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Resim Yükleyin
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Ýlgili Makaleler
The source of the helium visible lines in η Carinae
We assume that the helium-I lines emitted by the massive binary systemη Carinae are formed in the acceleration zone of the less-massivesecondary star. We calculate the Doppler shift of the lines as afunction of orbital phase and of several parameters of the binarysystem. We find that a good fit is obtained if the helium lines areformed in the region where the secondary wind speed is vzone= 430 km s‑1. The acceptable binary eccentricity is inthe range 0.90 ≲ e ≲ 0.95, and the inclination angle (theangle between a line perpendicular to the orbital plane and the line ofsight) is in the range 40° ≲ i ≲ 55°. Lower values ofe require higher values of i, and vice versa. The binary system isoriented such that the secondary star is in our direction (closer to us)during periastron passage. The orbital motion can account in part to theDoppler shift of the peak in X-ray emission.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Ultraviolet Excited High-J Molecular Hydrogen in Photodissociation Regions
We have calculated synthetic interstellar cloud models to investigatethe formation and destruction of high-J molecular hydrogen inphotodissociation regions. The effects of five physical parameters (theincident ultraviolet [UV] intensity, H2 column density, cloudtemperature, total density, and H2 formation ratecoefficient) on the populations of H2 rotational levels areexplored. We have found that N(4)/N(0) is proportional to the incidentUV intensity IUV and the H2 molecular fraction fis simply related to the ratio of IUV and the hydrogendensity nH, implying a new method to derive IUVand nH with the observational parameters N(4)/N(0) and f,assuming an H2 formation rate R. High-resolution FUSE spectraof H2 toward three translucent sight lines (HD 110432, HD192639, and HD 185418) in the Milky Way and 24 diffuse sight lines inthe SMC and LMC are referenced to obtain N(4)/N(0) and f. Using ourmethod, we are able to derive IUV~10-40 for the translucentsight lines, when R is assumed to be 10 times higher than the Galacticvalue. Synthetic models for the Magellanic sight lines suggest thatIUV>=10, nH<=100 cm-3, andR~3×10-16 for the low H2 column densitysight lines (N[H2]<=1018 cm-2) andIUV>=10, nH<=1000 cm-3, andR~3×10-17 for the high H2 column densitysight lines (N[H2]>=1019 cm-2).
| Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
Using UV spectra obtained with FUSE, HST, and/or IUE together withhigher resolution optical spectra, we determine interstellar columndensities of 12CO, 13CO, and/or C2 for10 Galactic sight lines with E(B-V) ranging from 0.37 to 0.72. TheN(CO)/N(H2) ratio varies over a factor of 100 in this sample,due primarily to differences in N(CO). For a given N(H2 ),published models of diffuse and translucent clouds predict less CO thanis observed. The J=1-3 rotational levels of 12CO aresubthermally populated in these sight lines, with Textypically between 3 and 7 K. In general, there appears to be nosignificant difference between the excitation temperatures of12CO and 13CO. Fits to the higher resolution COline profiles suggest that CO (like CN) is concentrated in relativelycold, dense gas. We obtain C2 column densities from the F-X(0-0) band at 1341 Å (three sight lines; J=0-12), the F-X (1-0)band at 1314 Å (one sight line; J=0-12), the D-X (0-0) band at2313 Å (four sight lines; J=0-18), and the A-X (3-0) and (2-0)bands at 7719 and 8757 Å (seven sight lines; J=0-12). Comparisonsamong those column densities yield a set of mutually consistent bandf-values for the UV and optical C2 bands, but also revealsome apparent anomalies within the F-X (0-0) band. Both the kinetictemperature Tk inferred from the C2 rotationalpopulations and the excitation temperature T02(C2)are generally smaller than the correspondingT01(H2)-suggesting that C2 isconcentrated in colder, denser gas than H2. Incorporatingadditional column density data for K I, HD, CH, C2,C3, CN, and CO from the literature (for a total sample of 74sight lines), we find that (1) CO is most tightly correlated with CN;(2) the ratios 12CO/H2 and13CO/H2 both are fairly tightly correlated withthe density indicator CN/CH (but C2/H 2 is not);and (3) the ratio 12CO/13CO is somewhatanticorrelated with both CN/CH and N(CO). Sight lines with12CO/13CO below the average local Galactic valueof 12C/13C appear to sample colder, denser gas inwhich isotope exchange reactions have enhanced 13CO, relativeto 12CO.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. Based in part onobservations obtained with the Apache Point Observatory 3.5 m telescope,which is owned and operated by the Astrophysical Research Consortium.
| Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
We report on the abundance of interstellar neutral nitrogen (N I) for 30sight lines, using data from the Far Ultraviolet Spectroscopic Explorer(FUSE) and the Hubble Space Telescope (HST). N I column densities arederived by measuring the equivalent widths of several ultravioletabsorption lines and subsequently fitting those to a curve of growth. Wefind a mean interstellar N/H of 51+/-4 ppm. This is below the mean foundby Meyer et al. of 62+4-3 ppm (adjusted for adifference in f-values). Our mean N/H is similar, however, to the(f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for alarger sample of sight lines with larger hydrogen column densitiescomparable to those in this study. We discuss the question of whether ornot nitrogen shows increased gas-phase depletion in lines of sight withcolumn densities logN(Htot)>~21, as claimed by Knauth etal. The nitrogen abundance in the line of sight toward HD 152236 isparticularly interesting. We derive very small N/H and N/O ratios forthis line of sight that may support a previous suggestion that membersof the Sco OB1 association formed from an N-deficient region.
| Bright OB stars in the Galaxy. III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Hα, IR and radio analysis
Context: .Recent results strongly challenge the canonical picture ofmassive star winds: various evidence indicates that currently acceptedmass-loss rates, {dot M}, may need to be revised downwards, by factorsextending to one magnitude or even more. This is because the mostcommonly used mass-loss diagnostics are affected by "clumping"(small-scale density inhomogeneities), influencing our interpretation ofobserved spectra and fluxes. Aims: .Such downward revisions wouldhave dramatic consequences for the evolution of, and feedback from,massive stars, and thus robust determinations of the clumping propertiesand mass-loss rates are urgently needed. We present a first attemptconcerning this objective, by means of constraining the radialstratification of the so-called clumping factor. Methods: .To thisend, we have analyzed a sample of 19 Galactic O-type supergiants/giants,by combining our own and archival data for Hα, IR, mm and radiofluxes, and using approximate methods, calibrated to more sophisticatedmodels. Clumping has been included into our analysis in the"conventional" way, by assuming the inter-clump matter to be void.Because (almost) all our diagnostics depends on the square of density,we cannot derive absolute clumping factors, but only factors normalizedto a certain minimum. Results: .This minimum was usually found tobe located in the outermost, radio-emitting region, i.e., the radiomass-loss rates are the lowest ones, compared to {dot M} derived fromHα and the IR. The radio rates agree well with those predicted bytheory, but are only upper limits, due to unknown clumping in the outerwind. Hα turned out to be a useful tool to derive the clumpingproperties inside r < 3{ldots}5 Rstar. Our most importantresult concerns a (physical) difference between denser and thinnerwinds: for denser winds, the innermost region is more strongly clumpedthan the outermost one (with a normalized clumping factor of 4.1± 1.4), whereas thinner winds have similar clumping properties inthe inner and outer regions. Conclusions: .Our findings arecompared with theoretical predictions, and the implications arediscussed in detail, by assuming different scenarios regarding the stillunknown clumping properties of the outer wind.
| 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.
| The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ``ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments.
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
| Bright OB stars in the Galaxy. II. Wind variability in O supergiants as traced by Hα
We investigate the line-profile variability (lpv) of Hα for alarge sample of O-type supergiants (15 objects between O4 and O9.7), inan objective, statistically rigorous manner. We employed the TemporalVariance Spectrum (TVS) analysis, developed for the case of photosphericabsorption lines and modified by us to take into account the effects ofwind emission. By means of a comparative analysis we place constraintson the properties of this variability - quantified in terms of a meanand a newly defined fractional amplitude of deviations - as a functionof stellar and wind parameters. The results of our analysis show thatall the stars in the sample show evidence of significant lpv inHα, mostly dominated by processes in the wind. The variationsoccur between zero and 0.3 v_&infy; (i.e., below 1.5 R_star ), in goodagreement with results from similar studies. A comparison between theobservations and corresponding line-profile simulations indicates thatfor stars with intermediate wind densities the properties of theHα variability can be explained by simple models consisting ofcoherent or broken shells (blobs) uniformly distributed over the windvolume, with an intrinsic scatter in the maximum density contrast ofabout a factor of two. For stars at lower and higher wind densities, onthe other hand, we found certain inconsistencies between theobservations and our predictions, most importantly concerning the meanamplitude and the symmetry properties of the TVS. This disagreementmight be explained by the presence of coherent large-scale structures,partly confined in a volume close to the star. Interpreted in terms of avariable mass-loss rate, the observed variations of Hα indicatechanges of ±4% with respect to the mean value of dot M for starswith stronger winds and of ± 16% for stars with weaker winds. Theeffect of these variations on the corresponding wind momenta is ratherinsignificant (less than 0.16 dex), increasing only the local scatterwithout affecting the Wind Momentum Luminosity Relationship.
| Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution
In this paper we have analyzed 25 Galactic O and early B-stars by meansof H and K band spectroscopy, with the primary goal to investigate towhat extent a lone near-IR spectroscopy is able to recover stellar andwind parameters derived in the optical. Most of the spectra have beentaken with subaru-ircs, at an intermediate resolution of 12 000, andwith a very high S/N, mostly on the order of 200 or better. In order tosynthesize the strategic H/He lines, we have used our recent,line-blanketed version of fastwind (Puls et al. 2005, A&A, 435,669). In total, seven lines have been investigated, where for two starswe could make additional use of the Hei2.05 singlet which has beenobserved with irtf-cshell. Apart from Brγ and Heii2.18, the otherlines are predominately formed in the stellar photosphere, and thusremain fairly uncontaminated from more complex physical processes,particularly clumping. First we investigated the predicted behaviour ofthe strategic lines. In contradiction to what one expects from theoptical in the O-star regime, almost all photospheric H/Hei/Heii H/Kband lines become stronger if the gravity decreases. Concerning H andHeii, this finding is related to the behaviour of Stark broadening as afunction of electron density, which in the line cores is different formembers of lower (optical) and higher (IR) series. Regarding Hei, thepredicted behaviour is due to some subtle NLTE effects resulting in astronger overpopulation of the lower level when the gravity decreases.We have compared our calculations with results from the alternative NLTEmodel atmosphere code cmfgen (Hillier & Miller 1998, ApJ, 496, 407).In most cases, we found reasonable or nearly perfect agreement. Only theHei2.05 singlet for mid O-types suffers from some discrepancy, analogouswith findings for the optical Hei singlets. For most of our objects, weobtained good fits, except for the line cores of Brγ in earlyO-stars with significant mass-loss. Whereas the observations showBrγ mostly as rather symmetric emission lines, the models predicta P Cygni type profile with strong absorption. This discrepancy (whichalso appears in lines synthesized by cmfgen) might be an indirect effectof clumping. After having derived the stellar and wind parameters fromthe IR, we have compared them to results from previous optical analyses.Overall, the IR results coincide in most cases with the optical oneswithin the typical errors usually quoted for the correspondingparameters, i.e., an uncertainty in T_eff of 5%, in log g of 0.1 dex andin {dot M} of 0.2 dex, with lower errors at higher wind densities.Outliers above the 1-σ level where found in four cases withrespect to log g and in two cases for {dot M}.
| Velocity Dispersion of the High Rotational Levels of H2
We present a study of the high rotational bands (J>=2) ofH2 toward four early-type Galactic stars: HD 73882, HD192639, HD 206267, and HD 207538. In each case, the velocity dispersion,characterized by the spectrum fitting parameter b, increases with thelevel of excitation, a phenomenon that has previously been detected bythe Copernicus and IMAPS observatories. In particular, we show with 4σ confidence that for HD 192639 it is not possible to fit all Jlevels with a single b-value and that higher b-values are needed for thehigher levels. The amplitude of the line broadening, which can be ashigh as 10 km s-1, makes explanations such as inhomogeneousspatial distribution unlikely. We investigate a mechanism in which thebroadening is due to the molecules that are rotationally excited throughthe excess energy acquired after their formation on a grain(H2 formation pumping). We show that different dispersionswould be a natural consequence of this mechanism. We note, however, thatsuch a process would require a formation rate 10 times higher than whatwas inferred from other observations. In view of this result, and of thedifficulty in accounting for the velocity dispersion as thermalbroadening (T would be around 10,000 K), we conclude then that we aremost certainly observing some highly turbulent warm layer associatedwith the cold diffuse cloud. Embedded in a magnetic field, it could beresponsible for the high quantities of CH+ measured in thecold neutral medium.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| Line profile variability in the spectra of Oef stars. II. HD 192281, HD 14442 and HD 14434
We present the very first analysis of the spectroscopic variability ofthe three rapidly rotating Oef stars HD 192281 (O5(ef)), HD 14442(O5.5ef) and HD 14434 (O6.5(ef)). Radial velocities of the He IIλ 4541 line reveal no evidence of binarity on time scales of afew days, or from one year to the next, for any of the targets. The HeII λ 4686 double-peaked emission and, to some extent, the Hβabsorption line display significant profile variability in the spectraof all three stars. Data gathered during different observing runs spreadover six years reveal a rather stable time scale for HD 192281 and HD14442, whereas the variability pattern changes significantly from oneyear to the other. The case of HD 14434 is less clear as no obvious timescale emerges from our analysis. In a tentative way to interpret thisvariability, stellar rotation remains a possible clock for HD 192281 andHD 14442. However, currently available models addressing stellarrotation fail to explain some crucial aspects of the observedvariability behaviour, which appear to be even more complex in the caseof HD 14434.Based on observations collected at the Observatoire de Haute-Provence,France.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| Tiny-scale molecular structures in the Magellanic Clouds. I. FUSE, HST and VLT observations
The objective of this series of two papers is to investigate small-scalemolecular structures in the Magellanic Clouds (hereafter MCs). We reporton the FUSE detections of the HD and CO molecules on the lines of sighttowards three Large Magellanic stars: Sk -67D05, Sk -68D135, and Sk-69D246. HD is also detected for the first time on the lines of sighttowards two Small Magellanic Cloud stars: AV 95 and Sk 159. While the HDand CO abundances are expected to be lower in the Large Magellanic Cloudwhere molecular fractions are a third of the Galactic value and wherethe photodissociation flux is up to thousands of times as large, wereport an average HD/H2 ratio of 1.4±0.5 ppm and aCO/H2 ratio ranging from 0.8 to 2.7 ppm similar to theGalactic values. We tentatively identify a deuterium reservoir(hereafter D-reservoir) towards the Small Magellanic Cloud, along thelight path to AV 95. We derive a D/H ratio ranging from 1.×10-6 to 1.1× 10-5. Combining FUSE andHST/STIS data we also analyzed the H2, Cl I, Cl II, Fe II, SII, C I, C I*, and C I** content, when available. High resolution VLTobservations of Na I, Ca II, and Ca I were obtained in support of thelower resolution FUSE and STIS data for three targets in order tounravel the sightline velocity structures. These observations form theonly such set of detections in the Magellanic Clouds to date and allowus to investigate in detail some of the physical properties of theintervening molecular gas. Our detection of the HD and CO molecules inthe Magellanic Clouds is an argument for the existence of dense(nH > 100 cm-3) components. Furthermore, wedemonstrate that these components are probably extremely small molecularclumps (possibly as small as 10-2 pc) or filaments similar tothe tiny-scale atomic structures (TSAS) recently observed in the halo ofour Galaxy by Richter et al. (\cite{Richter2003}). For these fivesightlines, we also detected molecular hydrogen originating in theGalactic disk. From these observations we conclude that tiny-scalemolecular filamentary structures are present in the disk of the Galaxyas well.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Stellar and wind parameters of Galactic O-stars. The influence of line-blocking/blanketing
We have re-analyzed the Galactic O-star sample from \citet{puls96} bymeans of line-blanketed NLTE model atmospheres in order to investigatethe influence of line-blocking/blanketing on the derived parameters. Theanalysis has been carried out by fitting the photospheric and wind linesfrom H and He. In most cases we obtained a good fit, but we have alsofound certain inconsistencies which are probably related to a stillinadequate treatment of the wind structure. These inconsistenciescomprise the line cores of Hγ and Hβ insupergiants (the synthetic profiles are too weak when the mass-loss rateis determined by matching Hα) and the ``generalizeddilution effect'' (cf. \citealt{vo89}) which is still present in He I4471 of cooler supergiants and giants.Compared to pure H/He plane-parallel models we found a decrease ineffective temperatures which is largest at earliest spectral types andfor supergiants (with a maximum shift of roughly 8000 K). This findingis explained by the fact that line-blanketed models of hot stars havephotospheric He ionization fractions similar to those from unblanketedmodels at higher Teff and higher log g. Consequently, anyline-blanketed analysis based on the He ionization equilibrium resultsin lower Teff-values along with a reduction of either log gor helium abundance (if the reduction of log g is prohibited by theBalmer line wings). Stellar radii and mass-loss rates, on the otherhand, remain more or less unaffected by line-blanketing.We have calculated ``new'' spectroscopic masses and compared them withprevious results. Although the former mass discrepancy \citep{h92}becomes significantly reduced, a systematic trend for masses below 50Msun seems to remain: The spectroscopically derived valuesare smaller than the ``evolutionary masses'' by roughly 10Msun. Additionally, a significant fraction of our samplestars stays over-abundant in He, although the actual values were foundto be lower than previously determined.Also the wind-momentum luminosity relation (WLR) changes because oflower luminosities and almost unmodified wind-momentum rates. Comparedto previous results, the separation of the WLR as a function ofluminosity class is still present but now the WLR for giants/dwarfs isconsistent with theoretical predictions.We argue that the derived mass-loss rates of stars withHα in emission are affected by clumping in the lowerwind region. If the predictions from different and independenttheoretical simulations (\citealt {Vink00, Paul03, puls03a}) that theWLR should be independent of luminosity class were correct, a typicalclumping factor <ρ2>/<ρ>2 ≈5 should be derived by ``unifying'' the different WLRs.Based upon observations obtained at the INT and the European SouthernObservatory, La Silla, Chile. The INT is operated on the island of LaPalma by the ING in the Spanish Observatorio de El Roque de losMuchachos of the Instituto de Astrofísica de Canarias.Appendix A in only available in electronic form athttp://www.edpsciences.org
| Minimizing Strong Telluric Absorption in Near-Infrared Stellar Spectra
We have obtained high-resolution spectra (R~25,000) of an A star overvarying air mass to determine the effectiveness of telluric removalwithin the limit of a high signal-to-noise ratio. The near-infrared lineHe I at 2.058 μm, which is a sensitive indicator of physicalconditions in massive stars, supergiants, H II regions, and YSOs,resides among pressure-broadened telluric absorption from CO2and water vapor that varies with both time and observed air mass. Ourstudy shows that in the limit of bright stars at high resolution,accuracies of 5% are typical for high-air-mass observations (greaterthan 1.9), improving to a photon-limited accuracy of 2% at smaller airmasses (less than 1.15). We find that by using the continuum betweentelluric absorption lines of a rovibrational fan, a photon-limited 1%accuracy is achievable.
| Bright OB stars in the Galaxy. I. Mass-loss and wind-momentum rates of O-type stars: A pure H\alpha analysis accounting for line-blanketing
We study mass-loss and wind momentum rates of 29 Galactic O-type starswith luminosity classes I, III and V by means of a pure H\alpha profileanalysis and investigate to what extent the results compare to thoseoriginating from a state-of-the-art, complete spectral analysis. Ourinvestigation relies on the approximate method developed by\citet{Puls96} which we have modified to account for the effects ofline-blanketing. Effective temperatures and gravities needed to obtainquantitative results from such a simplified approach have been derivedby means of calibrations based on most recent spectroscopic NLTEanalyses and models of Galactic stars by \citet{Repo03} and\citet{Martins02}. Comparing (i) the derived wind-densities to thosedetermined by \citet{Repo03} for eleven stars in common and (ii) theWind-momentum Luminosity Relationship (WLR) for our sample stars tothose derived by other investigations, we conclude that our approximateapproach is actually able to provide consistent results. Additionally,we studied the consequences of ``fine tuning'' some of the direct andindirect parameters entering the WLR, especially by accounting fordifferent possible values of stellar reddening and distances. Combiningour data set with the corresponding data provided by \citet{Herrero02}and \citet{Repo03} we finally study the WLR for the largest sample ofGalactic O-type stars gathered so far, including an elaborate errortreatment. The established disagreement between the theoreticalpredictions and the ``observed'' WLRs being a function of luminosityclass is suggested to be a result of wind clumping. Different strategiesto check this hypothesis are discussed, particularly by comparing theH\alpha mass-loss rates with the ones derived from radio observations.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.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/410/905
| Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and SpaceTelescope Imaging Spectrograph observations of the weak interstellar N Iλ1160 doublet toward 17 high-density sight lines[N(Htot)>=1021 cm-2]. When combinedwith published data, our results reveal variations in the fractional N Iabundance showing a systematic deficiency at large N(Htot).At the FUSE resolution (~20 km s-1), the effects ofunresolved saturation cannot be conclusively ruled out, although O Iλ1356 shows little evidence of saturation. We investigated thepossibility that the N I variability is due to the formation ofN2 in our mostly dense regions. The 0-0 band of thec'41Σ+u-X1Σ+gtransition of N2 at 958 Å should be easily detected inour FUSE data; for 10 of the denser sight lines, N2 is notobserved at a sensitivity level of a few times 1014cm-2. The observed N I variations are suggestive of anincomplete understanding of nitrogen chemistry.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985, and the NASA/ESA HubbleSpace Telescope, obtained from the Multimission Archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
| Abundances and Physical Conditions in the Interstellar Gas toward HD 185418
We present a study of the abundances and physical conditions in theinterstellar gas toward the moderately reddened B0.5 V star HD 185418.This star is located at (l,b)=(53deg,-2.2d) and has anestimated distance of 790 pc and a reddening E(B-V)=0.50.This work is based on the analysis of Far Ultraviolet SpectroscopicExplorer (FUSE) and Hubble Space Telescope/Space Telescope ImagingSpectrograph (HST/STIS) spectra covering the range from 912 to 1361Å. We used both sets of far-ultraviolet data, together withhigh-resolution optical data, to derive accurate gas-phase columndensities for important interstellar species such as C I, C I*, C I**, NI, O I, O I*, Mg II, S I, S III, Cl I, Mn II, Fe II, Ni II, Cu II, GeII, Kr I, and CO. Numerous lines of H2 are present in theFUSE spectra, with a kinetic temperature for the lowest rotationallevels T01=100+/-15 K. Analysis of the C I fine-structureexcitation at that temperature implies an average local density ofhydrogen nH=6.3+/-2.5 cm-3. Chemical argumentsbased on analyses of CH, CH+, and C2 indicate thata fraction of the CH (>=20%) is formed as a by-product ofnonequilibrium CH+ synthesis. The electron densityne, derived under the assumption of photoionizationequilibrium, ranges from 0.03 to 0.32 cm-3 for the sixdifferent neutral/first-ion ratios considered. The relatively complexcomponent structure seen in high-resolution spectra of K I, Na I, and CaII, the relatively low average volume densities, the modest molecularfraction f(H2)=0.44, and the measured depletions all indicateno evidence for the presence of translucent components in the lightpath. The line of sight rather contains a mixture of cold and warmneutral diffuse gas. The detection of Si III and S III indicates thepresence of some ionized gas (at a 1% level) along the line of sight. Wecould not determine unambiguously whether this ionized gas wasphysically related to the neutral components.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555. Theobservations are associated with proposal 8241. Based in part onobservations obtained with the Apache Point Observatory 3.5 m telescope,which is owned and operated by the Astrophysical Research Consortium.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Oxygen Gas-Phase Abundance Revisited
We present new measurements of the interstellar gas-phase oxygenabundance along the sight lines toward 19 early-type Galactic stars atan average distance of 2.6 kpc. We derive O I column densities fromHubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS)observations of the weak 1355 Å intersystem transition. We derivetotal hydrogen column densities [N(HI)+2N(H2)] using HST/STISobservations of Lyα and Far Ultraviolet Spectroscopic Explorer(FUSE) observations of molecular hydrogen. The molecular hydrogencontent of these sight lines ranges fromf(H2)=2N(H2)/[N(HI)+2N(H2)]=0.03 to0.47. The average of6.3×1021 cm-2 mag-1 with astandard deviation of 15% is consistent with previous surveys. The meanoxygen abundance along these sight lines, which probe a wide range ofGalactic environments in the distant interstellar medium, is106 (O/H)gas=408+/-13 (1 σ in the mean). Wesee no evidence for decreasing gas-phase oxygen abundance withincreasing molecular hydrogen fraction, and the relative constancy of(O/H)gas suggests that the component of dust containing theoxygen is not readily destroyed. We estimate that, if 60% of the dustgrains are resilient against destruction by shocks, the distantinterstellar total oxygen abundance can be reconciliated with the solarvalue derived from the most recent measurements of 106(O/H)gassolar=517+/-58 (1 σ). We note thatthe smaller oxygen abundances derived for the interstellar gas within500 pc or from nearby B star surveys are consistent with a localelemental deficit.
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| 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.
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