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A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. I. Characterizing the Variability
We present the results of a systematic search for large-scalespectroscopic variability in apparently single Wolf-Rayet (WR) starsbrighter than v ~ 12.5. In this first paper we characterize the variousforms of variability detected and distinguish several separate groups.For each star in our sample, we obtained 4-5 high-resolution spectrawith signal-to-noise ratio ~100. Our ultimate goal is to identify newcandidates presenting variability that potentially comes from corotatinginteraction regions (CIRs). Out of a sample of 25 stars, 10 were foundto display large-scale changes of which four are of CIR-type (WR 1,WR 115, WR 120, and WR 134). The star WR 134 wasalready known to show such changes from previous studies. Three WN8stars present a different type of large-scale variability and we believedeserve a group of their own. Also, all three WC9d stars in our samplepresent large-scale variability, but it remains to be checked if theseare binaries, as many dust-making WR stars are double. Finally, of theremaining stars, 10 were found to show small-amplitude spectral changes,which we attribute to normal line-profile variability due toinhomogeneities in the wind, and five were found to show no spectralvariability, as far as can be concluded from the data in hand. Follow-upstudies are required to identify potential periods for our candidatesshowing CIR-type changes and eventually estimate a rotation rate forthese WR stars.
| The Neutral Counterpart of an Uncatalogued Nebula and a Probable Interstellar Bubble around WR 53 in the Centaurus Region
From the inspection of optical images at l = 307° we have found anuncatalogued nebula of about 9 arcmin in radius, which we namedG307.27+0.27. The analysis of the HI-21cm line emission distributionrevealed an expanding HI shell that encircles the optical emissionregion. The shell, placed at a kinematical distance of 4+/-1 kpc, issuggested to be the HI counterpart of the optical nebula. COobservations at radio wavelengths, and far and mid IR data, allowed usto detect molecular gas and interstellar dust associated with thestructures. The presence of an O6 star at a spectrophotometric distancecompatible with that of the shell supports a stellar wind/HII regionorigin for the whole structure. We also report a probable HIinterstellar bubble related to the Wolf-Rayet star WR 53. Its physicalparameters are similar to the parameters of other HI bubbles aroundmassive stars.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| An Effelsberg HI study of the ISM around WR 126, WR 154 and WR 155
The neutral hydrogen distribution has been studied in the direction ofthree Galactic Wolf-Rayet (WR) stars using the 100 m Effelsberg radiotelescope. Cavities in the HI distribution, regions of low HIemissivity, are observed over a 8-9 km s-1, velocity rangefor WR 126 (≡ST 2), WR 154 (≡HD 213049) and WR 155(≡HD 214419). These minima are interpreted as the observable 21-cmHI line counterpart of interstellar bubbles created by the winds of theWR stars and their progenitors. The HI cavities are elongated structuresdepicting an axial ratio ranging from 1.3 (WR 155) to 3 (WR 126). The WRstars are always eccentric with respect to either the geometric centreof the HI cavity or the absolute minimum inside it. This offset rangesfrom 50% to 80% of the HI hole's minor axis. The major axis of thesestructures range from 13 (WR 155) to 27 pc (WR 126), while the missingHI mass amounts to 45-50 Mȯ (WR 126), 60Mȯ (WR 155) and 85 Mȯ (WR 154).Figures 1, 3, 5 are only available in electronic form athttp://www.edpsciences.org
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars
The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksecXMM-Newton, observation, implying an upper limit to the X-ray luminosityof LX < 2.5x 1030 erg s-1 andto the X-ray to bolometric luminosity ratio ofLX/Lbol < 4*E-9. This confirmsindications from earlier less sensitive measurements that there has beenno convincing X-ray detection of any single WC star. This lack ofdetections is reinforced by XMM-Newton, and CHANDRA observations of WCstars. Thus the conclusion has to be drawn that the stars withradiatively-driven stellar winds of this particular class areinsignificant X-ray sources. We attribute this to photoelectronicabsorption by the stellar wind. The high opacity of the metal-rich anddense winds from WC stars puts the radius of optical depth unity athundreds or thousands of stellar radii for much of the X-ray band. Webelieve that the essential absence of hot plasma so far out in the windexacerbated by the large distances and correspondingly high ISM columndensities makes the WC stars too faint to be detectable with currenttechnology. The result also applies to many WC stars in binary systems,of which only about 20% are identified X-ray sources, presumably due tocolliding winds.
| Wolf-Rayet star parameters from spectral analyses
The Potsdam non-LTE code for expanding atmospheres, which accounts forclumping and iron-line blanketing, has been used to establish a grid ofmodel atmospheres for WC stars. A parameter degeneracy is discovered forearly-type WC models which do not depend on the `stellar temperature'.15 Galactic WC4-7 stars are analyzed, showing a very uniform carbonabundance (He:C = 55:40) with only few exceptions.
| Stellar and wind properties of LMC WC4 stars. A metallicity dependence for Wolf-Rayet mass-loss rates
We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based(2.3 m MSSSO, NTT) spectroscopy to determine the physical parameters ofsix WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellarparameters are revised significantly relative to Gräfener et al.(\cite{Grafener1998}) based on improved observations and moresophisticated model atmosphere codes, which account for line blanketingand clumping. We find that stellar luminosities are revised upwards byup to 0.4 dex, with surface abundances spanning a lower range of 0.1 leC/He le 0.35 (20-45% carbon by mass) and O/He le 0.06 (<=10% oxygenby mass). Relative to Galactic WC5-8 stars at known distance, andanalysed in a similar manner, LMC WC4 stars possess systematicallyhigher stellar luminosities, ~ 0.2 dex lower wind densities, yet asimilar range of surface chemistries. We illustrate how theclassification C III lambda 5696 line is extremely sensitive to winddensity, such that this is the principal difference between the subtypedistribution of LMC and Galactic early-type WC stars. Temperaturedifferences do play a role, but carbon abundance does not affect WCspectral types. We illustrate the effect of varying temperature andmass-loss rate on the WC spectral type for HD 32257 (WC4, LMC) and HD156385 (WC7, Galaxy) which possess similar abundances and luminosities.Using the latest evolutionary models, pre-supernova stellar masses inthe range 11-19 Msun are anticipated for LMC WC4 stars, with7-14 Msun for Galactic WC stars with known distances. Thesevalues are consistent with pre-cursors of bright type-Ic supernovae suchas SN 1998bw (alias GRB 980425) for which a minimum total mass of C andO of 14 Msun has been independently derived. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer, and NASA-ESA Hubble Space Telescope. Also based onobservations collected at the European Southern Observatory in program63.H-0683, and at the Australian National University Siding SpringObservatory.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| The Effect of Binarity and Metallicity in the Spectra of WC and WO Stars
A statistical analysis of the main emission lines common to the WC andWO stars is made based on an extensive set of spectral data. To definethe trends in equivalent width ( Wλ), line ratios, andline widths, median values are derived for single-spectrum stars ofdifferent spectral class. We find that in Galactic WO and WC4 stars,Wλ (C IV 581 nm) is smaller compared to inextragalactic objects. In both Galactic and extragalactic stars,Wλ (O V 559 nm) smoothly increases towards early WCand WO stars. It is argued that differences in stellar wind structure,in combination with the ambient metallicity, may be the cause of theanomalies. Variation of the profile of the 465 nm blend indicates asubstantial contribution of He II 468 nm for the WCE and WO stars. Inaddition, we comment on the carbon abundances in relation to theevolutionary status of these objects. We also give an estimate of theOB/WR continuum flux ratio in composite-spectrum systems.
| Kinematical Structure of Wolf-Rayet Winds. I.Terminal Wind Velocity
New terminal wind velocities for 164 Wolf-Rayet stars (from the Galaxyand LMC) based on PCyg profiles of lambda1550 CIV resonance line werederived from the archive high and low resolution IUE spectra availableform the INES database. The high resolution data on 59 WR stars (39 fromthe Galaxy and 20 from LMC) were used to calibrate the empiricalrelation lambda_min^Abs- lambda_peak^Emis vs terminal wind velocity,which was then used for determinations of the terminal wind velocitiesfrom the low resolution IUE data. We almost doubled the previous mostextended sample of such measurements. Our new measurements, based onhigh resolution data, are precise within 5-7%. Measurements, based onthe low resolution spectra have the formal errors of approx 40-60%. Acomparison of the present results with other determinations suggestshigher precision of approx 20%. We found that the terminal windvelocities for the Galactic WC and WN stars correlate with the WRspectral subtype. We also found that the LMC WN stars have winds slowerthan their Galactic counterparts, up to two times in the case of the WNEstars. No influence of binarity on terminal wind velocities was found.Our extended set of measurements allowed us to test application of theradiation driven wind theory to the WR stars. We found that, contrary toOB stars, terminal wind velocities of the WR stars correlate only weaklywith stellar temperature. We also note that the terminal to escapevelocity ratio for the WR stars is relatively low: 2.55 pm 1.14 for theGalactic WN stars and 1.78 pm 0.70 for the Galactic WCs. This ratiodecreases with temperature of WR stars, contrary to what is observed inthe case of OB stars. The presented results show complex influence ofchemical composition on the WR winds driving mechanism efficiency. Ourkinematical data on WR winds suggest evolutionary sequence: WNL -->WNE --> WCE --> WCL.
| A Mid-Infrared Spectral Survey of Galactic Wolf-Rayet Stars
We present 8-13 μm spectra at resolution R~600 of 29 northernGalactic Wolf-Rayet stars, including the first ever reportedmid-infrared (MIR) spectrum for many. Among the subtypes of the starsstudied were 14 WC, 13 WN, 1 WN/WC, and an additional reclassified WN.Lines of He I and He II, along with fine-structure lines of Ne II and SIV, are strongly present in 22 of the sources observed, while six of thesources exhibit the powerful emission of heated circumstellar carbondust. We point out similarities between our spectra and Infrared SpaceObservatory (ISO) observations of several of the same sources and notean unresolved discrepancy between the two data sets for the WC6 star WR146. We investigate the diagnostic power of MIR He I and He II lines forsubtype discrimination and find the line ratio Wλ(9.7μm He II)/Wλ(11.3 μm He I+He II) can providemoderate discrimination within the WN and WC types, though the smallnumber of stars with corresponding line pairs detected made suchassessment difficult.
| The VIIth catalogue of galactic Wolf-Rayet stars
The VIIth catalogue of galactic PopulationI Wolf-Rayet stars providesimproved coordinates, spectral types and /bv photometry of known WRstars and adds 71 new WR stars to the previous WR catalogue. This censusof galactic WR stars reaches 227 stars, comprising 127 WN stars, 87 WCstars, 10 WN/WC stars and 3 WO stars. This includes 15 WNL and 11 WCLstars within 30 pc of the Galactic Center. We compile and discuss WRspectral classification, variability, periodicity, binarity, terminalwind velocities, correlation with open clusters and OB associations, andcorrelation with Hi bubbles, Hii regions and ring nebulae. Intrinsiccolours and absolute visual magnitudes per subtype are re-assessed for are-determination of optical photometric distances and galacticdistribution of WR stars. In the solar neighbourhood we find projectedon the galactic plane a surface density of 3.3 WR stars perkpc2, with a WC/WN number ratio of 1.5, and a WR binaryfrequency (including probable binaries) of 39%. The galactocentricdistance (RWR) distribution per subtype shows RWRincreasing with decreasing WR subtype, both for the WN and WC subtypes.This RWR distribution allows for the possibility ofWNE-->WCE and WNL-->WCL subtype evolution.
| Exospheric models for the X-ray emission from single Wolf-Rayet stars
We review existing ROSAT detections of single Galactic Wolf-Rayet (WR)stars and develop wind models to interpret the X-ray emission. The ROSATdata, consisting of bandpass detections from the ROSAT All-Sky Survey(RASS) and some pointed observations, exhibit no correlations of the WRX-ray luminosity (LX) with any star or wind parameters ofinterest (e.g. bolometric luminosity, mass-loss rate or wind kineticenergy), although the dispersion in the measurements is quite large. Thelack of correlation between X-ray luminosity and wind parameters amongthe WR stars is unlike that of their progenitors, the O stars, whichshow trends with such parameters. In this paper we seek to (i) test byhow much the X-ray properties of the WR stars differ from the O starsand (ii) place limits on the temperature TX and fillingfactor fX of the X-ray-emitting gas in the WR winds. Adoptingempirically derived relationships for TX and fXfrom O-star winds, the predicted X-ray emission from WR stars is muchsmaller than observed with ROSAT. Abandoning the TX relationfrom O stars, we maximize the cooling from a single-temperature hot gasto derive lower limits for the filling factors in WR winds. Althoughthese filling factors are consistently found to be an order of magnitudegreater than those for O stars, we find that the data are consistent(albeit the data are noisy) with a trend of fx ∝(Mν&infy;)-1 in WR stars, as is also the casefor O stars.
| Mass-loss rates of Wolf-Rayet stars as a function of stellar parameters
Clumping-corrected mass-loss rates of 64 Galactic Wolf-Rayet (WR) starsare used to study the dependence of mass-loss rates, momentum transferefficiencies and terminal velocities on the basic stellar parameters andchemical composition. The luminosities of the WR stars have beendetermined either directly from the masses, using the dependence of L onmass predicted by stellar evolution theory, or they were determined fromthe absolute visual magnitudes and the bolometric corrections. For thispurpose we improved the relation between the bolometric correction andthe spectral subclass. (1) The momentum transfer efficiencies η(i.e. the ratio between the wind momentum loss and radiative momentumloss) of WR stars are found to lie in the range of 1.4 to 17.6, with themean value of 6.2 for the 64 program stars. Such values can probably beexplained by radiative driving due to multiple scattering of photons ina WR wind with an ionization stratification. However, there may be aproblem in explaining the driving at low velocities. (2) We derived thelinear regression relations for the dependence of the terminal velocity,the momentum transfer efficiency and the mass-loss rates on luminosityand chemical composition. We found a tight relation between the terminalvelocity of the wind and the parameters of the hydrostatic core. Thisrelation enables the determination of the mass of the WR stars fromtheir observed terminal velocities and chemical composition with anaccuracy of about 0.1 dex for WN and WC stars. Using evolutionary modelsof WR stars, the luminosity can then be determined with an accuracy of0.25 dex or better. (3) We found that the mass-loss rates(&mathaccent "705Frelax dot;) of WR stars depend strongly onluminosity and also quite strongly on chemical composition. For thecombined sample of WN and WC stars we found that &mathaccent"705Frelax dot; in Mȯyr-1 can be expressed as&mathaccent "705Frelax dot; ≃ 1.0 ×10-11(L/L ȯ)1.29Y1.7Z0.5 (1) with an uncertainty of σ = 0.19dex (4) The new mass-loss rates are significantly smaller than adoptedin evolutionary calculations, by about 0.2 to 0.6 dex, depending on thecomposition and on the evolutionary calculations. For H-rich WN starsthe new mass-loss rates are 0.3 dex smaller than adopted in theevolutionary calculations of Meynet et al. (1994). (5) The lowermass-loss rates, derived in this paper compared to previously adoptedvalues, facilitate the formation of black holes as end points of theevolution of massive stars. However they might create a problem inexplaining the observed WN/WC ratios, unless rotational mixing ormass-loss due to eruptions is important.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The neon abundance in WC stars - I. ISO SWS spectroscopy of WR146 (WC6+O)
We present infrared spectroscopy in the wavelength range 2.6-20mum ofthe WC6+O binary system WR146 (MR112) obtained with the Short WavelengthSpectrometer (SWS) on the Infrared Space Observatory (ISO). These dataare combined with ground-based optical (INT) and near-IR (UKIRT)spectroscopy and theoretical continuum distributions revealing E(B-V) =2.8mag, a continuum optical light ratio (WR:O) = 1:2 +/- 1, a probableO8.5V companion and a distance of 750 pc. We obtain C/He=0.15 by numberfor the WC6 star from an analysis of near-IR recombination lines, afactor of 2 higher than previously estimated by Eenens & Williams.We find a WC6 terminal wind velocity of 2700kms^-1 from the observed[Neiii] 15.5-μm profile, while results from test WCE modelatmospheric calculations and radio flux measurements imply M = 2.6 x 10^- 5M / yr^-1. An analysis of the [Neiii] 15.5-μm line yields a lowerlimit to the neon abundance of Ne/He=3.4x10^-3 by number. Our ISOspectra show no wind emission in either [Neii] 12.81 μm or [Nev]14.32 μm from which we estimate Neiv/Neiii <= 1. The resultingbound on the neon abundance in the WC6 star is thus 3.4x10^-3<=Ne/He<=6.8x10^-3. This result is in close accord with the predicted valueof Ne/He~6x10^-3 from stellar evolution models of WC stars, during whichthe products of interior nuclear burning are revealed at the stellarsurface.
| Catalogue of stars in the northern Milky Way having H-alpha in emission
Not Available
| An IRAS-based Search for New Dusty Late-Type WC Wolf-Rayet Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS..100..413C&db_key=AST
| Spectral analyses of 25 Galactic Wolf-Rayet stars of the carbon sequence.
We present a grid of helium-carbon models for Wolf-Rayet (WR) stars ofthe carbon sequence (WC) with β_ C_=0.2 (carbon mass fraction),thus extending our previously released grid with β_C_=0.6 to adifferent chemical composition. The WR model atmospheres are based onthe so-called standard assumptions. The calculations account for non-LTEradiation transfer in spherically expanding atmospheres. Helium andcarbon are represented by detailed model atoms, especially concerningthe ions Ciii and Civ. Using the model grids 25 Galactic WC stars ofintermediate subtype (WC5 to WC8) are analyzed. Subsequently we performfine analyses by calculating several individual models for each of theprogram stars. Temperatures, radii, mass-loss rates and terminalvelocities are determined together with the carbon to helium ratio. Theanalyzed WC stars are found to form two groups, which can bedistinguished by the strength of their emission lines. Stars with weaklines (WC-w) have effective temperatures close to 50kK and their windsare relatively thin, forming the continuous spectrum in regions withsmall expansion velocities. WC stars with strong lines (WC-s) havehigher effective temperatures (60 to 100kK, referring to the coreradius) and thick winds. Thus there is a strong analogy to thedistribution of the early-type WN stars (WNE-w and WNE-s, respectively).For the WC stars we determine luminosities between 10^4.7^ and10^5.5^Lsun_ and mass-loss rates from 10^-4.8^ to10^-3.9^Mȯ/yr. The carbon mass fraction varies from 0.2 to 0.6. Nocorrelation is found between the carbon abundance and any of the stellarparameters (e.g. temperature, luminosity) or the spectral subtype. Theevolution of WR stars is discussed by comparing the results of ouranalyses with evolutionary tracks.
| Terminal Velocities of Wolf-Rayet Star Winds from Low Resolution IUE Spectra
Attracted by the simplicity of the recently published by Prinja (1994)method of determination of terminal wind velocities in hot stars fromlow resolution IUE spectra we investigate its application to WR stars.With a large sample of low resolution IUE spectra of WR stars we foundeven simpler, that is linear instead of square, empirical relationbetween Delta lambda as defined by Prinja (1994) and terminal windvelocity -- vinfty. Using this new empirical relation wepresent vinfty for a sample of 85 galactic and LMC stars, 19of them determined for the first time. We almost tripled the number ofterminal velocity determinations for LMC WR stars. The comparison withother determinations shows that this simple method is accurate to within10-20%. We confirm the correlation between terminal velocity and WCsubtype. We also show that terminal velocities of WN stars are lowerthan that of WCE. A comparison between galactic and LMC stars shows thatthe LMC WN stars have slower winds in most of WN subtypes.
| The ROSAT PSPC survey of the Wolf-Rayet stars
Not Available
| Low resolution IUE spectra of Wolf-Rayet stars.
We present uniformly reduced and measured equivalent widths, FWHM andobserved line fluxes for 94 "single" WR stars (34 galactic WN, 22galactic WC, 31 LMC WN and 7 LMC WC) based on the archive IUE spectra ofWR stars gathered from different observational runs and from differentepochs. The spectra are used for spectral classification in theultraviolet region and for searching correlations among the strength andwidths of emission lines of different ions. Some correlations withoptical and near IR lines observed by other authors are given as well.The set of spectra we use is almost complete to 12 magnitude andrepresentative according to spectral subtype of WR stars.
| Improved bolometric corrections for WR stars from cluster membership and evolutionary models
Evolutionary models allow an assignment of both a mass and a luminosityto a Wolf-Rayet (WR) star in a cluster, and hence allow a determinationof the Bolometric Correction (B.C.). We plot all cluster WR stars in theM_v_,logM/Msun_ diagram of Smith & Maeder (1989); thismore than doubles the number of stars and extends the subclassesrepresented to include WN4.5, WC9 and WO2. The same method applied to WRstars in associations yields large uncertainties but includes usefullimits on subclasses not represented in clusters: WN2, 3 and 8 and WC 5and 6. The increased sample and expanded subclass range give adiscrimination in the B.C. that was not previously possible. The B.C.'sderived for WN stars range from -4.0 to -6.0 with the expected trend oflarger values for stars with higher excitation spectra. For WC stars,there is little evidence for a similar trend; most observationspresented here are consistent with B.C.=-4.5, as before. The convergenceof B.C. values derived from evolutionary and atmospheric models isextremely satisfactory, giving increased confidence in both methods.
| Spectrophotometry of Wolf-Rayet stars. I - Continuum energy distributions
All available low-resolution IUE spectra are assembled for Galactic,LMC, and SMC W-R stars and are merged with ground-based optical and NIRspectra in order to collate in a systematic fashion the shapes of theseenergy distributions over the wavelength range 0.1-1 micron. They can beconsistently fitted by a power law of the form F(lambda) isapproximately equal to lambda exp -alpha over the range 1500-9000 A toderive color excesses E(B-V) and spectral indices by removing the 2175-Ainterstellar absorption feature. The WN star color excesses derived arefound to be in good agreement with those of Schmutz and Vacca (1991) andKoesterke et al. (1991). Significant heterogeneity in spectral indexvalues was generally seen with any given subtype, but the groupsconsisting of the combined set of Galactic and LMC W-R stars, theseparate WN and WC sequences, and the Galactic and LMC W-R stars allshowed a striking and consistent Gaussian-like frequency distribution ofvalues.
| A new survey of nebulae around Galactic Wolf-Rayet stars in the northern sky
Interference filter CCD images have been obtained in H-alpha andforbidden O III 5007 A for 62 Wolf-Rayet (W-R) stars, representing acomplete survey of nebulae around Galactic W-R stars in the northernsky. We find probable new ring nebulae around W-R stars number 113, 116and 132, and possible new ring nebulae around W-R stars number 133 and153. All survey images showing nebulosities around W-R stars arepresented in this paper. New physical information is derived from theimproved images of known ring nebulae. The absence of ring nebulaearound most W-R stars is discussed.
| Near-infrared spectroscopy of Galactic Wolf-Rayet stars
We present high-quality near-IR spectra of 24 Galactic WR stars, of abroad range of subtypes, selected as having known distances. The datacover the region 0.97-1.12 micron and include the 10830-A He I triplet.Measurements of He I and He II lines, together with the absolutemagnitudes, yield temperatures, luminosities, mass-loss rates, andterminal velocities for our sample. We extend that sample by includingresults for a further 12 stars of known distance, taken from theliterature. The spectroscopic mass-loss rates are in excellent agreementwith those estimated from radio data, and exceed the 'single-scatteringlimit' by large factors. Mass-loss rates depend only weakly on mass, butthere is a statistically significant correlation between surface massflux and temperature. Terminal velocities correlate loosely with subtypefor both WR sequences. Comparison with core-helium-burning massluminosity tracks suggests that the spectroscopic luminosities may besystematically too faint by about 0.5 dex. We suggest that the WNC starsin our sample may represent an intermediate phase between WNL and WCEsubtypes.
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