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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 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

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

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

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 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 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.

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.

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.

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.

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

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/

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

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

We present HST/GHRS echelle observations of multiple interstellar lines of CI, MgI, CrII, and ZnII towards both stars in the mu Cru binary system. Despite large differences in the profiles of the neutral species, no significant variations between the stars are seen in the CrII and ZnII line profiles. In particular, the ZnII absorption observed at -8.6 km/sec towards mu Cru is constant despite greatly enhanced columns of the neutral species at this velocity towards mu^1 Cru. An analysis of the fine-structure excitation of CI in this cloud implies that the density is n_H < 250 cm^{-3}. From the lack of variation in the (optical) NaI D2 line profiles towards mu^1 and mu^2 Cru in spectra taken 21 months apart, we can place a lower limit to the size of the structures of ~10 AU. These results are discussed in the context of recent radio and optical studies of apparently pervasive high density small-scale interstellar structure., 10 pages, 2 figures, to appear in the Astrophysical Journal (Letters)