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Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made byHST STIS and FUSE to determine the column densities and rotationalexcitation temperatures for carbon monoxide and molecular hydrogen,respectively, along 23 sight lines to Galactic O and B stars. Thereddening values range from E(B-V)=0.07 to 0.62, sampling the diffuse totranslucent interstellar medium (ISM). We find that the H2column densities range from 5×1018 to8×1020 cm-2 and the CO from upper limitsaround 2×1012 cm-2 to detections as high as1.4×1016 cm-2. CO increases with increasingH2, roughly following a power law of factor ~2. TheCO/H2 column density ratio is thus not constant, ranging from10-7 to 10-5, with a mean value of3×10-6. The sample segregates into ``diffuse'' and``translucent'' regimes, the former with molecular fraction <~0.25and AV/d<1 mag kpc-1. The mean CO/H2for these two regimes are 3.6×10-7 and9.3×10-6, respectively, significantly lower than thecanonical dark cloud value of 10-4. Six sight lines show theisotopic variant 13CO, and the isotopic ratio we observe(~50-70) is consistent with, if perhaps a little below, the average12C/13C for the ISM at large. The averageH2 rotational excitation temperature is 74+/-24 K, agreeingwell with previous studies, and the average CO temperature is 4.1 K,with some sight lines showing temperatures as high as 6.4 K. The higherexcitation CO is observed with higher column densities, consistent withthe effects of photon trapping in clouds with densities in the 20-100cm-3 range. We discuss the implications for the structure ofthe diffuse/translucent regimes of the ISM and the estimation ofmolecular mass in galaxies.
| Ultraviolet Excited High-J Molecular Hydrogen in Photodissociation Regions
We have calculated synthetic interstellar cloud models to investigatethe formation and destruction of high-J molecular hydrogen inphotodissociation regions. The effects of five physical parameters (theincident ultraviolet [UV] intensity, H2 column density, cloudtemperature, total density, and H2 formation ratecoefficient) on the populations of H2 rotational levels areexplored. We have found that N(4)/N(0) is proportional to the incidentUV intensity IUV and the H2 molecular fraction fis simply related to the ratio of IUV and the hydrogendensity nH, implying a new method to derive IUVand nH with the observational parameters N(4)/N(0) and f,assuming an H2 formation rate R. High-resolution FUSE spectraof H2 toward three translucent sight lines (HD 110432, HD192639, and HD 185418) in the Milky Way and 24 diffuse sight lines inthe SMC and LMC are referenced to obtain N(4)/N(0) and f. Using ourmethod, we are able to derive IUV~10-40 for the translucentsight lines, when R is assumed to be 10 times higher than the Galacticvalue. Synthetic models for the Magellanic sight lines suggest thatIUV>=10, nH<=100 cm-3, andR~3×10-16 for the low H2 column densitysight lines (N[H2]<=1018 cm-2) andIUV>=10, nH<=1000 cm-3, andR~3×10-17 for the high H2 column densitysight lines (N[H2]>=1019 cm-2).
| Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
Using UV spectra obtained with FUSE, HST, and/or IUE together withhigher resolution optical spectra, we determine interstellar columndensities of 12CO, 13CO, and/or C2 for10 Galactic sight lines with E(B-V) ranging from 0.37 to 0.72. TheN(CO)/N(H2) ratio varies over a factor of 100 in this sample,due primarily to differences in N(CO). For a given N(H2 ),published models of diffuse and translucent clouds predict less CO thanis observed. The J=1-3 rotational levels of 12CO aresubthermally populated in these sight lines, with Textypically between 3 and 7 K. In general, there appears to be nosignificant difference between the excitation temperatures of12CO and 13CO. Fits to the higher resolution COline profiles suggest that CO (like CN) is concentrated in relativelycold, dense gas. We obtain C2 column densities from the F-X(0-0) band at 1341 Å (three sight lines; J=0-12), the F-X (1-0)band at 1314 Å (one sight line; J=0-12), the D-X (0-0) band at2313 Å (four sight lines; J=0-18), and the A-X (3-0) and (2-0)bands at 7719 and 8757 Å (seven sight lines; J=0-12). Comparisonsamong those column densities yield a set of mutually consistent bandf-values for the UV and optical C2 bands, but also revealsome apparent anomalies within the F-X (0-0) band. Both the kinetictemperature Tk inferred from the C2 rotationalpopulations and the excitation temperature T02(C2)are generally smaller than the correspondingT01(H2)-suggesting that C2 isconcentrated in colder, denser gas than H2. Incorporatingadditional column density data for K I, HD, CH, C2,C3, CN, and CO from the literature (for a total sample of 74sight lines), we find that (1) CO is most tightly correlated with CN;(2) the ratios 12CO/H2 and13CO/H2 both are fairly tightly correlated withthe density indicator CN/CH (but C2/H 2 is not);and (3) the ratio 12CO/13CO is somewhatanticorrelated with both CN/CH and N(CO). Sight lines with12CO/13CO below the average local Galactic valueof 12C/13C appear to sample colder, denser gas inwhich isotope exchange reactions have enhanced 13CO, relativeto 12CO.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. Based in part onobservations obtained with the Apache Point Observatory 3.5 m telescope,which is owned and operated by the Astrophysical Research Consortium.
| Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
We report on the abundance of interstellar neutral nitrogen (N I) for 30sight lines, using data from the Far Ultraviolet Spectroscopic Explorer(FUSE) and the Hubble Space Telescope (HST). N I column densities arederived by measuring the equivalent widths of several ultravioletabsorption lines and subsequently fitting those to a curve of growth. Wefind a mean interstellar N/H of 51+/-4 ppm. This is below the mean foundby Meyer et al. of 62+4-3 ppm (adjusted for adifference in f-values). Our mean N/H is similar, however, to the(f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for alarger sample of sight lines with larger hydrogen column densitiescomparable to those in this study. We discuss the question of whether ornot nitrogen shows increased gas-phase depletion in lines of sight withcolumn densities logN(Htot)>~21, as claimed by Knauth etal. The nitrogen abundance in the line of sight toward HD 152236 isparticularly interesting. We derive very small N/H and N/O ratios forthis line of sight that may support a previous suggestion that membersof the Sco OB1 association formed from an N-deficient region.
| Chlorine in the Galactic Interstellar Medium: Revised f-Values with the Far Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph
Cl I is the atomic species most directly coupled to molecular hydrogendue to its chemistry. Its weakest lines are thereby probably the besttracer of optically thick H2 components in diffuse clouds. Wereport on the empirical determination of the oscillator strengths forfour Cl I absorption lines predicted to be weak and often detectedtoward moderately reddened sight lines observed with the Far UltravioletSpectroscopic Explorer (FUSE). We compared our oscillator strengthestimates with the oscillator strength calculations listed in Morton. Wefind that our empirical oscillator strength values for the Cl I 1004,1079, 1090, and 1094 Å lines differ from the theoreticalpredictions by factors of ~3.1, 1.2, 2.4, and 0.42, respectively. Webriefly discuss the value of Cl I as tracer of molecular gas for ourstar sample.Based on observations made with the NASA/ESA Hubble Space Telescope(HST), obtained from the Data Archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| Physical Conditions in the Interstellar Medium toward HD 185418
We have developed a complete model of the hydrogen molecule as part ofthe spectral simulation code Cloudy. Our goal is to apply this tospectra of high-redshift star-forming regions where H2absorption is seen, but where few other details are known, to understandits implication for star formation. The microphysics of H2 isintricate, and it is important to validate these numerical simulationsin better understood environments. This paper studies a well-definedline of sight through the Galactic interstellar medium (ISM) as a testof the microphysics and methods we use. We present a self-consistentcalculation of the observed absorption-line spectrum to derive thephysical conditions in the ISM toward HD 185418, a line of sight withmany observables. We deduce density, temperature, local radiation field,cosmic-ray ionization rate, and chemical composition and compare theseconclusions with conditions deduced from analytical calculations. Wefind a higher density and similar abundances, and we require acosmic-ray flux enhanced over the Galactic background value, consistentwith enhancements predicted by MHD simulations.
| Phosphorus in the diffuse interstellar medium
We present FUSE and HST/STIS measurements of the P ii column densitytoward Galactic stars. We analyzed P ii through the profile fitting ofthe unsaturated λ1125 and λ1533 lines and derived columndensities integrated along the sightlines as well as in individualresolved components. We find that phosphorus is not depleted along thosesightlines sampling the diffuse neutral gas. We also investigate thecorrelation existing between P ii and O i column densities and find thatthere is no differential depletion between these two specie.Furthermore, the ratio N(P ii)/N(O i) is consistent with the solar P/Ovalue, implying that P ii and O i coexist in the same gaseous phase andare likely to evolve in parallel. We argue that phosphorus, as traced byP ii, is an excellent neutral oxygen tracer in various physicalenvironments, except when ionization corrections are a significantissue. Thus, P ii lines (observable with FUSE, HST/STIS, or withVLT/UVES for the QSO sightlines) are particularly useful as a proxy forO i lines when these are saturated or blended.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| Tiny-scale molecular structures in the Magellanic Clouds. I. FUSE, HST and VLT observations
The objective of this series of two papers is to investigate small-scalemolecular structures in the Magellanic Clouds (hereafter MCs). We reporton the FUSE detections of the HD and CO molecules on the lines of sighttowards three Large Magellanic stars: Sk -67D05, Sk -68D135, and Sk-69D246. HD is also detected for the first time on the lines of sighttowards two Small Magellanic Cloud stars: AV 95 and Sk 159. While the HDand CO abundances are expected to be lower in the Large Magellanic Cloudwhere molecular fractions are a third of the Galactic value and wherethe photodissociation flux is up to thousands of times as large, wereport an average HD/H2 ratio of 1.4±0.5 ppm and aCO/H2 ratio ranging from 0.8 to 2.7 ppm similar to theGalactic values. We tentatively identify a deuterium reservoir(hereafter D-reservoir) towards the Small Magellanic Cloud, along thelight path to AV 95. We derive a D/H ratio ranging from 1.×10-6 to 1.1× 10-5. Combining FUSE andHST/STIS data we also analyzed the H2, Cl I, Cl II, Fe II, SII, C I, C I*, and C I** content, when available. High resolution VLTobservations of Na I, Ca II, and Ca I were obtained in support of thelower resolution FUSE and STIS data for three targets in order tounravel the sightline velocity structures. These observations form theonly such set of detections in the Magellanic Clouds to date and allowus to investigate in detail some of the physical properties of theintervening molecular gas. Our detection of the HD and CO molecules inthe Magellanic Clouds is an argument for the existence of dense(nH > 100 cm-3) components. Furthermore, wedemonstrate that these components are probably extremely small molecularclumps (possibly as small as 10-2 pc) or filaments similar tothe tiny-scale atomic structures (TSAS) recently observed in the halo ofour Galaxy by Richter et al. (\cite{Richter2003}). For these fivesightlines, we also detected molecular hydrogen originating in theGalactic disk. From these observations we conclude that tiny-scalemolecular filamentary structures are present in the disk of the Galaxyas well.
| The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS-32985.
| Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and SpaceTelescope Imaging Spectrograph observations of the weak interstellar N Iλ1160 doublet toward 17 high-density sight lines[N(Htot)>=1021 cm-2]. When combinedwith published data, our results reveal variations in the fractional N Iabundance showing a systematic deficiency at large N(Htot).At the FUSE resolution (~20 km s-1), the effects ofunresolved saturation cannot be conclusively ruled out, although O Iλ1356 shows little evidence of saturation. We investigated thepossibility that the N I variability is due to the formation ofN2 in our mostly dense regions. The 0-0 band of thec'41Σ+u-X1Σ+gtransition of N2 at 958 Å should be easily detected inour FUSE data; for 10 of the denser sight lines, N2 is notobserved at a sensitivity level of a few times 1014cm-2. The observed N I variations are suggestive of anincomplete understanding of nitrogen chemistry.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985, and the NASA/ESA HubbleSpace Telescope, obtained from the Multimission Archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
| Abundances and Physical Conditions in the Interstellar Gas toward HD 185418
We present a study of the abundances and physical conditions in theinterstellar gas toward the moderately reddened B0.5 V star HD 185418.This star is located at (l,b)=(53deg,-2.2d) and has anestimated distance of 790 pc and a reddening E(B-V)=0.50.This work is based on the analysis of Far Ultraviolet SpectroscopicExplorer (FUSE) and Hubble Space Telescope/Space Telescope ImagingSpectrograph (HST/STIS) spectra covering the range from 912 to 1361Å. We used both sets of far-ultraviolet data, together withhigh-resolution optical data, to derive accurate gas-phase columndensities for important interstellar species such as C I, C I*, C I**, NI, O I, O I*, Mg II, S I, S III, Cl I, Mn II, Fe II, Ni II, Cu II, GeII, Kr I, and CO. Numerous lines of H2 are present in theFUSE spectra, with a kinetic temperature for the lowest rotationallevels T01=100+/-15 K. Analysis of the C I fine-structureexcitation at that temperature implies an average local density ofhydrogen nH=6.3+/-2.5 cm-3. Chemical argumentsbased on analyses of CH, CH+, and C2 indicate thata fraction of the CH (>=20%) is formed as a by-product ofnonequilibrium CH+ synthesis. The electron densityne, derived under the assumption of photoionizationequilibrium, ranges from 0.03 to 0.32 cm-3 for the sixdifferent neutral/first-ion ratios considered. The relatively complexcomponent structure seen in high-resolution spectra of K I, Na I, and CaII, the relatively low average volume densities, the modest molecularfraction f(H2)=0.44, and the measured depletions all indicateno evidence for the presence of translucent components in the lightpath. The line of sight rather contains a mixture of cold and warmneutral diffuse gas. The detection of Si III and S III indicates thepresence of some ionized gas (at a 1% level) along the line of sight. Wecould not determine unambiguously whether this ionized gas wasphysically related to the neutral components.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555. Theobservations are associated with proposal 8241. Based in part onobservations obtained with the Apache Point Observatory 3.5 m telescope,which is owned and operated by the Astrophysical Research Consortium.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Oxygen Gas-Phase Abundance Revisited
We present new measurements of the interstellar gas-phase oxygenabundance along the sight lines toward 19 early-type Galactic stars atan average distance of 2.6 kpc. We derive O I column densities fromHubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS)observations of the weak 1355 Å intersystem transition. We derivetotal hydrogen column densities [N(HI)+2N(H2)] using HST/STISobservations of Lyα and Far Ultraviolet Spectroscopic Explorer(FUSE) observations of molecular hydrogen. The molecular hydrogencontent of these sight lines ranges fromf(H2)=2N(H2)/[N(HI)+2N(H2)]=0.03 to0.47. The average of6.3×1021 cm-2 mag-1 with astandard deviation of 15% is consistent with previous surveys. The meanoxygen abundance along these sight lines, which probe a wide range ofGalactic environments in the distant interstellar medium, is106 (O/H)gas=408+/-13 (1 σ in the mean). Wesee no evidence for decreasing gas-phase oxygen abundance withincreasing molecular hydrogen fraction, and the relative constancy of(O/H)gas suggests that the component of dust containing theoxygen is not readily destroyed. We estimate that, if 60% of the dustgrains are resilient against destruction by shocks, the distantinterstellar total oxygen abundance can be reconciliated with the solarvalue derived from the most recent measurements of 106(O/H)gassolar=517+/-58 (1 σ). We note thatthe smaller oxygen abundances derived for the interstellar gas within500 pc or from nearby B star surveys are consistent with a localelemental deficit.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| Gas-Phase Iron Abundances and Depletions in Translucent Interstellar Lines of Sight from Far Ultraviolet Spectroscopic Explorer Observations of Fe II Lines
Far Ultraviolet Spectroscopic Explorer (FUSE) wavelength coverageincludes several weak- to moderate-strength lines of Fe II, allowing thedetermination, through curve-of-growth analysis, of accurate Fe IIabundances and hence iron depletions. We have analyzed Fe II absorptionlines toward 18 of the reddened stars included in the FUSE survey ofmolecular hydrogen abundances in translucent clouds. Our analysis isbased on equivalent width measurements and curves of growth, aided bythe fact that some of the observed lines are weak enough to be on thelinear part of the curve of growth. In interpreting our abundance anddepletion results, we have combined our data with those of an earliersurvey of interstellar iron abundances and depletions in diffuse clouds,based on Copernicus data. The principal result of our survey is thatiron depletions, known from earlier work to increase with averageline-of-sight density for diffuse clouds, do not continue to increasewith either density or extinction in translucent clouds; i.e., there isno significant trend of increasing depletion with increasing extinctionor molecular fraction. This may be due to the fact that our data setdoes not probe lines of sight with greater average volume densities thanthose that were covered by the previous Copernicus-based survey of irondepletions. We conclude by reevaluating the definition of translucentclouds, based on the lack of enhanced iron depletions in our sample.
| Space Telescope Imaging Spectrograph Observations of Interstellar Oxygen and Krypton in Translucent Clouds
We have obtained high-resolution Hubble Space Telescope (HST) SpaceTelescope Imaging Spectrograph (STIS) observations of O I λ1356and Kr I λ1236 absorption in 11 sight lines characterized by highextinction, large H I column densities, and/or long path lengths.Previous Goddard High Resolution Spectrograph (GHRS) measurements ofthese weak features in seven relatively nearby diffuse clouds have shownno evidence for density-dependent depletion of either oxygen or kryptonand have yielded a weighted mean gas-phase abundance ratio oflog[N(O)/N(Kr)]GHRS=5.56+/-0.04. Our STIS measurements yielda lower weighted mean of log[N(O)/N(Kr)]STIS=5.48 thedifference is due primarily to several translucent sight lines in theSTIS data set that diverge from the GHRS value. These translucent cloudsight lines pass near dense, star-forming regions, notably the ρOph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas,should not be depleted much into grains, these cases suggest a trendtoward the enhanced oxygen depletion predicted for denser ISM clouds.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc. under NASAcontract NAS 5-26555.
| Rejection of the C-7 Diffuse Interstellar Band Hypothesis
Using the new high-resolution (~8 km s-1) echellespectrograph on the 3.5 m telescope at the Apache Point Observatory, wehave begun a high-sensitivity survey of the diffuse interstellar bands(DIBs) in a large sample of reddened stars. Now that we are 2 years intothis long-term survey, our sample includes over 20 reddened stars thatshow at least one of the DIBs that had been suggested to be caused byC-7, based on the gas-phase measurement of theC-7 spectrum by J. P. Maier's group. Thehigh-quality astronomical data from this larger sample of stars, alongwith the spectroscopic constants from the new laboratory work recentlyreported by Maier's group, have enabled us to examine more carefully theagreement between C-7 and the DIBs. We find thatnone of the C-7 bands match the DIBs in wavelengthor expected profile. One of the DIBs (λ5748) attributed toC-7 is actually a stellar line. The two strongestDIBs attributed to C-7 (λ6270 andλ4964) are not correlated in strength, so they cannot share thesame carrier. On the whole, we find no evidence supporting thehypothesis that C-7 is a carrier of the DIBs.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Intrinsic colour indices of O- and B- type stars in the Vilnius photometric system.
Not Available
| Precise equatorial coordinates of double and multiple systems - an astronomical support to the HIPPARCOS mission
Results are presented on determinations of precise positions (to theaccuracy required for their possible inclusion in the Hipparcos InputCatalogue) of 623 double and multiple stars (126 of them newlydiscovered) from the measurement of 39 plates taken at the ESO and theAstronomical Observatory of Torino, obtained between 1984 and 1987. Thetable includes the index number, the name of the discoverer, binarycomponent according to the designation by the Catalogue de Composantesd'etoiles Doubles et Multiples (CCDM), the magnitude of the componentsas given in the CCDM, epoch of observation, right ascension referred tothe J2000.0 equinox, declination, position angle referred to the J2000.0North Pole, angular separation, and DM and ADS numbers.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| Extinction law survey based on UV ANS photometry
The paper presents an extensive survey of interstellar extinction curvesderived from the ANS photometric measurements of early type starsbelonging to our Galaxy. This survey is more extensive and deeper thanany other one, based on spectral data. The UV color excesses aredetermined with the aid of 'artificial standards', a new techniqueproposed by the authors which allows the special check of Sp/L match ofa target and the selected standard. The results indicate that extinctionlaw changes from place to place.
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