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Ýlgili Makaleler
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| A census of the Carina Nebula - I. Cumulative energy input from massive stars
The Carina Nebula (NGC 3372) is our richest nearby laboratory in whichto study feedback through ultraviolet radiation and stellar winds fromvery massive stars during the formation of an OB association, at anearly phase in the evolution of the surrounding proto-superbubble beforesupernova explosions have influenced the environment. This feedback istriggering successive generations of new star formation around theperiphery of the nebula, while simultaneously evaporating the gas anddust reservoirs out of which young stars are trying to accrete material.This paper takes inventory of the combined effect from all the knownmassive stars that power the Carina Nebula through their total ionizingflux and integrated mechanical energy from their stellar winds. Carinais close enough and accessible enough that spectral types for individualstars are available, and many close binary and multiple systems haverecently been spatially resolved, so that one can simply add them.Adopting values from the literature for corresponding spectral types,the present-day total ionizing photon luminosity produced by the 65 Ostars and three WNL stars in Carina is QH~=1051s-1, the total bolometric luminosity of allstars earlier than B2 is 2.5 × 107Lsolar,and the total mechanical luminosity of stellar winds is LSW~=105Lsolar. The total QH was about 25per cent higher when η Carinae was on the main sequence, before itand its companion were surrounded by its obscuring dust shell; for thefirst 3Myr, the net ionizing flux of the 70 O stars in Carina was about150 times greater than in the Orion Nebula. About400-500Msolar has been contributed to the HII region bystellar wind mass-loss during the past 3Myr. Values for QHand LSW are also given for the individual clusters Tr14, 15and 16, and Bo10 and 11, which are more relevant on smaller spatialscales than the total values for the whole nebula.
| H I bubbles surrounding southern optical ring nebulae: Anon (WR 23) and RCW 52
We analyze the interstellar medium in the environs of two hot andmassive stars, HD 92809 (=WR 23, WC6) and LSS 1887 (O8V), which ionizethe optical ring nebulae Anon (WR 23) and RCW 52, respectively. Ouranalysis is based on neutral hydrogen (Hi) 21 cm line data, which revealinterstellar bubbles surrounding the massive stars and their opticalring nebulae. The Hi bubble related to WR 23 is 13.3 pc in radius and isexpanding at 10 km s-1. The associated atomic neutral mass is830 M_ȯ. The Hi structure related to LSS 1887 is about 6.3 pc inradius, has an expansion velocity of 7 km s-1 and anassociated atomic neutral mass of 100 M_ȯ. These Hi features arethe neutral counterparts of the optical ring nebulae and were mainlycreated by the action of the stellar winds of the massive stars on theirenvirons. The dynamical age of the Hi bubble around WR 23(7×105 yr) suggests that it was created during the WRphase of stellar evolution. However, the large tangential motions of WR23 and LSS 1887 suggest that part of the observed optical and Histructures may be due to a bow shock. The analysis of the distributionof emission in the far infrared and in the CO(1-0) molecular line in theenvirons of WR 23 and LSS 1887 reveals that there are also infrared andmolecular counterparts of the detected Hi bubbles.
| Observations of the Diffuse Far-Ultraviolet Background with the Far Ultraviolet Spectroscopic Explorer
We have used observations taken under the Far Ultraviolet SpectroscopicExplorer (FUSE) S405/505 channel realignment program to explore thediffuse far-ultraviolet (FUV; 1000-1200 Å) radiation field. Of the71 independent locations in that program, we have observed a diffusesignal in 32, ranging in brightness from 1600 to a maximum of2.9×105 photons cm-2 sr-1s-1 Å-1 in Orion. The FUSE data confirm thatthe diffuse FUV sky is patchy with regions of intense emission, usuallynear bright stars, but also with dark regions, even at low Galacticlatitudes. We find a weak correlation between the FUV flux and the 100μm ratio but with wide variations, perhaps due to differences in thelocal radiation field.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| CO-to-H2 Abundance Ratio of the Foreground Gas of the Carina Nebula
We analyze CO and H2 absorption lines of the foregroundmolecular cloud in the Carina nebula. We use HST-STIS(Hubble SpaceTelescope - Space Telescope Imaging Spectrograph) & IUE(International Ultraviolet Explorer) INES data to analyze the A-X(v=0→2) absorption band of CO for several hot stars toward theCarina nebula, while 9 stars of them have FUSE (Far UltravioletSpectroscopic Explorer) spectra to analyze the (v=0→4) vibrationalband in the Lyman series of H2. The column densities of COand H2 varies in the vicinity of N(CO) 1013cm-2 and N(H2) 1019cm-2, respectively. The resultant CO-to-H2abundance ratio is about 10-6. We investigate the variationof the abundance ratio according to the relative position of the targetstars to morphology the molecular cloud in the Carina nebula.
| The Carina Spiral Feature: Strömgren-Hβ photometry approach. I. The photometric data-base
A data-base collating all uvbybeta photometry available at present forO-B9 stars brighter than 10th visual magnitude in the field of theCarina Spiral Feature is presented. The completeness and homogeneity ofthe data-base are discussed.Based on CDS data.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/cgi-bin/qcat?J/A+A/410/523
| 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.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| Neutral Gas Bubbles Surrounding Southern Optical Ring Nebulae: Anon(WR 23) and RCW 52
Based on interferometric H I 21 cm line data we analyze the distributionof the neutral hydrogen in the environs of the optical ring nebulaearound WR 23 (WC6) and LS 1887 (O8V). We identify the interstellaratomic gas bubbles associated with the optical ring nebulae Anon(WR 23)and RCW 52.
| 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.
| 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.
| Chemical abundances and winds of massive stars in M31: a B-type supergiant and a WC star in OB 10
We present high quality spectroscopic data for two massive stars in theOB 10 association of M31, OB 10-64 (B0 Ia) and OB 10-WR1 (WC6). Mediumresolution spectra of both stars were obtained using the ISISspectrograph on the William Herschel Telescope. This is supplementedwith Hubble Space Telescope STIS UV spectroscopy and Keck I HIRES datafor OB 10-64. A non-local thermodynamic equilibrium (LTE) modelatmosphere and abundance analysis for OB 10-64 is presented, indicatingthat this star has similar photospheric CNO, Mg and Si abundances tosolar neighbourhood massive stars. A wind analysis of this early B-typesupergiant reveals a mass-loss rate ofMȯ=1.6×10-6Msolaryr-1,and v&infy;=1650kms-1. The corresponding windmomentum is in good agreement with the wind momentum-luminosityrelationship found for Galactic early-B supergiants. Observations of OB10-WR1 are analysed using a non-LTE, line-blanketed code, to revealapproximate stellar parameters of logL/Lsolar~5.7, T*~75kK,v&infy;~3000kms-1,Mȯ/(Msolaryr-1)~10-4.3adopting a clumped wind with a filling factor of 10 per cent.Quantitative comparisons are made with the Galactic WC6 star HD 92809(WR23) revealing that OB 10-WR1 is 0.4 dex more luminous, though it hasa much lower C/He ratio (~0.1 versus 0.3 for HD 92809). Our studyrepresents the first detailed, chemical model atmosphere analysis foreither a B-type supergiant or a Wolf-Rayet (WR) star in Andromeda, andshows the potential of how such studies can provide new information onthe chemical evolution of galaxies and the evolution of massive stars inthe local Universe.
| 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.
| Strömgren and Hβ photometry of O and B type stars in star-forming regions. III. Carina Spiral Feature
Strömgren and Hβ photometry of O and B type stars, generallybrighter than 9.5 mag is reported for the field of the Carina SpiralFeature. The observations are based on the PPM catalogue identificationsand are designed to improve the completeness of the existing uvbybetadata for the bright early-type stars in the field. We present new uvbyphotometry for 283 stars and Hβ photometry for 225 of them. Theseobservations are part of an ongoing effort to study the structure ofselected star-forming regions in the Milky Way by means of uvbybetaphotometry. A comparison of the new data to other uvbybeta data sets forthis field is presented. Based on data from the Strömgren AutomaticTelescope of the Copenhagen University Observatory, La Silla. Tables 1and 2 are only available in electronic form at the CDS via anonymous ftpto 130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Constraints on the Evolution of Massive Stars through Spectral Analysis. I. The WC5 Star HD 165763
Using a significantly revised non-LTE radiative transfer code thatallows for the effects of line blanketing by He, C, O, Si, and Fe, wehave performed a detailed analysis of the Galactic Wolf-Rayet (W-R) starHD 165763 (WR 111, WC5). Standard W-R models consistently overestimatethe strength of the electron scattering wings, especially on stronglines, so we have considered models where the wind is both homogeneousand clumped. The deduced stellar parameters for HD 165763 are asfollows: L=2.0x10^5 L_solar ,R_*=1.8 R_solar ,M=1.5x10^-5 M_solar yr^-1,V_&infy;=2300 km s^-1, N(C)/N(He)=0.4, X(C)=0.462, N(O)/N(He)=0.1,X(O)=0.154. The above results are for a clumped model in which thevolume filling factor of the clumps is assumed to be 0.1 and implies(assumes) a distance of 1.55 kpc for HD 165763. Similar results areobtained for homogeneous models except that the deduced mass loss islarger, approximately 5.0x10^-5 M_solar yr^-1. The clumped model givesconsistently better agreement with the line profiles, particularly thered wings, confirming the scenario that W-R winds are clumped. Becauseof the severe blending in WC stars and the simplicity of our approach,it is not feasible to deduce the actual filling factor or mass-lossrate. The quantity that can be determined from line strengths using ourmodels is M/f^1/2. Using the evolutionary models of Maeder and Meynet,we find that the stellar abundances and parameters are consistent with astar of initial mass 60 M_solar and that has a present mass 10 M_solar.The stellar parameters deduced for HD 165763 are significantly differentfrom earlier analyses. The deduced luminosity is a factor of 2 larger,and a smaller core radius is found. The smaller radius is in betteragreement with expectations from stellar evolution calculations. Both ofthese changes can be attributed to the effects of line blanketing. Thededuced C/He abundance is similar to earlier calculations, and the O/Heabundance had not previously been determined. The observed ironspectrum, principally due to Fe V and Fe VI, is well reproduced using asolar iron mass fraction, although a variation of at least a factor of 2about this value cannot be precluded. In particular, the modelsnaturally produce the Fe V emission feature centered on 1470 Å andthe complex Fe emission/absorption spectrum at shorter wavelengths.Also, Fe strongly modifies the line strengths and profile shapesshortward of 1800 Å and must be taken into account if we are tosuccessfully model this region. The reddening toward HD 165763 does notfollow the mean Galactic extinction law. We determine the reddening lawtoward HD 165763 by comparing our model continuum levels toobservations. In order to simultaneously match the UV, optical, andparticularly the infrared fluxes, we used the parameterized reddeninglaw of Cardelli, Clayton, and Mathis with E_B-V=0.3 and R=4.5, whereR=A_V/E_B-V. Based on both observational and theoretical suggestions wehave considered models in which the wind is still accelerating at largeradii. In particular, we discuss models in which the velocity law can becharacterized by beta=1 for r<10R_* but that undergo a substantialvelocity increase (~600 km s^-1) beyond 10R_*. These models appear togive slightly better fits to the line profiles, but the improvements aresmall, and it is difficult to gauge whether observational data requiresuch a velocity law. We cannot yet determine whether the winds of W-Rstars are driven by radiation pressure, because we neglect many higherlevels of iron in our model ions, and we do not include importantelements such as cobalt and nickel. However, the ``wind problem'' in W-Rstars is less severe than previously assumed if clumping occurs. For ourclumped model, the single scattering limit is only exceeded by a factorof 10 compared to 3 times this value for a homogeneous wind. Clumpingappears to be the key to explaining the apparent high mass-loss ratesdetermined for W-R stars and is extremely important in understanding howor even whether W-R winds are driven by radiation pressure. A reductionin W-R mass-loss rates has important implications for stellar evolutioncalculations.
| Radio Continuum Measurements of Southern Early-Type Stars. III. Nonthermal Emission from Wolf-Rayet Stars
The Australia Telescope Compact Array (ATCA) has been used to search forradio continuum emission at 2.4 and 1.4 GHz from a sample of 36 southernWolf-Rayet stars. Seven Wolf-Rayet stars were detected at 2.4 GHz, ofwhich two were also detected at 1.4 GHz. We have identified sixWolf-Rayet stars, WR 14, 39, 48, 90, 105, and 112, that have nonthermalemission. The ATCA data confirm that at least 40% of Wolf-Rayet starswith measured spectral indices have nonthermal emission at centimeterwavelengths. Properties of each of the six sources are discussed. Themeasured spectral indices are between 0 and -1.0, and the radioluminosities are of order 10^29 ergs s^-1. So far 10 confirmed sourcesof nonthermal emission are known, including the six ATCA detections andfour previously known cases, WR 125, 140, 146, and 147. In all cases,the nonthermal radio emission almost certainly originates from aninteraction between the Wolf-Rayet stellar wind and the wind from amassive companion star. The radio observations agree well withtheoretical predictions for colliding winds. Synchrotron emission occursfrom relativistic electrons accelerated in strong shocks. The nonthermalspectral indices are likely to be close to -0.5. For WR 39, the detectedradio emission is offset by ~3" from the optical position of WR 39 andby ~2" from the optical position of WR 38B. We suggest that the radioemission may originate from a wind-wind interaction between WR 39 and WR38B, although this is not confirmed. For WR 11, the radio spectral indexincreases from +0.3 between 3 and 6 cm to +1.2 between 13 and 20 cm.This is interpreted as evidence for a highly attenuated nonthermalcomponent that originates well within the ionized wind of the W-R starfrom an interaction with the wind of the O9 companion star.
| Hubble Space Telescope Detection of Optical Companions of WR 86, WR 146, and WR 147: Wind Collision Model Confirmed
Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of therado-binary Wolf-Rayet stars WR 146 and WR 147, as well as the 0.2"visual binary WR 86, resolve each of them into two very close opticalcomponents. The colors of these optical pairs are similar, indicatingthat they are likely to be physically bound WR + OB systems at the samedistance. Comparison of the locations of the optical components of WR146 and WR 147 with high-resolution radio maps strikingly demonstratesthat the nonthermal radio components arise between the optical binarycomponents, closer to the OB component than the WR. This is as expectedif the nonthermal radio emission results from the collision of thestellar winds of the binary components seen in the HST images. Thesimilar magnitudes and colors determined for the components of WR 86from our HST images, combined with an analysis of the unresolved,combined WC7 + OB optical spectrum, indicates an absolute magnitude forthe WC7 component of about M_V ~ -5. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
| 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.
| Radio Continuum Measurements of Southern Early-Type Stars. II. A Distance-limited Sample of Wolf-Rayet Stars
A distance-limited sample of southern Wolf-Rayet stars within 3 kpc ofthe Sun has been observed with the Australia Telescope Compact Array at8.64 and 4.80 GHz. Radio continuum flux densities at one or bothfrequencies were obtained for 10 sources and upper limits for 20; foursources are found to be thermal emitters on the basis of the observedspectral index. Five sources are classified as nonthermal. One sourcecould not be classified. We derive mass-loss rates for the thermalsources. After combining them with all existing radio mass-loss rates ofWolf-Rayet stars in the northern and southern hemisphere, we perform acomparison with mass-loss rates derived from optical emission lines. Thetwo methods lead to consistent results, which suggests either that theassumption of a spherically symmetric, stationary, homogeneous stellarwind is correct or that deviations from this assumption affect bothmethods in the same way. Wolf-Rayet mass-loss rates are surprisinglyuniform across spectral type. We find an average mass-loss rate of 4 x10-5 Mȯ yr-1 for all types observed, except for WC9 stars, whichhave rates that are lower by at least a factor of 2. An alternativeexplanation could be partial recombination of helium from He+ to He0 inthe radio region, which would lead to a reduced number of freeelectrons, and therefore reduced radio flux for WC9 stars. Mass-lossrates of 8 x 10-5 Mȯ yr-1 for late WN stars favored in recentstellar evolution models disagree with the observations of thesesubtypes. The results of this survey suggest that ~40% of all Wolf-Rayetstars with measured spectral index are nonthermal emitters at centimeterwavelengths. This percentage is nearly twice as high as that ofnonthermal emitters among OB stars and is higher than that previouslyestimated for WR stars. The nature of the nonthermal emission is stillnot fully understood. Possible causes of nonthermal emission arediscussed. In particular, we speculate that nonthermal emission mayarise from an interaction between a thermal WR wind and surroundingmaterial owing to a shell ejected during a previous evolutionary stageor owing the wind of a companion star.
| A Survey of Nebulae around Galactic Wolf-Rayet Stars in the Southern Sky. III. Survey Completion and Conclusions
We present the conclusion of a narrow-band optical CCD survey ofWolf-Rayet stars in the southern portion of the Milky Way. In this partof the survey we complete our survey of the southern Galaxy and reportthe detection of 10 new optical nebulae associated with Wolf-Rayetstars. This brings the final survey total to 40 Wolf-Rayet stars withassociated nebulae in 114 southern Galactic fields for a 35% detectionrate. Our results suggest that the Galactic environment has littleapparent effect on the detection rate of nebulae associated withWolf-Rayet stars. Indeed, a more important role in the production ofnebulae is likely to be played by the evolution of the central star. Thesurvey results also suggest a slightly higher incidence of nebuladetection around WN stars over WC stars, although nebulae associatedwith WC stars are noted as being generally larger and some may have beenmissed through being larger than the CCD array used. Indeed, theincreased rate of nebula detection compared to that of a northernGalactic survey can be accounted for solely through the fact that alarger region of sky around the Wolf-Rayet stars was imaged in oursouthern survey as compared to the northern survey. Larger nebulaeexisting around WC as opposed to WN stars are also consistent with thecurrent theory of the evolution of Wolf-Rayet stars from WN to WC.
| 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
| 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.
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| Takýmyýldýz: | Karina |
| Sað Açýklýk: | 10h41m38.32s |
| Yükselim: | -58°46'18.7" |
| Görünürdeki Parlaklýk: | 9.137 |
| özdevim Sað Açýklýk: | -9.6 |
| özdevim Yükselim: | 2.2 |
| B-T magnitude: | 9.546 |
| V-T magnitude: | 9.171 |
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