our analysis of the Si IV andC IV line widths yields a temperature of T = 10,450 +/- 3,400 K for this component. At this low temperature, neither Si IV nor C IV can be produced via collisions. We investigate several photoionization models to explain the intermediate-velocity Si IV and C IV absorption toward HD 14434. Photoionization models employing cooling of a hot (T ~ 10^6 K) diffuse plasma as the source of ionizing radiation reproduces the observed properties of the IVC toward HD 14434 quite well. The hot plasma responsible for the ionizing radiation in these models may be attributed to hot gas contained in a supershell in or near the Perseus Arm or from a more generally distributed hot ionized medium., 29 pages, 6 figures, accepted for publication in August 1, 2003 edition of The Astrophysical Journal

We present Hubble Space Telescope Imaging Spectrograph (HST/STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) observations of high ion interstellar absorption along the sight line to HD 116852. At a distance of 4.8 kpc, HD 116852 is an O9 III star lying in the low Galactic halo, -1.3 kpc from the plane of the Galaxy in the direction l = 304.9, b = -16.1. The STIS E140H grating observations provide high-resolution (FWHM = 2.7 km/s) spectra of the resonance doublets of Si IV, C IV, and N V. These data are complemented by medium-resolution (FWHM = 20 km/s) FUSE spectra of O VI. We find evidence for three distinct types of highly ionized gas present in the data. First, two narrow absorption components are resolved in the Si IV and C IV profiles, at approximate LSR velocities of -36 and -10 km/s. These narrow components appear to be produced in gas associated with the Norma and Sagittarius spiral arms, at approximate z-distances of -1.0 and -0.5 kpc, respectively. Second, we detect an intermediate-width component in C IV and Si IV, at 17 km/s, which we propose could arise at the conductive interface at the boundary between a low column density neutral or weakly ionized cloud and the surrounding hot medium. Finally, a broad collisionally ionized component of gas responsible for producing the smooth N V and O VI profiles is observed; such absorption is also present to a lesser degree in the profiles of Si IV and C IV. The broad O VI absorption is observed at a velocity displaced from the broad C IV component by almost 20 km/s, an amount large enough to suggest that the two ions may not co-exist in the same physical location., Comment: 39 pages, 6 figures; accepted for publication in January 10, 2003 edition of The Astrophysical Journal

Spectra obtained with the Hubble Space Telescope GHRS are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the diffuse interstellar clouds seen along the line of sight to the star 23 Ori. 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 < v < +26 km/s. Weaker low-velocity absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km/s < v < +30 km/s. Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km/s < v < -83 km/s. Weak absorption components from ionized gas are seen at intermediate velocities (-43 km/s < v < -4 km/s). The large range in n_e (from 0.04 cm^{-3} to 0.95 cm^{-3}) derived independently from nine pairs of neutral and singly ionized species in the SLV gas suggests that additional processes besides simple photoionization and radiative recombination affect the ionization balance. The adopted average SLV electron density, n_e = 0.15+/-0.05 cm^{-3}, implies a relatively large n_e/n_H ~ 0.01, and thus some ionization of hydrogen in these predominantly neutral components. Comparisons of the SLV depletions and n_H with those found for the strong ``component B'' (v ~ -14 km/s) blend toward zeta Oph hint at a possible relationship between depletion and local density for relatively cold interstellar clouds. Calcium appears to be more severely depleted in warm, low density gas than has generally been assumed. An appendix summarizes the most reliable oscillator strengths currently available for a number of the interstellar absorption lines analyzed in this work., 73 pages, Latex, aaspp4.sty; to appear in ApJS, vol. 124 (1999 Oct); also available from ftp://astro.uchicago.edu/pub/astro/welty/23ori/23ori9.ps.gz