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[Si II], [Fe II], [C II], and H2 Emission from Massive Star-forming Regions
We calculate the [Si II] 34.8 μm, [Fe II] 26.0 μm, and [C II] 158μm infrared fine-structure emission that may arise from ionized gas(H II regions) and/or associated neutral gas (photodissociation regions[PDRs]) in massive star-forming regions. Assuming thermal pressurebalance between an H II region and a PDR, the relative amounts offine-structure line emission from the H II region and PDR depend on theelectron density and resulting thermal pressure in the H II region,gas-phase abundances of the emitting species, and the UV spectrum fromthe stellar population producing the H II region. For normalmetallicity, we find that [C II] emission is always dominated by PDRs,while [Si II] and [Fe II] are dominated by PDRs for H II regions withelectron density ne>~10 cm-3. We also calculatethe H2 0-0 S(0), 0-0 S(1), 0-0 S(2), and 0-0 S(3) purerotational line emission arising from the PDR at the atomic-to-molecularinterface. The overall intensity of the H2 line emissiondirectly traces warm molecular mass, while H2 line ratiosconstrain the PDR temperature, gas density, and far-ultraviolet fieldstrength. Models of the integrated emission of [Si II], [Fe II], [C II],and H2 from Galactic and extragalactic star-forming regionsare presented for use in interpreting observations with Spitzer, ISO,SOFIA, and the Herschel Space Observatory. We compare our results withobservations of the Galactic source NGC 2023 (an individual HII/photodissociation region in Orion), the inner regions of the MilkyWay, and the central regions of the nearby star-forming spiral galaxyNGC 7331. We also compare our results with recently published similarwork by Abel and coworkers.
| 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.
| Large Area Mapping at 850 μm. IV. Analysis of the Clump Distribution in the Orion B South Molecular Cloud
We present results from a survey of a 1300 arcmin2 region ofthe Orion B South molecular cloud, including NGC 2024, NGC 2023, and theHorsehead Nebula (B33), obtained using the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).Submillimeter continuum observations at 450 and 850 μm are discussed.Using an automated algorithm, 57 discrete emission features (``clumps'')are identified in the 850 μm map. The physical conditions withinthese clumps are investigated under the assumption that the objects arein quasi-hydrostatic equilibrium. The best-fit dust temperature for theclumps is found to be Td=18+/-4 K, with the exception ofthose associated with the few known far-infrared sources residing in NGC2024. The latter internally heated sources are found to be much warmer.In the region surrounding NGC 2023, the clump dust temperatures agreewith clump gas temperatures determined from molecular line excitationmeasurements of the CO molecule. The bounding pressure on the clumpslies in the range log(k-1P cm3K-1)=6.1+/-0.3. The cumulative mass distribution is steep atthe high-mass end, as is the stellar initial mass function. Thedistribution flattens significantly at lower masses, with a turnoveraround 3-10 Msolar.
| Submillimeter imaging spectroscopy of the Horsehead nebula
We present 15 arcsecond resolution single-dish imaging of the Horseheadnebula in the Ci (1-0) and CO (4-3) lines, carried out using the CHAMParray at the Caltech Submillimeter Observatory (CSO). The data are usedtogether with supporting observations of the (2-1) transitions of the COisotopologues to determine the physical conditions in the atomic andmolecular gas via Photon Dominated Region (PDR) modeling. The CO(4-3)/(2-1) line ratio, which is an excellent tracer of the direction ofthe incoming UV photons, increases at the western and northern edges ofthe nebula, confirming that the illumination is provided mostly by thestars σ and ζ Orionis. The observed line intensities areconsistent with PDR models with an H nuclei volume density of 3{-} 7 × 104 cm-3. The models predict akinetic temperature of 12 K and a C18O fractional abundancewith respect to H atoms of 2.4 × 10-7 in the shieldedregion, which in turn imply a total molecular mass of 24 Mȯ in theC18O filament. The outer halo, devoid of C18O, buttraced by the Ci emission has a comparable density and contributesadditional 13 Mȯ of material, resulting in an upper limit of 37Mȯ for the total molecular mass of the nebula.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Blue Luminescence and the Presence of Small Polycyclic Aromatic Hydrocarbons in the Interstellar Medium
Blue luminescence (BL) was first discovered in a proto-planetary nebula,the Red Rectangle (RR), surrounding the post-AGB star HD 44179. BL hasbeen attributed to fluorescence by small, 3-4 ringed neutral polycyclicaromatic hydrocarbon (PAH) molecules and was thought to be unique to theRR environment, where such small molecules are actively being producedand shielded from the harsh interstellar radiation by a densecircumstellar disk. In this paper we present the BL spectrum detected inseveral ordinary reflection nebulae illuminated by stars havingtemperatures between 10,000 and 23,000 K. All these nebulae are known toalso exhibit the infrared emission features called aromatic emissionfeatures (AEFs) attributed to large PAHs. We present the spatialdistribution of the BL in these nebulae. In the case of Ced 112, the BLis spatially correlated with mid-IR emission structures attributed toAEFs. These observations provide evidence for grain processing andpossibly for in situ formation of small grains and large molecules fromlarger aggregates. Most importantly, the detection of BL in theseordinary reflection nebulae suggests that the BL carrier is anubiquitous component of the interstellar medium and is not restricted tothe particular environment of the RR.
| 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.
| 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.
| An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
In this paper we present a new method for deriving UV through IRextinction curves, based on the use of stellar atmosphere models toprovide estimates of the intrinsic (i.e., unreddened) stellar spectralenergy distributions (SEDs), rather than unreddened (or lightlyreddened) standard stars. We show that this ``extinction withoutstandards'' technique greatly increases the accuracy of the derivedextinction curves and allows realistic estimations of the uncertainties.An additional benefit of the technique is that it simultaneouslydetermines the fundamental properties of the reddened stars themselves,making the procedure valuable for both stellar and interstellar studies.Given the physical limitations of the models we currently employ, thetechnique is limited to main-sequence and mildly evolved B stars.However, in principle, it can be adapted to any class of star for whichaccurate model SEDs are available and for which the signatures ofinterstellar reddening can be distinguished from those of the stellarparameters. We demonstrate how the extinction without standards curvesmake it possible to (1) study the uniformity of curves in localizedspatial regions with unprecedented precision, (2) determine therelationships between different aspects of curve morphology, (3) producehigh-quality extinction curves from low color excess sight lines, and(4) derive reliable extinction curves for mid to late B stars, therebyincreasing spatial coverage and allowing the study of extinction in openclusters and associations dominated by such stars. The application ofthis technique to the available database of UV through IR SEDs, and tofuture observations, will provide valuable constraints on the nature ofinterstellar grains and on the processes that modify them, and it willenhance our ability to remove the multiwavelength effects of extinctionfrom astronomical energy distributions.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| A far UV study of interstellar gas towards HD 34078: High excitation H2 and small scale structure
To investigate the presence of small scale structure in the spatialdistribution of H2 molecules we have undertaken repeated FUSEUV observations of the runaway O9.5V star, HD 34078. In this paper wepresent five spectra obtained between January 2000 and October 2002.These observations reveal an unexpectedly large amount of highly excitedH2>. Column densities for H2 levels from (v =0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levelsare determined. These results are interpreted in the frame of a modelinvolving essentially two components: i) a foreground cloud (unaffectedby HD 34078) responsible for the H2 (J = 0, 1), CI, CH,CH+ and CO absorptions; ii) a dense layer of gas (n ≃104 cm-3) close to the O star and stronglyilluminated by its UV flux which accounts for the presence of highlyexcited H2. Our model successfully reproduces theH2 excitation, the CI fine-structure level populations aswell as the CH, CH+ and CO column densities. We also examinethe time variability of H2 absorption lines tracing each ofthese two components. From the stability of the J = 0, 1 and 2 dampedH2 profiles we infer a 3σ upper limit on column densityvariations Δ N(H2)/N(H2) of 5% over scalesranging from 5 to 50 AU. This result clearly rules out any pronouncedubiquitous small scale density structure of the kind apparently seen inHI. The lines from highly excited gas are also quite stable (equivalentto Δ N/N ≤30%) indicating i) that the ambient gas throughwhich HD 34078 is moving is relatively uniform and ii) that the gas flowalong the shocked layer is not subject to marked instabilities.Based on observations performed by the FUSE mission and at the CFHTtelescope.
| Investigation of the UV Radiation Process in NGC 2023
We observed the H2 absorption lines in the far-UV band towardHD 37903, which is the central star of the reflection nebula NGC 2023,and ran the radiative-collisional equilibrium program CLOUDY to simulatethe UV radiation process of NGC 2023. The input parameters are chosen togenerate the same ionization and excitation distribution of atoms andmolecules as observed, so that we can create a physical model for NGC2023.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Ion Chemistry in Photon-dominated Regions: Examining the [HCO+]/[HOC+]/[CO+] Chemical Network
HOC+ and CO+ have been detected toward twowell-known photon-dominated regions (PDRs), S140 and NGC 2023, using theArizona Radio Observatory 12 m telescope. The J=1-->0 transition ofHOC+ at 89 GHz and the two spin components of the N=2-->1line of CO+ near 236 GHz were observed, as well as theJ=1-->0 transitions of H13CO+ andHC18O+. The J=3-->2 line of HOC+ at268 GHz was also mapped across the Orion Bar. The[HCO+]/[HOC+] ratios determined in S140 and NGC2023 were ~12,408 and 1913, respectively, values indicative of quiescentmolecular gas rather than PDR sources, where the abundance ofHOC+ is thought to be enhanced. However, the beam in boththese measurements may contain material from the adjoining molecularcloud, favoring HCO+. Alternatively, the[HCO+]/[HOC+] ratio may vary with Av inPDR regions. The [CO+]/[HOC+] ratio in S140 andNGC 2023, at several positions in the Orion Bar, and in other PDRs, onthe other hand, falls uniformly in the range ~1-10. In addition, theline profiles of CO+ and HOC+ in the Orion Barlook remarkably similar. The syntheses of HOC+ andCO+ appear to be correlated in PDRs, most likely through thecommon precursor, C+. The reaction of C+ andH2O is thought to preferentially create HOC+, asopposed to HCO+, and C++OH or O2 leadsfavorably to CO+.
| 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.
| Radio Continuum Jets from Protostellar Objects
We have carried out a deep, 3.6 cm radio continuum survey of youngoutflow sources using the Very Large Array in its A configurationproviding subarcsecond resolution. The eight regions observed are Haro6-10 and L1527 IRS in Taurus, Haro 5a/6a in OMC 2/3, NGC 2023 MMS, NGC2264 IRS1, HH 108 IRAS/MMS in Serpens, L1228, and L1251A. In combinationwith our similar and previously published maps of eight otherstar-forming regions, we find only one region with a single source,while the other 15 regions have on average 3.9 nearby sources. Thissupports the view that isolated star formation is rare. We have selected21 objects, which are all young mostly Class I sources, and find abinary frequency of 33% in the separation range from 0.5" to 12". Thisis within the uncertainties comparable to the observed binary frequencyamong T Tauri stars in a similar separation range. Seven of the 21sources drive giant Herbig-Haro flows. Four of these seven are known tohave companions (three are triple systems), corresponding to 57%. Wediscuss these results in relation to the hypothesis that giantHerbig-Haro flows are driven by disintegrating multiple systems.
| 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.
| 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.
| The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction. I. Probing Small Molecular Polycyclic Aromatic Hydrocarbons
We have obtained new Hubble Space Telescope/Space Telescope ImagingSpectrograph spectra to search for structure in the ultravioletinterstellar extinction curve, with particular emphasis on a search forabsorption features produced by polycyclic aromatic hydrocarbons (PAHs).The presence of these molecules in the interstellar medium has beenpostulated to explain the infrared emission features seen in the 3-13μm spectra of numerous sources. Ultraviolet (UV) spectra are uniquelycapable of identifying specific PAH molecules. We obtained highsignal-to-noise ratio UV spectra of stars that are significantly morereddened than those observed in previous studies. These data put limitson the role of small (30-50 carbon atoms) PAHs in UV extinction and callfor further observations to probe the role of larger PAHs. PAHs are ofimportance because of their ubiquity and high abundance inferred fromthe infrared data, and also because they may link the molecular and dustphases of the interstellar medium. A presence or absence of UVabsorption bands due to PAHs could be a definitive test of thishypothesis. We should be able to detect a 20 Å wide feature downto a 3 σ limit of ~0.02 AV. No such absorption featuresare seen other than the well-known 2175 Å bump.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and13CO was detected toward ρ Oph A and χ Oph. Themeasurements were obtained at medium resolution with the Goddard HighResolution Spectrograph on the Hubble Space Telescope. Column densityratios, N(12CO)/N(13CO), of 125+/-23 and 117+/-35were derived for the sight lines toward ρ Oph A and χ Oph,respectively. A value of 1100+/-600 for the ratioN(12C16O)/N(12C18O) towardρ Oph A was also obtained. Absorption from vibrationally excitedH2 (v''=3) was clearly seen toward this star aswell. The ratios are larger than the isotopic ratios for carbon andoxygen appropriate for ambient interstellar material. Since for bothcarbon and oxygen the more abundant isotopomer is enhanced, selectiveisotopic photodissociation plays the key role in the fractionationprocess for these directions. The enhancement arises because the moreabundant isotopomer has lines that are more optically thick, resultingin more self-shielding from dissociating radiation. A simple argumentinvolving the amount of self-shielding [from N(12CO)] and thestrength of the ultraviolet radiation field permeating the gas (from theamount of vibrationally excited H2) shows that selectiveisotopic photodissociation controls the fractionation seen in these twosight lines, as well as the sight line to ζ Oph.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| 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.
| 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.
| 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.
| 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.
| Optical spectroscopic and 2MASS measurements of Stephenson Halpha stars
We present the results of spectroscopic observations for 52 objects fromthe list of Halpha emission stars of Stephenson(\cite{Ste86}). Out of six known T Tauri stars observed, five showedHalpha in emission and in one (StHa 40), Halpha changed from being in absorption to emission over a period of two years,accompanied by photometric and spectral type variability. We confirm theT Tauri nature of one Stephenson object (StHa 48) on the basis of thepresence of Halpha and Hbeta in emission, Li Ilambda6708 in absorption, infrared excess and X-ray emission. Among the52 objects observed, there were other emission line objects: 1 Ke star,1 BQ[ ] star, 2 galaxies and 2 Be stars. We present a higher-resolutionspectrum of StHa 62 showing permitted and forbidden lines in emissiontypical of BQ[ ] stars. Twenty five out of 30 newly observed objectsfailed to show Halpha in emission. We also present 2MASSobservations for 112 StHa objects. We suggest three Stephenson objects(StHa 52, 125 and 129) to be YSOs on the basis of 2MASS, IRAS and ROSATobservations. These and all other known YSOs amongst StHa stars arefound in regions of star-forming clouds in Taurus, Orion and Ophiuchus.YSOs at high galactic latitudes in other parts of the sky are thereforerare.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/963
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| The relation between far-UV and visible extinctions
For directions of sufficient reddening (/E(B-V)>~0.25), there is asimple relation between the slope of the extinction curve in the far-UVand /E(B-V). Regardless of direction, the far-UV extinction curve isproportional to 1/λn e-2E(B-V)/λ(/λ in μm, /n=4), in accordance with the idea that reddenedstars spectra are contaminated by scattered light (Zagury, 2001b). Thisrelation is not compatible with the standard theory of extinction whichstates that far-UV and visible extinctions are due to different classesof particle. In that model the two (far-UV and visible) extinctions varythus independently according to the proportion of each type of particle.In preceding papers I have shown that the standard theory cannot explainUV observations of nebulae, and is contradicted by the UV spectra ofstars with very low reddening: for how long shall the standard theory beconsidered as the interpretation of the extinction curve?
| The Distribution of Atomic Carbon in the Orion Giant Molecular Cloud 1
The large-scale distribution of the C I(3P1-3P0, 492 GHz) emissionline from Orion A and B giant molecular clouds has been imaged with theMount Fuji submillimeter-wave telescope. The total area observed isabout 15 deg2. The overall spatial and velocity structure ofthe C I line is found to be similar to that of the 13CO(J=1-0) line. The derived column density ratio N(C I)/N(CO) shows alarge variation ranging from 0.2 to 2.9 toward the cloud edges, whereasit is relatively constant between 0.1 and 0.2 toward the interior regionof the entire Orion clouds in both massive star-forming and dark cloudregions. This almost constant ratio suggests that C I can coexist withCO even in the deep inside of the molecular cloud.
| 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.''
| Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023
The reflection nebula NGC 2023 was observed by a rocket-borne long-slitimaging spectrograph in the 900-1400 Å bandpass on 2000 February11. A spectrum of the star, as well as that of the nebular scatteredlight, was recorded. Through the use of a Monte Carlo modeling process,the scattering properties of the dust were derived. The albedo is low,0.2-0.4, and decreasing toward shorter wavelengths, while the phasefunction asymmetry parameter is consistent with highlyforward-scattering grains, g~0.85. The decrease in albedo, while theoptical depth increases to shorter wavelengths, implies that the far-UVrise in the extinction curve is due to an increase in absorptionefficiency.
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