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Disentangling the Nature of the Radio Emission in Wolf-Rayet Stars
We present quasi-simultaneous, multi-frequency Very Large Arrayobservations at 4.8, 8.4, and 22.5 GHz of a sample of 13 Wolf-Rayet (WR)stars, aimed at disentangling the nature of their radio emission and thepossible detection of a non-thermal behavior in close binary systems. Wedetected 12 stars from our sample, for which we derived spectralinformation and estimated their mass-loss rates. From our data, weidentified four thermal sources (WR 89, 113, 138, and 141), and threesources with a composite spectrum (similar contribution of thermal andnon-thermal emission; WR 8, 98, and 156). On the other hand, from thecomparison with previous observations, we confirm the non-thermalspectrum of one (WR 105), and also found evidence of a compositespectrum for WR 79a, 98a, 104, and 133. Finally, we discuss the possiblescenarios to explain the nature of the emission for the observedobjects.
| Stellar and wind properties of massive stars in the central parsec of the Galaxy
Context: How star formation proceeds in the Galactic Center is a debatedquestion. Addressing this question will help us understand the origin ofthe cluster of massive stars near the supermassive black hole, and moregenerally, starburst phenomena in galactic nuclei. In that context, itis crucial to know the properties of young massive stars in the centralparsec of the Galaxy. Aims: The main goal of this study is toderive the stellar and wind properties of the massive stars orbiting thesupermassive black hole SgrAstar in two counter-rotatingdisks. Methods: We use non-LTE atmosphere models including windsand line-blanketing to reproduce H and K band spectra of these starsobtained with SINFONI on the ESO/VLT. Results: The GC massivestars appear to be relatively similar to other Galactic stars. Thecurrently known population of massive stars emit a total 6.0 ×1050 s-1 (resp. 2.3 × 1049s-1) H (resp. He I) ionising photons. This is sufficient toproduce the observed nebular emission and implies that, in contrast toprevious claims, no peculiar stellar evolution is required in theGalactic Center. We find that most of the Ofpe/WN9 stars are lesschemically evolved than initially thought. The properties of several WN8stars are given, as well as two WN/C stars confirmed quantitatively tobe stars in transition between the WN and WC phase. We propose thesequence (Ofpe/WN9 rightleftharpoons LBV) ? WN8 ? WN/C formost of the observed GC stars. Quantitative comparison with stellarevolutionary tracks including rotation favour high mass loss rates inthe Wolf-Rayet phase in these models. In the OB phase, these tracksnicely reproduce the average properties of bright supergiants in theGalactic Center.
| The massive eclipsing LMC Wolf-Rayet binary BAT99-129. I. Orbital parameters, hydrogen content and spectroscopic characteristics
BAT99-129 in the LMC is one among a handful ofextra-galactic eclipsing Wolf-Rayet binaries known. We present blue,medium-resolution, phase-dependent NTT-EMMI spectra of this system thatallow us to separate the spectra of the two components of the binary andto obtain a reliable orbital solution for both stars. We assign an O5Vspectral type to the companion, and WN3(h)a to the Wolf-Rayet component.We discuss the spectroscopic characteristics of the system: luminosityratio, radii, rotation velocities. We find a possible oversynchronousrotation velocity for the O star. Surprisingly, the extracted Wolf-Rayetspectrum clearly shows the presence of blueshifted absorption lines,similar to what has been found in all single hot WN stars in the SMC andsome in the LMC. We also discuss the presence of such intrinsic lines inthe context of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binaryevolution and GRB progenitors. Altogether, BAT99 129 is theextragalactic counterpart of the well-known Galactic WR binaryV444 Cygni.
| 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.
| Kinematical Structure of Wolf-Rayet Winds. II. Internal Velocity Scatter in WN Stars
The shortward edge of the absorption core velocities - v_black asdetermined from low resolution archived IUE spectra from the INESdatabase are presented for three P Cyg profiles of NV 1240, HeII 1640and NIV 1720 for 51 Galactic and 64 LMC Wolf-Rayet stars of the WNsubtype. These data, together with v_black of CIV 1550 line presented inNiedzielski and Skorzynski (2002) are discussed. Evidences are presentedthat v_black of CIV 1550 rarely displays the largest wind velocity amongthe four lines studied in detail and therefore its application as anestimator of the terminal wind velocity in WN stars is questioned. Anaverage v_black of several lines is suggested instead but it is pointedout that v_black of HeII 1640 usually reveals the highest observablewind velocity in Galactic and LMC WN stars. It is shown that thestratification strength decreases from WNL to WNE stars and that for WNLstars there exists a positive relation between v_black and theIonization Potential. The velocity scatter between v_black obtained fromdifferent UV lines is found to correlate well with the X-ray luminosityof single WN stars (correlation coefficient R=0.82 for the data obtainedfrom the high resolution IUE spectra) and therefore two clumpy windmodels of single WN stars are presented that allow the velocity scatterto persist up to very large distances from the stellar surface (r approx500-1000 R_*). These models are used to explain the specific features ofsingle WN stars like broad absorption troughs of strong lines havingdifferent v_black, X-ray fluxes, IR/radio continua and stratificationrelations.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| A Very Large Array 3.6 Centimeter Continuum Survey of Galactic Wolf-Rayet Stars
We report the results of a survey of radio continuum emission ofGalactic Wolf-Rayet (WR) stars north of δ=-46°. Theobservations were obtained at 8.46 GHz (3.6 cm) using the Very LargeArray, with an angular resolution of ~6"×9" and typical rms noiseof ~0.04 mJy beam-1. Our survey of 34 WR stars resulted in 15definite and five probable detections, 13 of these for the first time atradio wavelengths. All detections are unresolved (θ<~5"). Timevariations in flux are confirmed in the cases of WR 98a, 104, 105, and125. WR 79a and WR 89 are also variable in flux, and we suspect they arealso nonthermal emitters. Thus, of our sample 20%-30% of the detectedstars are nonthermal emitters. Average mass-loss rate determinationsobtained excluding definite and suspected nonthermal cases give similarvalues for WN (all subtypes) and WC5-7 stars[M(WN)=(4+/-3)×10-5 Msolar yr-1and M(WC5-7)=(4+/-2)×10-5 Msolaryr-1], while a lower value was obtained for WC8-9 stars[M(WC8-9)=(2+/-1)×10-5 Msolaryr-1]. Uncertainties in stellar distances largely contributeto the observed scatter in mass-loss rates. Upper limits to themass-loss rates were obtained in cases of undetected sources and forsources that probably show additional nonthermal emission.
| Stellar evolution with rotation. X. Wolf-Rayet star populations at solar metallicity
We examine the properties of Wolf-Rayet (WR) stars predicted by modelsof rotating stars taking account of the new mass loss rates for O-typestars and WR stars (Vink et al. \cite{Vink00}, \cite{Vink01}; Nugis& Lamers \cite{NuLa00}) and of the wind anisotropies induced byrotation. We find that the rotation velocities v of WR stars are modest,i.e. about 50 km s-1, not very dependent on the initial v andmasses. For the most massive stars, the evolution of v is very stronglyinfluenced by the values of the mass loss rates; below ~ 12 M_sun theevolution of rotation during the MS phase and later phases is dominatedby the internal coupling. Massive stars with extreme rotation may skipthe LBV phase.Models having a typical v for the O-type stars have WR lifetimes on theaverage two times longer than for non-rotating models. The increase ofthe WR lifetimes is mainly due to that of the H-rich eWNL phase.Rotation allows a transition WN/WC phase to be present for initialmasses lower than 60 M_sun. The durations of the other WR subphases areless affected by rotation. The mass threshold for forming WR stars islowered from 37 to 22 M_sun for typical rotation. The comparisons of thepredicted number ratios WR/O, WN/WC and of the number of transitionWN/WC stars show very good agreement with models with rotation, whilethis is not the case for models with the present-day mass loss rates andno rotation. As to the chemical abundances in WR stars, rotation bringsonly very small changes for WN stars, since they have equilibrium CNOvalues. However, WC stars with rotation have on average lower C/He andO/He ratios. The luminosity distribution of WC stars is also influencedby rotation.
| Wolf-Rayet Stars, Black Holes, and Gamma-Ray Bursters in Close Binaries
We consider the evolutionary status of observed close binary systemscontaining black holes and Wolf-Rayet (WR) stars. When the componentmasses and the orbital period of a system are known, the reason for theformation of a WR star in an initial massive system of two main-sequencestars can be established. Such WR stars can form due to the action ofthe stellar wind from a massive OB star (M OB≥50M ȯ),conservative mass transfer between components with close initial masses,or the loss of the common envelope in a system with a large (up to25) initial component mass ratio. The strong impact ofobservational selection effects on the creation of samples of closebinaries with black holes and WR stars is demonstrated. We estimatetheoretical mass-loss rates for WR stars, which are essential for ourunderstanding the observed ratio of the numbers of carbon and nitrogenWR stars in the Galaxy . We also estimate the minimum initial masses ofthe components in close binaries producing black holes and WR stars tobe 25M ȯ. The spatial velocities of systems with black holesindicate that, during the formation of a black hole from a WR star, themass loss reaches at least several solar masses. The rate of formationof rapidly rotating Kerr black holes in close binaries in the Galaxy is3×10-6 yr-1. Their formation may be accompanied by a burst ofgamma radiation, possibly providing clues to the nature of gamma-raybursts. The initial distribution of the component mass ratios for closebinaries is dNdq=dM 2/M 1 in the interval 0.04≲q 0≤1,suggesting a single mechanism for their formation.
| 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.
| Evolution of Wolf-Rayet Stars in Binary Systems: An Analysis of the Mass and Orbital-Eccentricity Distributions
We have undertaken a statistical study of the component mass ratios andthe orbital eccentricities of WR + O close binary, detachedmain-sequence (DMS), contact early-type (CE), and semidetached (SD)systems. A comparison of the characteristics of WR + O systems and ofDMS, CE, and SD systems has enabled us to draw certain conclusions aboutthe evolutionary paths of WR + O binaries and to demonstrate that up to90% of all known WR + O binaries formed as a result of mass transfer inmassive close O + O binary systems. Since there is a clear correlationbetween the component masses in SD systems with subgiants, the absenceof an anticorrelation between the masses of the WR stars and O stars inWR + O binaries cannot be considered evidence against the formation ofWR + O binaries via mass transfer. The spectroscopic transitionalorbital period P tr sp corresponding to the transition from nearlycircular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is14d for WR + O systems and 2d 3d for OB + OB systems. Theperiod range in which all WR + O orbits are circular &$(1mathop dlimits_. 6 ≤slant P ≤slant 14(d) ); is close to the range for SD systems with subgiants, &0mathop dlimits_. 7 ≤slant P ≤slant 15(d); . The large difference between the P tr sp values for WR + O and OB +OB systems suggests that a mechanism of orbit circularization additionalto that for OB + OB systems at the DMS stage (tidal dissipation of theorbital energy due to radiative damping of the dynamical tides) acts inWR + O binaries. It is natural to suggest mass transfer in the parent O+ O binaries as this supplementary orbit-circularization mechanism.Since the transitional period between circular and elliptical orbits forclose binaries with convective envelopes and ages of 5×109 yearsis &P_{tr} = 12mathop dlimits_. 4$; , the orbits of most known SD systems with subgiants had enough timeto circularize during the DMS stage, prior to the mass transfer. Thus,for most SD systems, mass transfer plays a secondary role incircularization of their orbits. In many cases, the initial orbitaleccentricities of the O + O binary progenitors of WR + O systems arepreserved, due to the low viscosity of the O-star envelopes and theshort timescale for their nuclear evolution until the primary O starfills its Roche lobe and the mass transfer begins. The mass transfer inthe parent O + O systems is short-lived, and the number of orbitalcycles during the early mass-transfer stage is relatively low (lowerthan for the progenitors of SD systems by three or four orders ofmagnitude). The continued transfer of mass from the less massive to themore massive star after the component masses have become equal leads tothe formation of a WR + O system, and the orbit's residual eccentricityincreases to the observed value. The increase of the orbitaleccentricity is also facilitated by variable radial mass loss via thewind from the WR star in the WR + O system during its motion in theelliptical orbit. The result is that WR + O binaries can haveconsiderable orbital eccentricities, despite their intense masstransfer. For this reason, the presence of appreciable eccentricitiesamong WR + O binaries with large orbital periods cannot be consideredfirm evidence against mass transfer in the parent O + O binary systems.Only for the WR + O binaries with the longest orbital periods (4 of 35known systems, or 11 %) can the evolution of the parent O + O binariesoccur without filling of the Roche lobe by the primary O star, beinggoverned by radial outflow in the form of the stellar wind and possiblyby the LBV phenomenon, as in the case of HD 5980.
| 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.
| First Detections of Molecular Gas Associated with the Wolf-Rayet Ring Nebula NGC 3199
This paper presents the first observations of molecular gas associatedwith the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. Thisincludes first observations of the molecules HCN, HCO+, CN,and HNC seen in any Wolf-Rayet ring nebula. Our observations immediatelysuggest the presence of high-density molecular gas (>104cm-3) in the nebula with significant amounts of associatedmolecular gas, which is in the form of clumpy ejecta and/or interstellarmaterial. Molecular CO gas was mapped across the optically brightportion of the nebula and out into the diffuse ionized component usingthe 12CO J=1-->0 line. CO gas is not seen within theoptically bright rim of NGC 3199 but adjacent to it. The opticalemission rim therefore appears to mark regions of photodissociation.Velocity components in the CO data are consistent with those seen inhigh-resolution optical spectra of the Hα line but extend beyondthe visible emission. A prior suggestion of the formation of the nebulavia a bow shock appears unlikely since Hipparcos measurements show theproper motion of WR 18 is almost at right angles to the directionrequired for the bow shock model. Instead, line splitting toward thenorth of the nebula suggests that a possible blowout of the Wolf-Rayetwind through surrounding ejecta may be responsible for some of thevelocity features observed. Preliminary estimates of molecularabundances in the nebula seen toward the central star are significantlyhigher than for the interstellar medium and are similar to those inplanetary nebulae, although CN is distinctly underabundant in comparisonto the very high values found in many planetary nebulae. The abundancesfound are consistent with the idea that at least a portion of themolecular material is associated with ejecta from the central star.Based on observations collected at the Swedish-ESO SubmillimetreTelescope (SEST) at the European Southern Observatory, La Silla, Chile.The Swedish-ESO Submillimetre Telescope is operated jointly by theEuropean Southern Observatory (ESO) and the Swedish National Facilityfor Radio Astronomy, Onsala Space Observatory, at Chalmers University ofTechnology.
| 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.
| Wolf-Rayet Stars and Relativistic Objects: Distinctions between the Mass Distributions in Close Binary Systems
The observed properties of Wolf-Rayet stars and relativistic objects inclose binary systems are analyzed. The final masses M CO f for thecarbon-oxygen cores of WR stars in WR + O binaries are calculated takinginto account the radial loss of matter via stellar wind, which dependson the mass of the star. The analysis includes new data on the clumpystructure of WR winds, which appreciably decreases the requiredmass-loss rates for the WR stars. The masses M CO f lie in the range (12)M ȯ (20 44)M ȯ and have a continuous distribution. Themasses of the relativistic objects M x are 1 20M ȯ and have abimodal distribution: the mean masses for neutron stars and black holesare 1.35 ± 0.15M ȯ and 8 10M ȯ, respectively, with agap from 2 4M ȯ in which no neutron stars or black holes areobserved in close binaries. The mean final CO-core mass is &$/line M _{CO}(f) = 7.4 - 10.3M_ ȯ$; , close to the mean mass for the black holes. This suggests that it isnot only the mass of the progenitor that determines the nature of therelativistic object, but other parameters as well-rotation, magneticfield, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binariesthat may have low-mass companions (main-sequence or subgiant M-A stars)are identified; these could be the progenitors of low-mass X-raybinaries with neutron stars and black holes.
| X-ray Nova Binary Systems
The physical properties of X-ray novae as close binary systems areanalysed. Observational data in X-ray, UV, optical, IR and radio rangesare summarized. Modern theoretical considerations of the problem ofX-ray novae, taking into account some new ideas and results, aredescribed. Properties of optical stars in X-ray novae are analysed. Dataabout the masses of black holes in X-ray binary systems are summarized.Possible evolutionary links between WR stars in close binary systems andX-ray novae are analysed.
| A search for nonthermal radio emission from OB and WR stars with RATAN-600
We have searched for nonthermal radio emission from 40 OB and WR stars.Enhanced nonthermal radio emission from an early-type star could beevidence for the presence of a collapsed companion, and thus for itsorigin as the result of a supernova explosion in a massive binary. Asshown in the evolutionary calculations of joint evolution of a neutronstar and a normal star in binaries (Lipunov & Prokhorov \cite{lp}),a considerable fraction of neutron stars in binary systems having anoptical companion must be in the ejector state. A neutron star in thisstate generates a relativistic wind like an isolated radio pulsar. Mostejectors in binary systems can not be identified as radio pulsarsbecause of absorption of radiowaves in the stellar wind of the normalcompanion, but instead, they may appear as sources of high-energy quantadue to the synchrotron radiation of relativistic particles (ejected bythe radio pulsar) in the magnetic field of a normal star (Lipunov &Prokhorov \cite{lp2}; Lipunov & Nazin \cite{naz}). In this case asource of nonthermal radiation in a wide range from radio to hardgamma-ray may appear as a result of a specific reflection effect in themagnetic field of the optical companion. Cyg X-3 and the periodicradioburster LS I +610303 may be examples of just this kind.To test this idea, measurements of radio flux densities in the rangefrom 0.96 to 21.7 GHz from selected OB and WR stars were made with theRATAN-600. No nonthermal radio emission from the selected stars weredetected.
| Observational constraints on the efficiency of acceleration in the optically thin parts of Wolf-Rayet winds
Wolf-Rayet stars have such strong winds that their inner regions areoptically thick, preventing us from seeing the hydrostatic stellarcores. One might expect considerable acceleration of the wind to occurin the optically thick part. However, we show empirically that at least50%, and in some cases up to 100%, of the wind's acceleration occurs inthe optically thin part of the WR wind.
| A dedicated photometric system for the classification of Wolf-Rayet stars
We present here tests of a five-filter photometric system aimed at WRclassification. In addition to the well-known easy separation betweenthe WN and WC spectral types, these tests indicate interestingpotentialities in the discrimination of subgroups among the WN and theWC which look well related to the classical subtypes. The proposedcombinations of filters (or derived ones) bear enough discriminatingpower to satisfy some evolutionary studies in crowded fields wherespectroscopic follow-up is not possible. Based on observations collectedat the European Southern Observatory, La Silla, Chile.
| 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. II. Photometry
Abundant {HIPPARCOS photometry over 3 years of 141 O and Wolf-Rayetstars, including 8 massive X-ray binaries, provides a magnificentvariety of light curves at the sigma ~ 1-5% level. Among the mostinteresting results, we mention: optical outbursts in HD 102567 (MXRB),coinciding with periastron passages; drastic changes in the light curveshape of HD 153919 (MXRB); previously unknown long-term variability ofHD 39680 (O6V:[n]pe var) and WR 46 (WN3p); unusual flaring of HDE 308399(O9V); ellipsoidal variations of HD 64315, HD 115071 and HD 160641;rotationally modulated variations in HD 66811=zeta Pup (O4Inf) and HD210839=lambda Cep (O6I(n)fp); dust formation episode in WR 121 (WC9). Ina statistical sense, the incidence of variability is slightly higheramong the WR stars, which might be explained by the higher percentage ofknown binary systems. Among the presumably single WR stars, thecandidate runaways appear to be more variable then the rest. Based ondata from the ESA Hipparcos astrometry satellite
| 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
| Large IRAS Shells Around Galactic Wolf-Rayet Stars and the O Star Phase of Wolf-Rayet Evolution
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....112.2828M
| A three-dimensional classification for WN stars
A three-dimensional classification for WN stars is presented using (1)the HeII 5411/HeI 5875 ratio as a primary indicator of ionization, (2)FWHM 4686 and EW 5411 as indicators of line width and strength, and (3)an oscillating Pickering decrement as an indicator of the presence ofhydrogen. All WN stars in the Galaxy and two-thirds of the LMC stars areclassified on the new system. Almost all spectra inspected fall smoothlyinto categories within which the spectra are very similar. Allionization subclasses show a tight correlation between line strength andwidth, with stars containing hydrogen at the weak, narrow end, and WN/Cstars near the strong, broad end. H^+/He^++ correlates with strength andwidth with a cut-off for the presence of hydrogen, which is slightlydependent on ionization subclass, at about FWHM 4686=30A and EW5411=25A. The correlations found indicate that high (initial) mass starsevolve as narrow-line stars from late to early ionization subclass.Lower (initial) mass stars evolve with increasing line strength andwidth, probably to earlier ionization subclass. The HeII 4686/NV,III4604-40 ratio shows a clear correlation with Galactocentric radius,presumably an effect of the Z gradient. CIV 5808/HeII 5411 shows no suchcorrelation. LMC WN stars can be classified without difficulty by thecriteria established for Galactic WN stars. While individual spectra ofa given subtype are similar in the two galaxies, the frequencydistributions over ionization subclass, over EW and FWHM in subclassesWN4 and WN5, and hydrogen content in subclasses WN6-8 are different. Theeffects are presumably due to metallicity, but the causal connection isunclear.
| Wolf-Rayet Stars. Physical, Chemical and Mass Loss Properties and Evolutionary Status
This paper reviews the current status of knowledge regarding the basicphysical and chemical properties of Wolf-Rayet stars; their overall massloss and stellar wind characteristics and current ideas about theirevolutionary status. WR stars are believed to be the evolved descendentsof massive O-type stars, in which extensive mass loss reveals successivestages of nuclear processed material: WN stars the products of interiorCNO-cycle hydrogen burning, and WC and WO stars the products of interiorhelium burning. Recent stellar evolution models, particularly thoseincorporating internal mixing, predict results which are in good accordwith the different chemical compositions observationally inferred forWN, WC and WO stars. WR stars exhibit the highest levels of mass lossamongst earlytype stars: mass loss rates, typically, lie in the range [110]×10‑5 Mȯyr‑1. Radiation pressure-driven windsincorporating multi-scattering in high ionisation-stratified winds maycause these levels, but additional mechanisms may also be needed.
| Quantitative Spectroscopy of Wolf-Rayet Stars and Related Objects
Not Available
| Fundamental parameters of Wolf-Rayet stars. V. The nature of the WN/C star WR 8.
A detailed study has been carried out on the Galactic WN/C star WR 8(HD62910), based on ultraviolet, optical and infrared spectroscopy. Ourstandard model analysis allows a determination of its stellar parametersand chemical abundances of hydrogen, helium, carbon, nitrogen andoxygen. We find that the WN and WC spectral features are consistent withformation in the same stellar wind and derive stellar parameters ofT_*_=48kK (T_eff_=32kK), log(L/Lsun_)=5.1,log(?(M)/Msun_/yr)=-4.2, vinfinity_=1590km/s.The chemistry of WR 8 (H/He=~0, C/He=~0.02, C/N=~3, C/O=~4 by number) isindeed found to be intermediate between normal WN and WC stars, andexplains the peculiar WC line spectrum of WR 8. In common with previousWN analyses (e.g. Crowther et al. 1995d) all high and low excitationfeatures cannot be simultaneously reproduced at present, which weattribute to the neglect of line blanketing. We also determine physicalparameters and C/He ratios for two other WN/C stars, WR145 (MR111) inthe Galaxy and Brey 29 (HDE 269485) in the LMC. The physical propertiesof these stars are found to be very similar to WR 8, with C/N ratiosalso estimated to be of order unity. Overall, our results confirm thatmixing does take place at the boundary of the convective He core inWolf-Rayet stars, with an estimated timescale of =~10^4^ years and arein excellent agreement with the theoretical predictions of Langer (1991,1994, private communication) in which slow mixing is produced throughsemi-convection.
| 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
| Chemical composition of Wolf-Rayet stars. II. Hydrogen-to-helium ratio.
The formation of HeII, HeI and HI lines in the winds of somerepresentative WN stars of different spectral subtypes has been modeled.Two different types of models were studied: the clumped winds and smoothwinds for both of which the standard velocity lawv=v_0_+(vinfinity_-v_0_) (1-R_*_/r)beta^ withβ=1 was assumed. The smooth winds model predicts about two timeslower IR fluxes than observed if one determines the matter density inthe winds through the observed values of radio fluxes. The increaseddensity smooth wind models with 1.5 times higher density as compared tothe radio-flux scaling are in reasonable agreement with IR fluxes andHeII and HeI line fluxes in the case of WN 5, WN 6 and WN 8 stars butfor other subtypes these models predict discrepant line fluxes. Theclumped wind models agree quite well with the most importantobservational data whereas somewhat lower mass loss rates are nowderived as compared to the smooth wind models. Theoretical line fluxeswere found by summing of the contributions from different layers of thewind. The statistical equilibria equations for level populations weresolved in the Sobolev approximation by taking into account the overlapof HeII and HI lines in the expanding medium. We used 40, 20 and 52level atomic models for HeII, HI and HeI respectively, whereas theinfluence of higher levels was taken into account through the correctionterms. On the basis of our modelling study we derived a simple formulafor the determination of hydrogen-to-helium ratios for WN stars whichwas used for concrete estimates for 28 stars. In all cases thehydrogen-to-helium ratios are lower than the mean cosmic value. Byinspection of the line fluxes of the neighboring HeII lines of (n-4),(n-6) and (n-8) series in the spectra of two WC stars we concluded thatno hydrogen seems to be present in their winds. The hydrogen-to-heliumratio is decreasing when going from late to early WN subtypes withstrong scatter existing among the stars of WN 6 and later subtypes.
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
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