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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.
| Two new evolved bipolar planetary nebulae in the solar neighbourhood
We present AAO/UKST Hα+[N II] narrow-band imagery and low- andmedium-resolution optical spectroscopy of RCW24 and RCW69. These nebulaewere previously classified as HII regions, but we now show them to betwo of the largest and nearest bipolar Type I PNe yet discovered.Distances were estimated using extinction-distance and kinematicmethods, and via a new Hα surface brightness-radius relation. Theadopted distances are 1.0 +/- 0.3kpc for RCW24 and 1.3 +/- 0.2kpc forRCW69. Both objects have enhanced nitrogen abundances, withlogN/O~=+0.44 for RCW24, and logN/O=+0.33 for RCW 69. Systemicvelocities and |z| distances are VLSR = +5 km s-1and |z| ~ 23pc for RCW 24, and VLSR = -33 km s-1and only |z| ~ 7pc for RCW 69. Both PNe originated from massiveprogenitors (>2.0-2.5Msolar), as deduced from theirchemical abundances, large ionized masses, small |z| distances, lowpeculiar velocities and relatively hot central stars. These two objectsform an important addition to the small sample of evolved bipolar PNe inthe solar neighbourhood.
| VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue 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 onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| FUSE Measurements of Far-Ultraviolet Extinction. II. Magellanic Cloud Sight Lines
We present an extinction analysis of nine reddened/comparison star pairsin the Large and Small Magellanic Clouds (LMC and SMC) based onFar-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date,just two LMC sight lines have probed dust grain composition and sizedistributions in the Magellanic Clouds using spectral data forwavelengths as short as 950 Å. We supplement these two with datafrom four regions distinguished by their IR through UV extinction curvesand grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinctcharacters of extinction in the Clouds and Milky Way, our results aregenerally analogous to those found for Galactic curves-namely, that theFUSE portions of each extinction curve are described reasonably well byFitzpatrick & Massa curves fitted only to longer wavelength data andlack any dramatic new extinction features, and any deviations from theCardelli, Clayton, & Mathis (CCM) formalism continue into FUVwavelengths. A maximum entropy method analysis of all of these curvessuggests that LMCAvg and SMC wing sight lines, whose extinctionparameters more closely resemble those for Galactic paths, require moresilicon and/or carbon in dust than current abundance measurements wouldindicate are available. The requirements for LMC2 and SMC bar sightlines do not fully tax the available reservoirs, in part because largegrains contribute less to the extinction in these directions. Anintermediate product of this extinction analysis is the measurement ofnew H2 abundances in the Magellanic Clouds. Collectivelyconsidering Cloud sight lines that possess significant H2column densities, E(B-V)/N(H I) ratios are reduced by significantfactors relative to the Galactic mean, whereas the correspondingE(B-V)/N(H2) values more closely resemble their Galacticcounterpart. These trends reflect the fact that among these sight linesf(H2) values are lower than those common in the Milky Way forpaths with similar degrees of reddening.Based on observations with the NASA-CNES-CSA Far-UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS-32985.
| Velocity Dispersion of the High Rotational Levels of H2
We present a study of the high rotational bands (J>=2) ofH2 toward four early-type Galactic stars: HD 73882, HD192639, HD 206267, and HD 207538. In each case, the velocity dispersion,characterized by the spectrum fitting parameter b, increases with thelevel of excitation, a phenomenon that has previously been detected bythe Copernicus and IMAPS observatories. In particular, we show with 4σ confidence that for HD 192639 it is not possible to fit all Jlevels with a single b-value and that higher b-values are needed for thehigher levels. The amplitude of the line broadening, which can be ashigh as 10 km s-1, makes explanations such as inhomogeneousspatial distribution unlikely. We investigate a mechanism in which thebroadening is due to the molecules that are rotationally excited throughthe excess energy acquired after their formation on a grain(H2 formation pumping). We show that different dispersionswould be a natural consequence of this mechanism. We note, however, thatsuch a process would require a formation rate 10 times higher than whatwas inferred from other observations. In view of this result, and of thedifficulty in accounting for the velocity dispersion as thermalbroadening (T would be around 10,000 K), we conclude then that we aremost certainly observing some highly turbulent warm layer associatedwith the cold diffuse cloud. Embedded in a magnetic field, it could beresponsible for the high quantities of CH+ measured in thecold neutral medium.
| FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far-ultravioletwavelengths (FUV; 1050-1200 Å) for nine Galactic sight lines. TheFUV extinction was measured using data from the Far UltravioletSpectroscopic Explorer. The sight lines were chosen for their unusualextinction properties in the infrared through the ultraviolet; that theyprobe a wide range of dust environments is evidenced by the large spreadin their measured ratios of total to selective extinction,RV=2.43-3.81. We find that extrapolation of the Fitzpatrick& Massa relationship from the ultraviolet appears to be a goodpredictor of the FUV extinction behavior. We find that predictions ofthe FUV extinction based on the Cardelli, Clayton, & Mathis (CCM)dependence on RV give mixed results. For the seven extinctioncurves well represented by CCM in the infrared through ultraviolet(x<8 μm-1), the FUV extinction is well predicted inthree sight lines, overpredicted in two sight lines, and underpredictedin two sight lines. A maximum entropy method analysis using a simplethree-component grain model shows that seven of the nine sight lines inthe study require a larger fraction of grain materials to be in dustwhen FUV extinction is included in the models. Most of the added grainmaterial is in the form of small (radii <~ 200 Å) grains.Based on observations with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| CHORIZOS: A χ2 Code for Parameterized Modeling and Characterization of Photometry and Spectrophotometry
We have developed CHi-square cOde for parameterRized modeling andcharacterIZation of phOtometry and Spectrophotmetry (CHORIZOS). CHORIZOScan use up to two intrinsic free parameters (e.g., temperature andgravity for stars, type and redshift for galaxies, or age andmetallicity for stellar clusters) and two extrinsic parameters (amountand type of extinction). The code uses χ2 minimization tofind all models compatible with the observed data in the modelN-dimensional (N=1, 2, 3, 4) parameter space. CHORIZOS can use eithercorrelated or uncorrelated colors as input and is specially designed toidentify possible parameter degeneracies and multiple solutions. Thecode is written in IDL and is available to the astronomical community.Here we present the techniques used, test the code, apply it to a fewwell-known astronomical problems, and suggest possible applications. Asa first scientific result from CHORIZOS, we confirm from photometry theneed for a revised temperature-spectral type scale for OB starspreviously derived from spectroscopy.
| Shocked Clouds in the Vela Supernova Remnant
Unusually strong high-excitation C I has been detected in 11 lines ofsight through the Vela supernova remnant (SNR) by means of UV absorptionline studies of IUE data. Most of these lines of sight lie near thewestern edge of the bright X-ray region of the SNR in a spatiallydistinct band approximately 1° by 4° oriented approximatelynorth-south. The high-excitation C I (denoted C I* and C I**) isinterpreted as evidence of a complex of shocked dense clouds interactingwith the SNR, because of the high pressures indicated in this region. Tofurther analyze the properties of this region of enhanced C I* and CI**, we present new HIRES-processed IRAS data of the entire Vela SNR. Atemperature map calculated from the HIRES IRAS data, based on atwo-component dust model, reveals the signature of hot dust at severallocations in the SNR. The hot dust is anticorrelated spatially withX-ray emission, as would be expected for a dusty medium interacting witha shock wave. The regions of hot dust are strongly correlated withoptical filaments, supporting a scenario of dense clouds interior to theSNR that have been shocked and are now cooling behind the supernovablast wave. With few exceptions, the lines of sight to the stronghigh-excitation C I pass through regions of hot dust and opticalfilaments. Possible mechanisms for the production of the unexpectedlylarge columns of high-excitation C I are discussed. Dense clouds on theback western hemisphere of the remnant may explain the relatively lowX-ray emission in the western portion of the Vela SNR due to the slowerforward shock velocity in regions where the shock has encountered thedense clouds. An alternate explanation for the presence of ground-stateand excited-state neutrals, as well as ionized species, along the sameline of sight is a magnetic precursor that heats and compresses the gasahead of the shock.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| A Compact Array imaging survey of southern bright-rimmed clouds
We have carried out a radio-wavelength imaging survey of 45bright-rimmed clouds (BRCs), using the Australia Telescope Compact Arrayto characterise the physical properties in their ionised boundarylayers. We detected radio emission from a total of 25 clouds and using acombination of Digitised Sky Survey and mid-infrared MSX 8 \mum imagesclassified the emission into that associated with the ionised cloudrims, that associated with embedded possible massive YSOs and thatunlikely to be associated with the clouds at all. A total of 18 cloudsdisplay radio emission clearly associated with the cloud rim and wedetermine the ionising photon flux illuminating these clouds and theelectron density and pressure of their ionised boundary layers. Using aglobal estimate for the interior molecular pressure of these clouds weshow that the majority are likely to be in pressure equilibrium andhence are currently being shocked by photoionisation-induced shocks. Weidentify those clouds where the predicted ionising photon flux isinconsistent with that derived from the observations and show thateither the spectral types of the stars illuminating the BRCs are earlierthan previously thought or that there must be additional ionisingsources within the HII regions. Finally, we identify the radio sourcesembedded within the clouds with infrared stellar clusters and show thatthey contain late O and early B-type stars, demonstrating that a numberof BRCs are intimately involved with high to intermediate-mass starformation.Full Figs. \ref{fig:images} and \ref{fig:sfo86dss} are only available inelectronic form at http://www.edpsciences.org
| New Elements for 80 Eclipsing Binaries
This research presents new elements for 80 eclipsing binaries found withthe help of the ASAS-3, Hipparcos and TASS databases.
| 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.
| 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.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| New infrared star clusters in the southern Milky Way with 2MASS
We carried out a 2MASS J, H and Ks survey of infrared starclusters in the Milky Way sector 230deg< l <350deg. This zone was the least studied in the literature,previously including only 12 infrared clusters or stellar groups with|b|< 10deg, according to the recent catalogue by Bica etal. (2003). We concentrated efforts on embedded clusters, which arethose expected in the areas of known radio and optical nebulae. Thepresent study provides 179 new infrared clusters and stellar groups,which are interesting targets for detailed future infrared studies. Thesample of catalogued infrared clusters and stellar groups in the Galaxyis now increased by 63%.
| Inferring Physical Conditions in Interstellar Clouds of H2
We have developed a code that models the formation, destruction,radiative transfer, and vibrational/rotational excitation ofH2 in a detailed fashion. We discuss generally how suchcodes, together with Far Ultraviolet Spectrographic Explorer (FUSE)observations of H2 in diffuse and translucent lines of sight,may be used to infer various physical parameters. We illustrate theeffects of changes in the major physical parameters (UV radiation field,gas density, metallicity), and we point out the extent to which changesin one parameter may be mirrored by changes in another. We provide ananalytic formula for the molecular fraction, fH2,as a function of cloud column density, radiation fields, and grainformation rate of H2. Some diffuse and translucent lines ofsight may be concatenations of multiple distinct clouds viewed together.Such situations can give rise to observables that agree with the data,complicating the problem of uniquely identifying one set of physicalparameters with a line of sight. Finally, we illustrate the applicationof our code to an ensemble of data, such as our FUSE survey ofH2 in the Large and Small Magellanic Clouds, in order toconstrain the elevated UV radiation field intensity and reduced grainformation rate of H2 in those low-metallicity environments.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Optical spectroscopy of X-Mega targets - IV. CPD - 59°2636: a new O-type multiple system in the Carina Nebula
High-resolution optical spectroscopy of CPD - 59°2636, one of theO-type stars in the open cluster Trumpler 16 in the Carina Nebula,reveals this object to be a multiple system displaying triple lineswhich we label as components A, B and C of spectral types O7 V, O8 V andO9 V, respectively. From our radial velocity measurements we find thatthe components A and B form a close binary with a period of 3.6284 d,and we obtain the first circular radial velocity orbit for this systemwith semi-amplitudes of 184 and 192 km s-1, leading tominimum masses of 10.1 and 9.7 Msolar. We find that thecomponent C is a single lined binary with a period of 5.034 d andsemi-amplitude of 48 km s-1. We also analyse the X-rayradiation from CPD - 59°2636, finding neither appreciableoverluminosity nor phase-related X-ray flux variations.
| 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.''
| Abundances and Physical Conditions in the Interstellar Gas toward HD 192639
We present a study of the abundances and physical conditions in theinterstellar gas toward the heavily reddened star HD 192639[E(B-V)=0.64], based on analysis of Far UltravioletSpectroscopic Explorer (FUSE) and Hubble Space Telescope/Space TelescopeImaging Spectrograph spectra covering the range from 912 to 1361Å. This work constitutes a survey of the analyses that can beperformed to study the interstellar gas by combining data from differentinstruments. Low-velocity (-18 to -8 km s-1) components areseen primarily for various neutral and singly ionized species such as CI, O I, S I, Mg II, Cl I, Cl II, Mn II, Fe II, and Cu II. Numerous linesof H2 are present in the FUSE spectra, with a kinetictemperature for the lowest rotational levels T01=90+/-10 K.Analysis of the C I fine-structure excitation implies an average localdensity of hydrogen nH=16+/-3 cm-3. The averageelectron density, derived from five neutral/first ion pairs under theassumption of photoionization equilibrium, is ne=0.11+/-0.02cm-3. The relatively complex component structure seen inhigh-resolution spectra of K I and Na I, the relatively low averagedensity, and the measured depletions all suggest that the line of sightcontains a number of diffuse clouds, rather than a single dense,translucent cloud. Comparisons of the fractions of Cl in Cl I and ofhydrogen in molecular form suggest a higher molecular fraction, in theregion(s) where H2 is present, than that derived consideringthe average line of sight. In general, such comparisons may allow theidentification and characterization of translucent portions of suchcomplex lines of sight. The combined data also show high-velocitycomponents near -80 km s-1 for various species that appear tobe predominantly ionized and may be due to a radiative shock. A briefoverview of the conditions in this gas will be given. Based onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal 8241.
| C18O abundance in the nearby globule Barnard 68
We have studied the radial variation of the CO abundance in the nearbyisolated globule Barnard 68 (B68). For this purpose, B68 was mapped inthe three rotational lines 13CO(J=1-0),C18O(J=1-0) and C18O(J=2-1). Using the recentdiscovery of Alves et al. (\cite{alves_01}) that the density structureof B68 agrees with the prediction for a pressure bound distribution ofisothermal gas in hydrostatic equilibrium (Bonnor-Ebert sphere), we showthat the flat CO column density distribution can be explained bymolecular depletion. By combining the physical model with the observedCO column density profile, it was found that the density dependence ofthe CO depletion factor fd can be well fitted with the lawfd=1+const. n(H2), which is consistent with anequilibrium between the accretion and the desorption processes. In thecloud centre, between 0.5% and 5% of all CO molecules are in the gasphase. Our observations suggest a kinetic temperature of ~8 K. Incombination with the assumption that B68 is a Bonnor-Ebert sphere, thisleads to a distance of 80 pc. The cloud mass consistent with thesevalues is 0.7 Msun, considerably less than previouslyestimated. We find in B68 no clear deviance of the near-infraredreddening efficiency of dust grains per unit H2 columndensity with respect to values derived in diffuse clouds. Based onobservations made with the Swedish-ESO Submillimetre Telescope (SEST),Chile.
| 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.
| Interstellar CN toward CH+-forming regions
Measurements on interstellar CN absorption are presented for stars inthree southern OB associations, NGC 2439, Vela OB1, and Cen OB1. CN isdetected in 21 out of 31 stars observed. The doublet ratio for the R(1)and P(1) lines of the (0, 0) band of the B 2Sigma+-X 2Sigma + violet system and acomparison of violet system data with measurements of the (1, 0) and (2,0) bands of the A 2Pi -X 2Sigma + redsystem are used to derive Doppler parameters and total column densities.Inferred CN column densities vary by more than an order of magnitude forlines of sight with similar CH column densities. Observations of the (0,0) band of the CH B 2Sigma --X 2Pisystem are used to revise previously published CH column densitiestoward the lines of sight studied in CN. Together with earlier resultson CH, CH+, and C2, the CN data presented hereprovide a homogeneous set of column densities and radial velocities ofdiatomic molecules in three individual translucent clouds. We use thesedata to study CN production via chemical models. Gas densities areinferred from models based on production via CH and C2 incool gas. Most sightlines in our sample test densities typical fordiffuse molecular gas (a few hundred cm-3 ) when theultraviolet flux permeating the gas is between 1 and 5 times the averageinterstellar flux. A few lines of sight indicate that CN is producedunder dark cloud conditions because relatively large densities areobtained or because this simple chemical scheme is unable to reproducethe observed CN columns. Low densities are indicated for directions withupper limits on CN. We add an ad hoc component of a number oflow-velocity (<10 km s-1) criss-crossing MHD shocks toexplain observed column densities of interstellar CH+. Theseshocks also produce about 10 to 30% of the total CH column along theline of sight.
| Photon-dominated Regions in Low-Ultraviolet Fields: A Study of the Peripheral Region of L1204/S140
We have carried out an in-depth study of the peripheral region of themolecular cloud L1204/S140, where the far-ultraviolet radiation and thedensity are relatively low. Our observations test theories ofphoton-dominated regions (PDRs) in a regime that has been littleexplored. Knowledge of such regions will also help to test theories ofphotoionization-regulated star formation. [C II] 158 μm and [O I] 63μm lines are detected by the Infrared Space Observatory at all 16positions along a one-dimensional cut in right ascension. Emission fromH2 rotational transitions J=2-->0 andJ=3-->1 at 28 and 17 μm, respectively, was also detected atseveral positions. The [C II], [O I], and H2intensities along the cut show much less spatial variation than do therotational lines of 12CO and other CO isotopes. The average[C II] and [O I] intensities and their ratio are consistent with modelsof PDRs with low values of far-ultraviolet radiation (G0) anddensity. The best-fitting model has G0~15 and densityn~103 cm-3. Standard PDR models underpredict theintensity in the H2 rotational lines by up to anorder of magnitude. This problem has also been seen in bright PDRs andattributed to factors, such as geometry and gas-grain drift, that shouldbe much less important in the regime studied here. The fact that we seethe same problem in our data suggests that more fundamental solutions,such as higher H2 formation rates, are needed.Also, in this regime of low density and small line width, the [O I] lineis sensitive to the radiative transfer and geometry. Using theionization structure of the models, a quantitative analysis oftimescales for ambipolar diffusion in the peripheral regions of the S140cloud is consistent with a theory of photoionization-regulated starformation. Observations of [C II] in other galaxies differ from boththose of high-G0 PDRs in our galaxy and the low-G0regions we have studied. The extragalactic results are not easilyreproduced with mixtures of high- and low-G0 regions.
| H2 and CO Emission from Disks around T Tauri and Herbig Ae Pre-Main-Sequence Stars and from Debris Disks around Young Stars: Warm and Cold Circumstellar Gas
We present ISO Short-Wavelength Spectrometer observations ofH2 pure-rotational line emission from the disks around low-and intermediate-mass pre-main-sequence stars as well as from youngstars thought to be surrounded by debris disks. The pre-main-sequencesources have been selected to be isolated from molecular clouds and tohave circumstellar disks revealed by millimeter interferometry. Wedetect ``warm'' (T~100-200 K) H2 gas around many sources,including tentatively the debris-disk objects. The mass of this warm gasranges from ~10-4 Msolar up to8×10-3 Msolar and can constitute anonnegligible fraction of the total disk mass. Complementary single-dish12CO 3-2, 13CO 3-2, and 12CO 6-5observations have been obtained as well. These transitions probe coolergas at T~20-80 K. Most objects show a double-peaked CO emission profilecharacteristic of a disk in Keplerian rotation, consistent withinterferometer data on the lower J lines. The ratios of the12CO 3-2/13CO 3-2 integrated fluxes indicate that12CO 3-2 is optically thick but that 13CO 3-2 isoptically thin or at most moderately thick. The 13CO 3-2lines have been used to estimate the cold gas mass. If aH2/CO conversion factor of 1×104 is adopted,the derived cold gas masses are factors of 10-200 lower than thosededuced from 1.3 millimeter dust emission assuming a gas/dust ratio of100, in accordance with previous studies. These findings confirm that COis not a good tracer of the total gas content in disks since it can bephotodissociated in the outer layers and frozen onto grains in the colddense part of disks, but that it is a robust tracer of the disk velocityfield. In contrast, H2 can shield itself fromphotodissociation even in low-mass ``optically thin'' debris disks andcan therefore survive longer. The warm gas is typically 1%-10% of thetotal mass deduced from millimeter continuum emission, but it canincrease up to 100% or more for the debris-disk objects. Thus, residualmolecular gas may persist into the debris-disk phase. No significantevolution in the H2, CO, or dust masses is found for starswith ages in the range of 106-107 yr, although adecrease is found for the older debris-disk star β Pictoris. Thelarge amount of warm gas derived from H2 raises the questionof the heating mechanism(s). Radiation from the central star as well asthe general interstellar radiation field heat an extended surface layerof the disk, but existing models fail to explain the amount of warm gasquantitatively. The existence of a gap in the disk can increase the areaof material influenced by radiation. Prospects for future observationswith ground- and space-borne observations are discussed. Based in parton observations with ISO, an ESA project with instruments funded by ESAmember states (especially the PI countries: France, Germany,Netherlands, and the United Kingdom) and with participation of ISAS andNASA.
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