Daniel E. Welty

A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in lines of sight with AV e1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J ¼ 0 and J ¼ 1, which contain e99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of � 10 21 cm � 2 , measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN

Average extinction curves and relative abundances for quasi-stellar object absorption-line systems at 1 ≤zabs < 2

We have studied a sample of 809 Mg II absorption systems with 1.0 ≤ z abs ≤ 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-A 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 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 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 a 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 N H1 ∼ 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: 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.

STUDIES OF THE DIFFUSE INTERSTELLAR BANDS. III. HD 183143

Echelle spectra of HD 183143 [B7Iae, E(B − V) = 1.27] were obtained on three nights, at a resolving power R = 38,000 and with a signal-to-noise ratio ≈ 1000 at 6400 A in the final, combined spectrum. A catalog is presented of 414 diffuse interstellar bands (DIBs) measured between 3900 and 8100 A in this spectrum. The central wavelengths, the widths (FWHM), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. Among the 414 bands, 135 (or 33%) were not reported in four previous, modern surveys of the DIBs in the spectra of various stars, including HD 183143. The principal result of this study is that the great majority of the bands in the catalog are very weak and fairly narrow. Typical equivalent widths amount to a few mA, and the bandwidths (FWHM) are most often near 0.7 A. No preferred wavenumber spacings among the 414 bands are identified which could provide clues to the identities of the large molecules thought to cause the DIBs. At generally comparable detection limits in both spectra, the population of DIBs observed toward HD 183143 is systematically redder, broader, and stronger than that seen toward HD 204827 (Paper II). In addition, interstellar lines of C2 molecules have not been detected toward HD 183143, while a very high value of N(C2)/E(B − V )i s observed toward HD 204827. Therefore, either the abundances of the large molecules presumed to give rise to the DIBs, or the physical conditions in the absorbing clouds, or both, must differ significantly between the two cases.

A High-Resolution Survey of Interstellar K I Absorption

We present high-resolution (FWHM ~0.4-1.8 km s-1) spectra, obtained with the AAT UHRF, the McDonald Observatory 2.7 m coud? spectrograph, and/or the KPNO coud? feed, of interstellar K I absorption toward 54 Galactic stars. These new K I spectra reveal complex structure and narrow, closely blended components in many lines of sight. Multicomponent fits to the line profiles yield estimates for the column densities, line widths, and velocities for 319 individual interstellar cloud components. The median component width (FWHM) and the true median separation between adjacent components are both 1.2 km s-1. The median and maximum individual component K I column densities, about 4 ? 1010 and 1012 cm-2, correspond to individual component hydrogen column densities of about 2 ? 1020 and 1021 cm-2 and E(B-V) ~ 0.03 and 0.17, respectively. If T is typically ~100 K, then at least half the individual components have subsonic internal turbulent velocities. We also reexamine the relationships between the column densities of K I, Na I, C I, Li I, Htot, H2, and CH. The four trace neutral species exhibit essentially linear relationships with each other over wide ranges in overall column density. If C is uniformly depleted by 0.4 dex, then Li, Na, and K are each typically depleted by 0.6-0.7 dex. The total line of sight values for N(K I) and N(Na I) show roughly quadratic dependences on N(Htot), but the relationships for the ensemble of individual clouds could be significantly steeper. These quadratic (or steeper) dependences appear to rule out significant contributions to the ionization from cosmic rays, X-rays, and/or charge exchange with C II in most cases. Charge exchange with negatively charged large molecules may often be more important than radiative recombination in neutralizing most singly ionized atomic species in cool H I clouds, however?suggesting that the true ne, nH, and thermal pressures may be significantly smaller than the values estimated by considering only radiative recombination. Both N(CH) and N(H2) are nearly linearly proportional to N(K I) and N(Na I) [except for 1015 cm-2 N(H2) 1019 cm-2, over which H2 makes the transition to the self-shielded regime]. Those relationships appear also to hold for many individual components and component groups, suggesting that high-resolution spectra of K I and Na I can be very useful for interpreting lower resolution molecular data. The scatter about all these mean relationships is generally small (0.1-0.2 dex), if certain consistently discrepant sight lines are excluded?suggesting that both the relative depletions and the relative ionization of Li, C, Na, and K are generally within factors of 2 of their mean values. Differences noted for sight lines in Sco-Oph, in the Pleiades, near the Orion Trapezium, and in the LMC and SMC may be due to differences in the strength and/or shape of the ambient radiation fields, perhaps amplified by the effects of charge transfer with large molecules.

The Diffuse Interstellar Clouds toward 23 Orionis

Spectra obtained with the Hubble Space Telescope 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 diffuse 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 different 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 v? +26 km s-1. Most typically severely depleted species are less depleted by factors of 2-4, compared to the cold, dense cloud pattern found, for example, in the main components toward ? Oph.For the two strongest SLV components, T ~ 100 K and the thermal pressure log (nHT) ~ 3.1 cm-3 K; we thus have nH ~ 10-15 cm-3 and a total thickness of 12-16 pc. The adopted average SLV electron density, ne = 0.15 ? 0.05 cm-3, implies a relatively large ne/nH ~ 0.01 and thus some ionization of hydrogen in these predominantly neutral components.?????Weaker low-velocity (WLV) absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km s-1 v? +30 km s-1. The depletions in the WLV gas are typically less severe by a factor of 2-3 than in the SLV gas and are somewhat similar to the warm cloud pattern seen in lines of sight with low reddening, low mean density, and/or low molecular fraction. If T ~ 3000 K for the WLV components, then we have log(nHT) ~ 4.7-4.8 cm-3 K, nH ~ 15-20 cm-3, ne ~ 0.2 cm-3, ne/nH ~ 0.01, and a total thickness of 0.7-0.9 pc.?????Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km s-1 v? -83 km s-1. While the depletions in these ionized components are uncertain owing to unobserved ionization stages, aluminum (typically severely depleted) is probably depleted there by only a factor ~3, even at cloud velocities in excess of 100 km s-1. The individual high-velocity components typically have T ~ 8000 ? 2000 K, ne = nH ~ 0.4-0.5 cm-3, thermal pressure log(2neT) ~ 3.7-4.0 cm-3 K, and thicknesses of order 0.1 pc.?????Weak absorption components from ionized (H II) gas are seen in C II, Mg II, and Si III at intermediate velocities (-43 km s-1 v? -4 km s-1). Broad, weak absorption from the higher ions S III, C IV, Si IV, and N V is centered at -5 km s-1 v? +6 km s-1. No obvious absorption is discerned from a circumstellar H II region around 23 Ori itself.?????The large range in ne (from 0.04 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. Charge exchange with protons may reduce the abundances of S I, Mn I, and Fe I; dissociative recombination of CH+ may help to enhance C I. The large apparent fractional ionization in the SLV and WLV gas may be due to an enhanced flux of X-rays in the Orion region, to mixing of neutral and ionized gas at the boundary of the Orion-Eridanus bubble, or perhaps (in part) to charge exchange between singly ionized atomic species and large molecules (in which case the true ne would be somewhat smaller). Comparisons of the SLV depletions and nH with those found for the strong component B (v? ~ -14 km s-1) blend toward ? 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.

Hubble Space Telescope Observations of Element Abundances in Low-Redshift Damped Lyα Galaxies and Implications for the Global Metallicity-Redshift Relation

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 Lyα 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) on board the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with z < 0.4. In all 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-weighted mean Zn metallicity versus 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 instead of the Zn limits. The corresponding estimates of the z = 0 intercept of the metallicity-redshift relation are -0.74 ± 0.15, -0.75 ± 0.18, and -0.71 ± 0.13, respectively. Roughly similar results are obtained if survival analysis or an unbinned N-weighted nonlinear χ2 approach is used. Thus, the N-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.

STUDIES OF DIFFUSE INTERSTELLAR BANDS V. PAIRWISE CORRELATIONS OF EIGHT STRONG DIBs AND NEUTRAL HYDROGEN, MOLECULAR HYDROGEN, AND COLOR EXCESS

We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and E B?V . The DIBs are centered at ?? 5780.5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearsons correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width (EW) of ?5780.5 is better correlated with column densities of H than with E B?V or H2, confirming earlier results based on smaller data sets. We show that the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not correlated well enough with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H2, and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest that they arise from different molecules found in diffuse H regions with very little H2 (molecular fraction f < 0.01). Of the 133 stars with available data in our study, there are three with significantly weaker ?5780.5 than our mean H-?5780.5 relationship, all of which are in regions of high radiation fields, as previously noted by Herbig. The correlations will be useful in deriving interstellar parameters when direct methods are not available. For instance, with care, the value of N(H) can be derived from W ?(5780.5).

Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud

We have analyzed both high-resolution optical absorption-line spectra and UV spectra obtained with IUE of the LMC SN 1987A, in order to determine abundances and physical conditions in the various neutral interstellar clouds along the line of sight to the supernova (SN). We have used a flat-fielding procedure to enhance the signal-to-noise ratios (S/Ns) and the reliability of weak features in the UV spectra and have modeled the UV line profiles using the component structure derived from the higher resolution, high-S/N optical spectra of Ca II and Na I. Fits to the Ca II, Ca I, and Na I absorption-line profiles reveal (at least) 46 components, at velocities -24 km s-1v296 km s-1, which can be associated with the 10 component groups discernible in the lower resolution UV spectra. From the UV spectra, we determined component-group column densities for C I, Mg I, Mg II, Al II, Si II, P II, Cl I, Ti?II, Cr II, Mn II, Fe II, Ni II, and Zn II?with 1 ? uncertainties less than 0.1 dex in many cases. These are the most extensive and accurate abundances yet measured for the neutral ISM in the LMC. The component velocities, the patterns of relative elemental abundances [X/Zn] and [X/Fe], and various diagnostic ratios have been used to estimate the locations and physical characteristics [N(H), T, n] of these component groups. (Systematic differences among the diagnostic ratios make the derived physical properties somewhat uncertain, however.) The components at low velocities (5 km s-1v23 km s-1) have relative abundances and values for the diagnostic ratios very similar to those found for warm, diffuse Galactic disk clouds and likely are due to a mixture of warm and cool gas in the Galactic disk. The components at velocities 56 km s-1v90 km s-1 are due to a mixture of warm and cool gas, apparently with negligible depletions, in the Galactic halo. The two intermediate-velocity component groups (109 km s-1v140 km s-1 and 155 km s-1v176 km s-1) both have relative abundances similar to those found for Galactic halo clouds. These warm (T4500 K), partially ionized clouds are probably located in the Galactic halo and in the LMC, respectively. The components at velocities 191 km s-1v225 km s-1 also have relative abundances similar to those in the halo clouds but are likely due to gas in the LMC, perhaps very close to the SN. The component groups at 238 km s-1v255 km s-1 and 265 km s-1v270 km s-1 are probably located on opposite sides of the main LMC component group (at velocities 275 km s-1v296 km s-1) (using absorption-line data for several other adjacent lines of sight and the structure inferred from SN light-echo observations). Although the relative abundances and diagnostic ratios for those three LMC groups are similar to those found for warm, low-density Galactic disk clouds, the widths of individual components seen in very high resolution spectra of Na I and K I imply that T is generally less than about 1500 K. Higher N(Na I)/N(Ca II) ratios, the presence of CH, and the C I fine structure level populations suggest that the main LMC group contains both cool and warm gas. For the LMC components, the total N(H) estimated from the observed relative abundances and inferred depletions is consistent with the value obtained from Ly? absorption toward the neighboring star Sk -69?203, after accounting for differences in reddening and for an overall subsolar metallicity of 0.2-0.3 dex for the LMC ISM. Since the relative abundance patterns determined for stars and gaseous nebulae in both the SMC and the LMC appear to be similar to the solar pattern (for the elements whose interstellar abundances we have considered), the similarities in relative gas-phase interstellar abundances in our Galaxy and in the Magellanic Clouds suggest that the dust depletions follow similar patterns as well?despite differences in metallicity and dust-to-gas ratio among the three galaxies. These local relative abundance/depletion patterns may thus be used to infer total (gas+dust) abundances for QSO absorption-line systems at various redshifts.

The Magellanic Mopra Assessment (MAGMA). I. the molecular cloud population of the large magellanic cloud

We present the properties of an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) mapped at 11?pc resolution in the CO(1-0) line. Targets were chosen based on a limiting CO flux and peak brightness as measured by the NANTEN survey. The observations were conducted with the ATNF Mopra Telescope as part of the Magellanic Mopra Assessment. We identify clouds as regions of connected CO emission and find that the distributions of cloud sizes, fluxes, and masses are sensitive to the choice of decomposition parameters. In all cases, however, the luminosity function of CO clouds is steeper than dN/dLL ?2, suggesting that a substantial fraction of mass is in low-mass clouds. A correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. We argue that the correlation between virial mass and CO luminosity is the result of comparing two covariant quantities, with the correlation appearing tighter on larger scales where a size-linewidth relation holds. The virial parameter (the ratio of a clouds kinetic to self-gravitational energy) shows a wide range of values and exhibits no clear trends with the CO luminosity or the likelihood of hosting young stellar object (YSO) candidates, casting further doubt on the assumption of virialization for molecular clouds in the LMC. Higher CO luminosity increases the likelihood of a cloud harboring a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming the close link between giant molecular clouds and massive star formation.

VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds

We discuss the abundances of interstellar CH, CH+, and CN in the Magellanic Clouds, derived from spectra of seven SMC and 13 LMC stars obtained (mostly) with the VLT UVES. CH and/or CH+ have now been detected toward three SMC and nine LMC stars; CN is detected toward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly all the optical detections of these molecular species in interstellar media beyond the Milky Way. In the LMC, the CH/H2 ratio is comparable to that found for diffuse Galactic molecular clouds in four sight lines but is lower by factors of 2.5-4.0 in two others. In the SMC, the CH/H2 ratio is comparable to the local Galactic value in one sight line but is lower by factors of 10-15 in two others. The abundance of CH in the Magellanic Clouds thus appears to depend on local physical conditions and not just on metallicity. In both the SMC and the LMC, the observed relationships between the column density of CH and those of CN, CH+, Na I, and K I are generally consistent with the trends observed in our Galaxy. Using existing data for the rotational populations of H2 in these sight lines, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. The inferred temperatures range from about 45 to 90 K, the radiation fields range from about 1 to 900 times the typical local Galactic field, and the densities (in most cases) lie between 100 and 600 cm-3. Densities estimated from the observed N(CH), under the assumption that CH is produced via steady state gas-phase reactions, are considerably higher than those derived from H2. Much better agreement is found by assuming that the CH is made via the (still undetermined) process(es) responsible for the observed CH+. A significant fraction of the CH and CH+ in diffuse molecular material in the SMC and LMC may be produced in photon-dominated regions. The excitation temperature obtained from the populations of the two lowest CN rotational levels toward Sk -67 2 is quite consistent with the temperature of the cosmic microwave background radiation measured with COBE. Toward most of our targets, the UVES spectra also reveal absorption at velocities corresponding to the Magellanic Clouds ISM from several of the strongest of the diffuse interstellar bands (DIBs; at 5780, 5797, and 6284 A). On average, the three DIBs are weaker by factors of 7-9 (LMC) and about 20 (SMC), compared to those typically observed in Galactic sight lines with similar N(H I), presumably due to the lower metallicities and stronger radiation fields in the LMC and SMC. The three DIBs are also weaker (on average, but with some exceptions), by factors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in the Magellanic Clouds. The detection of several of the so-called C2 DIBs toward Sk 143 and Sk -67 2 with strengths similar to those in comparable Galactic sight lines, however, indicates that no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or for all sight lines) in the Magellanic Clouds.