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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.
| 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.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Hyperfine structure and interstellar curves of growth
The effects of the hyperfine structure (HFS) that is present in someinterstellar absorption lines are investigated in the case of a singleabsorbing cloud. If the respective total equivalent widths of two ormore unresolved HFS multiplets measured in relatively low-resolutionspectra are analysed specifically by means of a curve of growth, thecolumn density N(X) and the linewidth parameter b(X) inferred forabsorbing species X in the cloud will generally be in error if the HFSis ignored. The fundamental physical effect is the reduced linesaturation that arises because the total column density is divided amongthe HFS levels of the ground atomic level, each of which generally givesrise to an HFS line at a different wavelength. For nuclear spins I= 3/2and I= 5/2, theoretical curves of growth are calculated for some of theresonance lines of some alkali atoms, for each of four illustrativechoices of the parameter α=Δ/b, the ratio of the HFSsplitting in the ground atomic level to the linewidth. Applications ofthe results to interstellar absorption by NaI, KI and AlIII areemphasized. HFS is, fortunately, unimportant for most interstellarlines, however. Among the 35 elements that have been detected in diffuseclouds via interstellar absorption in the ultraviolet/optical spectralregion, the most abundant isotope of each of 25 of these shows no HFS,because I= 0 or, in the relevant ground atomic level, J= 0.
| 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).
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| An empirical temperature calibration for the Δ a photometric system . I. The B-type stars
We establish an empirical effective temperature calibration of mainsequence, luminosity class V to III B-type stars for the Δ aphotometric system which was originally developed to detect magneticchemically peculiar objects of the upper main sequence (early B-type toearly F-type) at 5200 Å. However, this system provides the index(g_1-y) which shows an excellent correlation with (B-V) as well as (b-y)and can be used as an indicator of the effective temperature. This issupplemented by a very accurate color-magnitude diagram, y or V versus(g_1-y), which can be used, for example, to determine the reddening,distance and age of an open cluster. This makes the Δ aphotometric system an excellent tool to investigate theHertzsprung-Russell-Diagram (HRD) in more detail. Using thereddening-free parameters and already established calibrations withinthe Strömgren uvbyβ, Geneva 7-color and Johnson UBV systems, apolynomial fit of third degree for the averaged effective temperaturesto the individual (g_1-y)0 values was derived. For thispurpose, data from the literature as well as new observations were takenresulting in 225 suitable bright normal B-type objects. The statisticalmean of the error for this sample is 238 K which is sufficient toinvestigate the HRD of distant galactic open clusters as well asextragalactic aggregates in the future.
| Dust Dynamics in Compressible Magnetohydrodynamic Turbulence
We calculate the relative grain-grain motions arising from interstellarmagnetohydrodynamic (MHD) turbulence. The MHD turbulence includes bothfluid motions and magnetic fluctuations. While the fluid motionsaccelerate grains through hydrodrag, the electromagnetic fluctuationsaccelerate grains through resonant interactions. We consider bothincompressive (Alfvén) and compressive (fast and slow) MHD modesand use descriptions of MHD turbulence obtained by Cho and Lazarian in2002. Calculations of grain relative motion are made for realistic graincharging and interstellar turbulence that are consistent with thevelocity dispersions observed in diffuse gas, including cutoff of theturbulence from various damping processes. We show that fast modesdominate grain acceleration and can drive grains to supersonicvelocities. Grains are also scattered by gyroresonance interactions, butthe scattering is less important than acceleration for grains movingwith sub-Alfvénic velocities. Since the grains are preferentiallyaccelerated with large pitch angles, the supersonic grains will bealigned with long axes perpendicular to the magnetic field. We comparegrain velocities arising from MHD turbulence with those arising fromphotoelectric emission, radiation pressure, and H2 thrust. Weshow that for typical interstellar conditions, turbulence should preventthese mechanisms from segregating small and large grains. Finally,gyroresonant acceleration is bound to preaccelerate grains that arefurther accelerated in shocks. Grain-grain collisions in the shock maythen contribute to the overabundance of refractory elements in thecomposition of Galactic cosmic rays.
| 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.
| 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.
| 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.
| Transitions from Autoionized Single-ionized Tin States: A Theoretical Study of the 5s5p (3Po) nl (nl = 5d, 6s) Levels of Sn II
Lines corresponding to several transitions from autoionized states ofsingle-ionized tin were identified in a laser-produced plasma generatedby 10640 Å irradiation of an Sn target at a flux of2×1010 W cm-2. Spectra were recorded andanalyzed between 2000 and 7000 Å. Theoretical analysis of Sn IIwas extended using relativistic Hartree-Fock calculations andconfiguration interactions in an intermediate-coupling scheme with thesupport of the Cowan code. Our calculations support the experimentalvalue obtained by Schectman et al. in 2000 of the lifetime of5s25d2D3/2 and the absorptionoscillator strength of the resonance transition to this level at 1400.45Å. The parametric description of 5s5p (3Po)nl (nl=5d, 6s) of Sn II levels is improved by taking into account thefar 5p3 configuration mixing effects. The results obtained inthis study will allow a substantial improvement in the interpretation ofthe data of the ultraviolet spectrum of Sn II observed by the GoddardHigh Resolution Spectrograph aboard the Hubble Space Telescope.
| Observations of Rotationally Resolved C3 in Translucent Sight Lines
The rotationally resolved spectrum of theA1Πu<--X1Σ+g000-000 transition of C3, centered at 4051.6 Å, hasbeen observed along 10 translucent lines of sight. To interpret thesespectra, a new method for the determination of column densities andanalysis of excitation profiles involving the simulation and fitting ofobserved spectra has been developed. The populations of lower rotationallevels (J<=14) in C3 are best fitted by thermaldistributions that are consistent with the kinetic temperaturesdetermined from the excitation profile of C2. Just as in thecase of C2, higher rotational levels (J>14) ofC3 show increased nonthermal population distributions inclouds that have been determined to have total gas densities below ~500cm-3.
| The Chemical Composition and Gas-to-Dust Mass Ratio of Nearby Interstellar Matter
We use recent results on interstellar gas toward nearby stars andinterstellar by-products within the solar system to select among theequilibrium radiative transfer models of the nearest interstellarmaterial presented in Slavin & Frisch. For the assumption thatO/H~400 parts per million, models 2 and 8 are found to yield good fitsto available data on interstellar material inside and outside of theheliosphere, with the exception of the Ne abundance in the pickup ionand anomalous cosmic-ray populations. For these models, the interstellarmedium (ISM) at the entry point to the heliosphere hasn(H0)=0.202-0.208 cm-3,n(He0)=0.0137-0.0152 cm-3, and ionizationsχ(H)=0.29-0.30, χ(He)=0.47-0.51. These best models suggest thatthe chemical composition of the nearby ISM is ~60%-70% subsolar if S isundepleted. Both H0 and H+ need to be includedwhen evaluating abundances of ions found in warm diffuse clouds. Models2 and 8 yield an H filtration factor of ~0.46. Gas-to-dust mass ratiosfor the ISM toward ɛ CMa are Rgd=178-183 for solarabundances of Holweger or Rgd=611-657 for an interstellarabundance standard 70% solar. Direct observations of dust grains in thesolar system by Ulysses and Galileo yield Rgd~=115 for models2 and 8, supporting earlier results (Frisch and coworkers). If the localISM abundances are subsolar, then gas and dust are decoupled over smallspatial scales. The inferred variation in Rgd over parseclength scales is consistent with the fact that the ISM near the Sun ispart of a dynamically active cluster of cloudlets flowing away from theSco-Cen association. Observations toward stars within ~500 pc show thatRgd correlates with the percentage of the dust mass that iscarried by iron, suggesting that an Fe-rich grain core (by mass) remainsafter grain destruction. Evidently large dust grains (>10-13g) and small dust grains (<10-13 g) are not well mixedover parsec length spatial scales in the ISM. It also appears that verysmall C-dominated dust grains have been destroyed in the ISM withinseveral parsecs of the Sun, since C appears to be essentiallyundepleted. However, if gas-dust coupling breaks down over the cloudlifetime, the missing mass arguments applied here to determineRgd and dust grain mineralogy are not appropriate.
| Nonthermal Chemistry in Diffuse Clouds with Low Molecular Abundances
High-quality archival spectra of interstellar absorption from C I towardnine stars, taken with the Goddard High Resolution Spectrograph on theHubble Space Telescope, were analyzed. Our sample was supplemented bytwo sight lines, 23 Ori and β1 Sco, for which the C Imeasurements of Federman, Welty, & Cardelli were used. Directionswith known CH+ absorption, but only upper limits onabsorption from C2 and CN, were considered for our study.This restriction allows us to focus on regions where CH+chemistry dominates the production of carbon-bearing molecules. Profilesynthesis of several multiplets yielded column densities and Dopplerparameters for the C I fine-structure levels. Equilibrium excitationanalyses, using the measured column densities as well as the temperaturefrom H2 excitation, led to values for gas density. Thesedensities, in conjunction with measurements of CH, CH+,C2, and CN column densities, provided estimates for theamount of CH associated with CH+ production, which in turnset up constraints on the present theories for CH+ formationin this environment. We found for our sample of interstellar clouds thaton average 30%-40% of the CH originates from CH+ chemistry,and in some cases it can be as high as 90%. A simple chemical model forgas containing nonequilibrium production of CH+ was developedfor the purpose of predicting column densities for CH, CO,HCO+, CH+2, andCH+3 generated from large abundances ofCH+. Again, our results suggest that nonthermal chemistry isnecessary to account for the observed abundance of CH and probably thatof CO in these clouds.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 NAS 5-26555.
| 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.
| Interstellar Silicon Abundance
We present 34 measurements of silicon gas phase column densities in theinterstellar medium. We have used spectra containing the SiII 1808 Angline which were obtained with the Goddard High Resolution Spectrograph(GHRS) aboard the Hubble Space Telescope (HST). Extinction curveparameters are determined for analyzed lines of sight and relationbetween Si/H ratio and extinction parameters is discussed. We find theabundance of gas phase silicon in diffuse clouds to be lower than thesolar value by a factor of four.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are 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/411/447
| Intermediate- and High-Velocity Ionized Gas toward ζ Orionis
We combine near-UV spectra obtained with the Hubble Space Telescope GHRSechelle with far-UV spectra obtained with the Interstellar MediumAbsorption Profile Spectrograph and Copernicus to study the abundancesand physical conditions in the predominantly ionized gas seen at highvelocity (-105 km s-1<~vsolar<~-65 kms-1) and at intermediate velocity (-60 kms-1<~vsolar<~-10 km s-1) alongthe line of sight to the star ζ Ori. We have high-resolution(FWHM~3.3-4.5 km s-1) and/or high signal-to-noise ratiospectra for at least two significant ions of C, N, Al, Si, S, andFe-enabling accurate estimates for both the total N(H II) and theelemental depletions. C, N, and S have essentially solar relativeabundances; Al, Si, and Fe appear to be depleted by about 0.8, 0.3-0.4,and 0.95 dex, respectively, relative to C, N, and S. While various ionratios would be consistent with collisional ionization equilibrium (CIE)at temperatures of 25,000-80,000 K, the widths of individualhigh-velocity absorption components indicate that T~9000+/-2000 K-so thegas is not in CIE. Analysis of the C II fine-structure excitationequilibrium, at that temperature, yields estimates for the densities(ne~nH~0.1-0.2 cm-3), thermal pressures(2nHT~2000-4000 cm-3 K), and thicknesses (0.5-2.7pc) characterizing the individual clouds. We compare the abundances andphysical properties derived for these clouds with those found for gas atsimilar velocities toward 23 Ori and τ CMa and also with severaldifferent models for shocked gas. While the shock models can reproducesome features of the observed line profiles and some of the observed ionratios, there are also significant differences between the models andthe data. The measured depletions suggest that roughly 10% of the Al,Si, and Fe originally locked in dust in the preshock medium may havebeen returned to the gas phase, consistent with recent predictions forthe destruction of silicate dust in a 100 km s-1 shock. Theobserved near-solar gas-phase abundance of carbon, however, appears tobe inconsistent with the predicted longer timescales for the destructionof graphite grains. Based in part on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| The Heavy-Element Enrichment of Lyα Clouds in the Virgo Supercluster
Using high signal-to-noise ratio echelle spectra of 3C 273 obtained withthe Space Telescope Imaging Spectrograph (resolution of 7 kms-1 FWHM), we constrain the metallicities of two Lyαclouds in the vicinity of the Virgo Cluster. We detect C II, Si II, andSi III absorption lines in the Lyα absorber atzabs=0.00530. Previous observations with the Far UltravioletSpectroscopic Explorer have revealed Lyβ-Lyθ absorptionlines at the same redshift, thereby accurately constraining the H Icolumn density. We model the ionization of the gas and derive[C/H]=-1.2+0.3-0.2, [Si/C]=0.2+/-0.1, andlognH=-2.8+/-0.3. The model implies a small absorberthickness, ~70 pc, and thermal pressure p/k~40 cm-3 K. It ismost likely that the absorber is pressure confined by an external mediumbecause gravitational confinement would require a very high ratio ofdark matter to baryonic matter. Based on a sample of Milky Way sightlines in which carbon and silicon abundances have been reliably measuredin the same interstellar cloud (including new measurements presentedherein), we argue that it is unlikely that the overabundance of Sirelative to C is due to depletion onto dust grains. Instead, thisprobably indicates that the gas has been predominately enriched byejecta from Type II supernovae. Such enrichment is most plausiblyprovided by an unbound galactic wind, given the absence of knowngalaxies within a projected distance of 100 kpc and the presence ofgalaxies capable of driving a wind at larger distances (e.g., H I1225+01). Such processes have been invoked to explain the observedabundances in the hot, X-ray-emitting gas in Virgo. However, the sightline to 3C 273 is more than 10° away from the X-ray emission region.We also constrain the metallicity and physical conditions of the Virgoabsorber at zabs=0.00337 in the spectrum of 3C 273 based ondetections of O VI and H I and an upper limit on C IV. If this absorberis collisionally ionized, the OVI/CIV limit requiresT>~105.3 K in the O VI-bearing gas. For either collisionalionization or photoionization, we find that [O/H]>~-2.0 atzabs=0.00337. Based on observations with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| An ultra-high-resolution study of the interstellar medium towards Orion
We report ultra-high-resolution observations (R~9×105)of NaI, CaII, KI, CH and CH+ for interstellar sightlinestowards 12 bright stars in Orion. These data enable the detection ofmany more absorption components than previously recognized, providing amore accurate perspective on the absorbing medium. This is especially sofor the line of sight to the Orion nebula, a region not previouslystudied at very high resolution. Model fits have been constructed forthe absorption-line profiles, providing estimates for the columndensity, velocity dispersion and central velocity for each constituentvelocity component. A comparison between the absorption occurring insightlines with small angular separations has been used, along withcomparisons with other studies, to estimate the line-of-sight velocitystructures. Comparisons with earlier studies have also revealed temporalvariability in the absorption-line profile of ζ Ori, highlightingthe presence of small-scale spatial structure in the interstellar mediumon scales of ~10au. Where absorption from both Na0 andK0 is observed for a particular cloud, a comparison of thevelocity dispersions measured for each of these species providesrigorous limits on both the kinetic temperature and turbulent velocityprevailing in each cloud. Our results indicate the turbulent motions tobe subsonic in each case. Na0/Ca+ abundance ratiosare derived for individual clouds, providing an indication of theirphysical state.
| A High-Resolution Survey of Interstellar K I Absorption
We present high-resolution (FWHM ~0.4-1.8 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar K Iabsorption toward 54 Galactic stars. These new K I spectra revealcomplex structure and narrow, closely blended components in many linesof sight. Multicomponent fits to the line profiles yield estimates forthe column densities, line widths, and velocities for 319 individualinterstellar cloud components. The median component width (FWHM) and thetrue median separation between adjacent components are both <~1.2 kms-1. The median and maximum individual component K I columndensities, about 4×1010 and 1012cm-2, correspond to individual component hydrogen columndensities of about 2×1020 and 1021cm-2 and E(B-V)~0.03 and 0.17, respectively. If T istypically ~100 K, then at least half the individual components havesubsonic internal turbulent velocities. We also reexamine therelationships between the column densities of K I, Na I, C I, Li I,Htot, H2, and CH. The four trace neutral speciesexhibit essentially linear relationships with each other over wideranges in overall column density. If C is uniformly depleted by 0.4 dex,then Li, Na, and K are each typically depleted by 0.6-0.7 dex. The totalline of sight values for N(K I) and N(Na I) show roughly quadraticdependences on N(Htot), but the relationships for theensemble of individual clouds could be significantly steeper. Thesequadratic (or steeper) dependences appear to rule out significantcontributions to the ionization from cosmic rays, X-rays, and/or chargeexchange with C II in most cases. Charge exchange with negativelycharged large molecules may often be more important than radiativerecombination in neutralizing most singly ionized atomic species in coolH I clouds, however-suggesting that the true ne,nH, and thermal pressures may be significantly smaller thanthe values estimated by considering only radiative recombination. BothN(CH) and N(H2) are nearly linearly proportional to N(K I)and N(Na I) [except for 1015cm-2<~N(H2)<~1019cm-2, over which H2 makes the transition to theself-shielded regime]. Those relationships appear also to hold for manyindividual components and component groups, suggesting thathigh-resolution spectra of K I and Na I can be very useful forinterpreting lower resolution molecular data. The scatter about allthese mean relationships is generally small (<~0.1-0.2 dex), ifcertain consistently ``discrepant'' sight lines are excluded-suggestingthat both the relative depletions and the relative ionization of Li, C,Na, and K are generally within factors of 2 of their mean values.Differences noted for sight lines in Sco-Oph, in the Pleiades, near theOrion Trapezium, and in the LMC and SMC may be due to differences in thestrength and/or shape of the ambient radiation fields, perhaps amplifiedby the effects of charge transfer with large molecules.
| Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons
With the high-resolution spectroscopy now available in the optical andsatellite UV, it is possible to determine the neutral/ionized columndensity ratios for several different elements in a single cloud.Assuming ionization equilibrium for each element, one can make severalindependent determinations of the electron density. For the clouds forwhich such an analysis has been carried out, these different estimatesdisagree by large factors, suggesting that some process (or processes)besides photoionization and radiative recombination might play animportant role in the ionization balance. One candidate process is thecollision of ions with dust grains. Making use of recent workquantifying the abundances of polycyclic aromatic hydrocarbon moleculesand other grains in the interstellar medium, as well as recent modelsfor grain charging, we estimate the grain-assisted ion recombinationrates for several astrophysically important elements. We find that theserates are comparable to the rates for radiative recombination forconditions typical of the cold neutral medium. Including grain-assistedion recombination in the ionization equilibrium analysis leads toincreased consistency in the various electron density estimates for thegas along the line of sight to 23 Orionis. However, not all of thediscrepancies can be eliminated in this way; we speculate on some otherprocesses that might play a role. We also note that grain-assistedrecombination of H+ and He+ leads to significantlylower electron fractions than usually assumed for the cold neutralmedium.
| Far Ultraviolet Spectroscopic Explorer Observations of Interstellar Gas toward the Small Magellanic Cloud Star Sk 108
We discuss the interstellar absorption lines found in Far UltravioletSpectroscopic Explorer spectra of the Wolf-Rayet binary Sk 108, which islocated in the northeastern part of the main ``bar'' of the SmallMagellanic Cloud. The spectra cover the 988-1187 Å wavelengthrange at a resolution of about 12,000 and a signal-to-noise ratio of20-40. We use detailed component information from higher resolutionnear-UV and optical spectra to model the far-UV lines of similarlydistributed species. Both the Galactic and SMC gas toward Sk 108 seem tobe predominantly neutral, although a significant fraction of the SMC gasis ionized. The column densities of P II, S II, and Ar I are consistentwith essentially solar ratios, relative to N(Zn II), in both theGalactic and SMC gas; the column density of N I remains somewhatuncertain. Molecular hydrogen is present in the Galactic gas, withproperties similar to those found in low mean density Galactic lines ofsight and in the Galactic gas toward several other LMC and SMC stars. Wereport a tentative detection of H2 in the SMC gas for J=1 and3, with rotational level populations consistent with an excitationtemperature on the order of 1000 K-similar to the H2 found indiffuse Galactic gas toward ζ Puppis. Strong absorption from N III,S III, and Fe III has revealed a significant ionized component,particularly in the SMC; O VI is present, but relatively weak,especially in the Galactic gas. The N(C IV)/N(O VI) ratio variessomewhat within the SMC, suggesting that several processes maycontribute to the observed high ion abundances.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Surface abundances of light elements for a large sample of early B-type stars - I. Spectral observations of 123 stars; measurements of hydrogen and helium lines; infrared photometry
High-resolution spectral observations of 123 B0-B5 stars in the mainsequence evolutionary phase were obtained at two observatories, namelythe McDonald Observatory (McDO) and the Crimean AstrophysicalObservatory (CrAO). Accurate equivalent widths W of two Balmer lines,Hβ and Hγ, and ten Hei lines were obtained for all the stars,as well as of the Heiiλ4686 line for the hottest ones. A carefulanalysis of the measured equivalent widths was performed. It is shownthat there is a very good agreement between the W values derived fromthe McDO and CrAO spectra for 14 common stars. A comparison withpublished data leads to the conclusion that the W values measuredearlier by some authors for strong Hei lines are very likely to beunderestimated. Infrared photometric observations in the J, H, and Kbands were performed for 70 programme stars. All these data will be usedin other papers: in particular for the Teff and loggdetermination and for the He, C, N and O abundance analyses.
| Modeling the Variable Chromosphere of α Orionis
A spectral analysis of the prototypical red supergiant star α Orithat is based on near-UV, optical, and near-IR high-dispersion spectraobtained between 1992 September and 1999 July with the Space TelescopeImaging Spectrograph and the Goddard High Resolution Spectrograph on theHubble Space Telescope, the Utrecht Echelle Spectrograph, and the SoFinEchelle Spectrograph is presented. With detailed non-LTE radiativetransfer calculations in spherical geometry, we model the meanconditions in the stellar chromosphere from Hα and the Mg IIresonance doublet. The Hα absorption line emerges from an extendedchromosphere. Temporal changes of its velocity structure are determinedfrom detailed fits to near-UV Si I lines, and chromospheric expansionvelocities around 4 km s -1 are found in 1992, whereas thechromosphere was collapsing onto the photosphere with a velocity of 5 kms -1 in 1998-1999. The Hα core depth is correlated overtime with weaker depression changes seen in prominent TiO band headsthat dominate the optical spectrum. From elaborate spectral synthesiscalculations, we isolate unblended metal absorption lines in the near-IRand determine Teff=3500 K and log(g)=-0.5 for solarmetallicity and 12+/-0.5 km s -1 for macrobroadening andvsini. Semiempirical fits yield chromospheric temperatures not in excessof 5500 K, but with long-term changes by ~400 K. The model extends over5000 Rsolar and requires supersonic microturbulence valuesranging to 19 km s -1, in strong contrast with thephotospheric value of only 2 km s -1. We observe Dopplershifts of 4-8 km s -1 in the scattering cores of manydouble-peaked near-UV emission lines which correlate with changes in theintensity ratio of their emission components. The red emissioncomponents were much stronger in 1992, indicating a phase of enhancedchromospheric outflow, for which we determine a spherical mass-loss rateof 6×10-7 Msolar yr -1. Wepresent a discussion of chromospheric pulsation in this massive star.Detailed modeling of the observed Mg II h and k line asymmetry is alsopresented. We demonstrate that a chromospheric Mn I blend stronglycontributes to this puzzling asymmetry. Based in part on observationswith the NASA/ESA Hubble Space Telescope obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contractNAS 5-26555.
| Interstellar Carbon Abundance
We present 10 new measurements of carbon gas phase column density in theinterstellar medium. We have used spectra made with the Goddard HighResolution Spectrograph aboard the Hubble Space Telescope containing theCII 1334.5 Ang and CII* 1335.7 Ang lines. The continuum reconstructionmethod has been used to obtain the carbon column density from theLorentzian damped lines. Extinction curve parameters are determined inselected directions and relation between C/H ratio and extinctionparameters is discussed. A correlation has been found between C/H andthe strength of the 2175 Ang bump. Unlike previous results, we noticethat C/H changes with fractional abundance of molecular hydrogen,f(H_2). The average value of C/H=3.55*10^{-4} for lines of sight withf(H_2)<1*10^{-3} is the same as solar photospheric abundance fromGrevese and Noels (1993) and may represent the real cosmic carbonabundance.
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