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Searching for hidden Wolf-Rayet stars in the Galactic plane - 15 new Wolf-Rayet stars
We report the discovery of 15 previously unknown Wolf-Rayet (WR) starsfound as part of an infrared (IR) broad-band study of candidate WR starsin the Galaxy. We have derived an empirically based selection algorithmwhich has selected ~5000 WR candidate stars located within the Galacticplane drawn from the Galactic Legacy Infrared Mid-Plane SurveyExtraordinaire (mid-IR) and Two-Micron All-Sky Survey (near-IR)catalogues. Spectroscopic follow-up of 184 of these reveals 11nitrogen-rich (WN) and four carbon-rich (WC) WR stars. Early WC subtypesare absent from our sample and none shows evidence for circumstellardust emission. Of the candidates which are not WR stars, ~120 displayedhydrogen emission-line features in their spectra. Spectral featuressuggest that the majority of these are in fact Bsupergiants/hypergiants, ~40 of these are identified Be/B[e] candidates.Here, we present the optical spectra for six of the newly detected WRstars, and the near-IR spectra for the remaining nine of our sample.With a WR yield rate of ~7 per cent and a massive star detection rate of~65 per cent, initial results suggest that this method is one of themost successful means for locating evolved, massive stars in the Galaxy.
| Direct constraint on the distance of γ2 Velorum from AMBER/VLTI observations
Context: . Interferometry can provide spatially resolved observations ofmassive star binary systems and their colliding winds, which thus farhave been studied mostly with spatially unresolved observations. Aims: . We present the first AMBER/VLTI observations, taken at orbitalphase 0.32, of the Wolf-Rayet and O (WR+O) star binary systemγ2 Velorum and use the interferometricobservables to constrain its properties. Methods: . TheAMBER/VLTI instrument was used with the telescopes UT2, UT3, and UT4 onbaselines ranging from 46 m to 85 m. It delivered spectrally dispersedvisibilities, as well as differential and closure phases, with aresolution R=1500 in the spectral band 1.95-2.17 μm. We interpretthese data in the context of a binary system with unresolved components,neglecting in a first approximation the wind-wind collision zone fluxcontribution. Results: . Using WR- and O-star synthetic spectra,we show that the AMBER/VLTI observables result primarily from thecontribution of the individual components of the WR+O binary system. Wediscuss several interpretations of the residuals, and speculate on thedetection of an additional continuum component, originating from thefree-free emission associated with the wind-wind collision zone (WWCZ),and contributing at most to the observed K-band flux at the 5% level.Based on the accurate spectroscopic orbit and the Hipparcos distance,the expected absolute separation and position angle at the time ofobservations were 5.1±0.9 mas and 66±15°,respectively. However, using theoretical estimates for the spatialextent of both continuum and line emission from each component, we infera separation of 3.62+0.11-0.30 mas and a positionangle of 73+9-11°, compatible with theexpected one. Our analysis thus implies that the binary system lies at adistance of 368+38-13 pc, in agreement with recentspectrophotometric estimates, but significantly larger than theHipparcos value of 258+41-31 pc. Based on observations collected at the European Southern Observatory,Paranal, Chile, within the guaranteed time programme 074.A-9025(A).
| 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
| Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR140
We use hydrodynamical models of the wind-collision region in thearchetype colliding-wind system WR140 to determine the spatial andspectral distributions of the radio, X-ray, and γ-ray emissionfrom shock-accelerated electrons. Our calculations are for orbital phase0.837 when the observed radio emission is close to maximum. Using theobserved thermal X-ray emission at this phase in conjunction with theradio emission to constrain the mass-loss rates, we find that the O starmass-loss rate is consistent with the reduced estimates for O4-5supergiants by Fullerton, Massa & Prinja, and the wind-momentumratio, η = 0.02. This is independent of the opening angle deducedfrom radio very long baseline interferometry observations of the WCRthat we demonstrate fail to constrain the opening angle.We show that the turnover at ~3 GHz in the radio emission is due tofree-free absorption, since models based on the Razin effect have anunacceptably large fraction of energy in non-thermal electrons. We findthat the spectral index of the non-thermal electron energy distributionis flatter than the canonical value for diffusive shock acceleration,namely p < 2. Several mechanisms are discussed that could lead tosuch an index. Our inability to obtain fits to the radio data with p> 2 does not exclude the possibility of shock modification, butstronger evidence than that which currently exists is necessary for itssupport.Tighter constraints on p and the nature of the shocks in WR140 will beobtained from future observations at MeV and GeV energies, for which wegenerally predict lower fluxes than those in previous works. Since thehigh stellar photon fluxes prevent the acceleration of electrons beyondγ >~ 105-106, TeV emission fromcolliding-wind binary systems will provide unambiguous evidence ofpion-decay emission from accelerated ions. We finish by commenting onthe emission and physics of the multiple wind collisions in densestellar clusters, paying particular attention to the Galactic Centre.
| Models of forbidden line emission profiles from axisymmetric stellar winds
A number of strong infrared forbidden lines have been observed inseveral evolved Wolf-Rayet (WR) star winds, and these are important forderiving metal abundances and testing stellar evolution models. Inaddition, because these optically thin lines form at large radius in thewind, their resolved profiles carry an imprint of the asymptoticstructure of the wind flow. This work presents model forbidden lineprofile shapes formed in axisymmetric winds. It is well known that anoptically thin emission line formed in a spherical wind expanding atconstant velocity yields a flat-topped emission profile shape. Simulatedforbidden lines are produced for a model stellar wind with anaxisymmetric density distribution that treats the latitudinal ionizationself-consistently and examines the influence of the ion stage on theprofile shape. The resulting line profiles are symmetric about linecentre. Within a given atomic species, profile shapes can vary betweencentrally peaked, doubly peaked, and approximately flat-topped inappearance depending on the ion stage (relative to the dominant ion) andviewing inclination. Although application to WR star winds isemphasized, the concepts are also relevant to other classes of hot starssuch as luminous blue variables and Be/B[e] stars.
| The Origin of Structures in Wolf-Rayet Winds: FUSE Observations of WR 135
We report the detection with FUSE of strong, highly blueshiftedabsorption features appearing in the absorption troughs of practicallyall major P Cygni profiles in the presumably single Wolf-Rayet star WR135. These features also appear in the shock-sensitive O VIλλ1032, 1038 doublet, coincident both in time and invelocity space with the rest of the lower ionization species. Choosingbetween two alternative interpretations (large-scale, coherentstructures vs. localized, random shocks), we favor the latter. Theabsolute value of the velocity as well as the velocity dispersion in theshocked region, the density of the shocked gas, and the timescales ofthe observed variability allow us to relate the observed shocks to theincidence of numerous overdense clumps (blobs) in the wind of a hot,massive star.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 Rich Population of X-Ray-emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
Recent optical and infrared studies have revealed that the heavilyreddened starburst cluster Westerlund 1 (Wd 1) contains at least 22Wolf-Rayet (W-R) stars, constituting the richest W-R population of anyGalactic cluster. We present results of a sensitive Chandra X-rayobservation of Wd 1 that detected 12 of the 22 known W-R stars and themysterious emission-line star W9. The fraction of detected WN stars isnearly identical to that of WC stars. The WN stars WR-A and WR-B, aswell as W9, are exceptionally luminous in X-rays and have similar hard,heavily absorbed X-ray spectra with strong Si XIII and S XV emissionlines. The luminous high-temperature X-ray emission of these three starsis characteristic of colliding-wind binary systems, but their binarystatus remains to be determined. Spectral fits of the X-ray-brightsources WR-A and W9 with isothermal plane-parallel shock models requirehigh absorption column densities, log NH=22.56(cm-2), and yield characteristic shock temperatureskTs~3 keV (Ts~35 MK).
| An Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])
We present new Spitzer IRS observations of HD 164270 (WC9, WR103). Aquantitative analysis of the UV, optical, near-, and mid-IR spectra ofHD 164270 is presented, allowing for line blanketing and wind clumping,revealing T*~48 kK, logL/Lsolar~4.9, andM˙~10-5 Msolar yr-1 for a volumefilling factor of f~0.1. Our models predict that He is partiallyrecombined in the outer stellar wind, such that recent radio-derivedmass-loss rates of WC9 stars have been underestimated. We obtainC/He~0.2 and O/He~0.01 by number from optical diagnostics. Mid-IRfine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm areobserved, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. Fromthese we obtain Ne/He~Ne+/He=2.2×10-3 bynumber, 7 times higher than the solar value (as recently derived byAsplund et al.), and S/He~S2+/He=5.1×10-5 bynumber. From a comparison with similar results for other WC subtypes weconclude that WC9 stars are as chemically advanced as earlier subtypes.We consider why late WC stars are exclusively observed inhigh-metallicity environments. In addition, we compare theUV/optical/mid-IR spectroscopic morphology of HD 164270 with theplanetary nebula central star BD +30 3639 ([WC9]). Their UV and opticalsignatures are remarkably similar, such that our quantitativecomparisons confirm similarities in stellar temperature, wind densities,and chemistry first proposed by Smith & Aller, in spite ofcompletely different evolutionary histories, with HD 164270 presently afactor of 10 more massive than BD +30 3639. At mid-IR wavelengths, thedust from the dense young nebula of BD +30 3639 completely dominates itsappearance, in contrast with HD 164270.
| Five WC9 stars discovered in the AAO/UKST Hα survey
We report the discovery of five massive Wolf-Rayet (WR) stars resultingfrom a programme of follow-up spectroscopy of candidate emission-linestars in the Anglo-Australian Observatory United Kingdom SchmidtTelescope (AAO/UKST) Southern Galactic Plane Hα survey. The6195-6775 Åspectra of the stars are presented and discussed. A WC9class is assigned to all five stars through comparison of their spectrawith those of known late-type WC stars, bringing the known total numberof Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibitnear-infrared (NIR) excesses characteristic of hot dust emission (asseen in the great majority of known WC9 stars), we find that two of thestars show no discernible evidence of such excesses. This increases thenumber of known WC9 stars without NIR excesses to seven. Reddenings anddistances for all five stars are estimated.
| On the metallicity dependence of Wolf-Rayet winds
We have performed a pilot study of mass loss predictions for late-typeWolf-Rayet (WR) stars as a function of metal abundance, over a rangebetween 10-5 ≤ (Z/Zȯ) ≤ 10. We findthat the winds of nitrogen-rich Wolf-Rayet stars are dominated by ironlines, with a dependence of mass loss on Z similar to that of massive OBstars. For more evolved, carbon-rich, WR stars the wind strength is alsofound to be dependent on the Fe abundance, so that they depend on thechemical environment of the host galaxy, but with a mass lossmetallicity dependence that is less steep than for OB stars. Our findingthat WR mass loss is Z-dependent is a new one, with importantconsequences for black hole formation and X-ray population studies inexternal galaxies. A further finding of our study is that the Zdependence of C-rich WR stars becomes weaker at metallicities belowZ/Zȯ ⪉ 1/10, and mass loss no longer declines oncethe metal abundance drops below (Z/Zȯ) ≃10-3. This is the result of an increased importance ofradiative driving by intermediate mass elements, such as carbon. Incombination with rapid rotation and/or proximity to the Eddington limit- likely to be relevant for massive Population iii stars - this effectmay indicate a role for mass loss in the appearance and evolution ofthese objects, as well as a potential role for stellar winds inenriching the intergalactic medium of the early Universe.
| A spectroscopic search for the non-nuclear Wolf-Rayet population of the metal-rich spiral galaxy M 83
We present a catalogue of non-nuclear regions containing Wolf-Rayetstars in the metal-rich spiral galaxy M 83 (NGC 5236). From a total of283 candidate regions identified using He ii λ4686 imaging withVLT-FORS2, Multi Object Spectroscopy of 198 regions was carried out,confirming 132 WR sources. From this sub-sample, an exceptional contentof ~1035 ± 300 WR stars is inferred, with N(WC)/N(WN) ~ 1.2,continuing the trend to larger values at higher metallicity amongstLocal Group galaxies, and greatly exceeding current evolutionarypredictions at high metallicity. Late-type stars dominate the WCpopulation of M 83, with N(WC8-9)/N(WC4-7) = 9 and WO subtypes absent,consistent with metallicity dependent WC winds. Equal numbers of late toearly WN stars are observed, again in contrast to current evolutionarypredictions. Several sources contain large numbers of WR stars. Inparticular, #74 (alias region 35 from de Vaucouleurs et al.) contains~230 WR stars, and is identified as a Super Star Cluster from inspectionof archival HST/ACS images. Omitting this starburst cluster would resultin revised statistics of N(WC)/N(WN) ~ 1 and N(WC8-9)/N(WC4-7) ~ 6 forthe "quiescent" disk population. Including recent results for thenucleus and accounting for incompleteness in our spectroscopic sample,we suspect the total WR population of M 83 may exceed 3000 stars.
| Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD192641
We present the results of an optical spectroscopic study of the massiveWolf-Rayet (WR) binary HD192641 = WR137. These 1986-2000 data cover thedust-formation maximum in 1997. Combining all available measurements ofradial velocities, we derive, for the first time, a spectroscopic orbitwith period 4766 +/- 66 d (13.05 +/- 0.18 yr). The resulting masses,adopting i= 67 °, are MO= 20 +/- 2Msolar forthe O component and MWR= 4.4 +/- 1.5Msolar for theWR component. These appear, respectively, approximately normal and onthe low side for the given spectral types. Analysis of the intensemultisite spectroscopic monitoring in 1999 shows that theCIIIλ5696 and CIVλλ5802/12 lines have the highestintrinsic variability levels. The periodogram analysis yields asmall-amplitude modulation in the absorption troughs of theCIVλλ5802/12 and HeIλ5876 lines with a period of0.83 d, which could be related either to pulsations or large-scalerotating structures as seen in the WN4 star EZ Canis Majoris (WR6).Wavelet analysis of the strong emission lines of CIIIλ5696 andCIVλλ5802/12 enabled us to isolate and follow for severalhours small structures (emission subpeaks) associated with densityenhancements within the wind of the Wolf-Rayet star. Cross-correlatingthe variability patterns seen in different lines, we find a weak butsignificant correlation between the variability in emission lines withdifferent ionization potentials, i.e. in lines formed at differentdistances from the WR stellar core. Adopting a β wind-velocity law,from the motion of individual subpeaks we find β~ 5, which issignificantly larger than the canonical value β~= 1 found in O starwinds.
| Inferring hot-star-wind acceleration from Line Profile Variability
The migration of profile sub-peaks identified in time-monitored opticalemission lines of Wolf-Rayet (WR) star spectra provides a directdiagnostic of the dynamics of their stellar winds via a measured ΔvLOS/Δ t, a line-of-sight velocity change per unittime. Inferring the associated wind acceleration scale from such anapparent acceleration then relies on the adopted intrinsic velocity ofthe wind material at the origin of this variable pattern. Such acharacterization of the Line Emission Region (LER) is in principlesubject to inaccuracies arising from line optical depth effects andturbulence broadening. In this paper, we develop tools to quantify sucheffects and then apply these to reanalyze the LER properties oftime-monitored WR stars. We find that most program lines can be fittedwell with a pure optically thin formation mechanism, that the observedline-broadening is dominated by the finite velocity extent of the LER,and that the level of turbulence inferred through Line ProfileVariability (lpv) has only a minor broadening effect in the overallprofile. Our new estimates of LER velocity centroids are systematicallyshifted outwards closer to terminal velocity compared to previousdeterminations, now suggesting WR-wind acceleration length scales βR* of the order of 10-20 Rȯ, a factor of afew smaller than previously inferred. Based on radiation-hydrodynamicssimulations of the line-driven-instability mechanism, we computesynthetic lpv for Ciii5696 Å for WR 111. The results match wellthe measured observed migration of 20-30 m s-2, equivalent toβ R* 20 Rȯ. However, our modelstellar radius of 19 Rȯ, typical of an O-typesupergiant, is a factor 2-10 larger than generally expected for WR coreradii. Such small radii leave inferred acceleration scales to be moreextended than expected from dynamical models of line driving, whichtypically match a “beta” velocity lawv(r)=v&infy; (1-R*/r)β, withβ ≈ 1-2; but the severity of the discrepancy is substantiallyreduced compared to previous analyses. We conclude with a discussion ofhow using lines formed deeper in the wind would provide a strongerconstraint on the key wind dynamics in the peak acceleration region,while also potentially providing a diagnostic on the radial variation ofwind clumping, an issue that remains crucial for reliable determinationof O-star mass loss rates.
| 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.
| The influence of binaries on galactic chemical evolution
Understanding the galaxy in which we live is one of the greatintellectual challenges facing modern science. With the advent of highquality observational data, the chemical evolution modeling of ourgalaxy has been the subject of numerous studies in the last years.However, all these studies have one missing element which is theevolution of close binaries. Reason: their evolution is very complex andsingle stars only perhaps can do the job. (Un)Fortunately at present weknow that the majority of the observed stars are members of a binary ormultiple system and that certain objects can only be formed throughbinary evolution. Therefore galactic studies that do not account forclose binary evolution may be far from realistic.Because of the large expertise developed through the years in stellarevolution in general and binary evolution in particular at the BrusselsAstrophysical Institute, we found ourselves in a privileged position tobe the first to do chemical evolutionary simulations with the inclusionof detailed binary evolution. The complexity of close binary evolutionhas kept many astronomers from including binary stars into theirstudies. However, it is not always the easiest way of living that givesyou the most excitement and satisfaction.
| Circular Polarization of Starlight
Of the 7500 stars cited in the Catalog of starlight polarization, thosewhich satisfy the condition P obs % and A V 0m.5 are selected. It ispresumed that the selected stars (n=216) have circularly polarizedlight.
| Cosmic Rays Acceleration in Wolf-Rayet Stellar Winds
Popescu et al (2004) gave a model for the observed cosmic rays between5×1015 and 3×1018 eV. Their source ispresumed to be the supernova of stars that explode in their winds. Theobserved cosmic rays abundance at the source are affected by spallationin the supernova shell, by the difference in ionization degree (beingone or two times ionized) at the injection in the supernova shock, thestars with initial masses 15MSun≤M≤30MSunhaving a different contribution to them than the stars with30MSun≤M≤50MSun, this being 2:1 for theelements with Z≥6. Still, the abundances after these corrections aredifferent by a factor Zi/ZHe, where Ziis the atomic number for the element i. This paper is dedicated to theexplanation of this factor and its physical meanings by consideringthat, prior to the shock injection, the wind particles are radiativeaccelerated.
| The chemical evolution of the Galaxy: the importance of stars with an initial mass larger than 40 Msolar
In the present paper we investigate in how far stars with an initialmass larger than 40 Msolar affect the chemical enrichment ofthe Galaxy. We illustrate the importance for chemical yields of a mostup-to-date treatment of the various stellar wind mass loss episodes instellar evolutionary codes and we discuss the effects of a possiblesupernova-like outburst prior to massive black hole formation.
| 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.
| The effects of binaries on the evolution of Wolf-Rayet type spectral features in starbursts
In the present paper we investigate in detail the effects of binarieswith initial period between 1 day and 10 years on theoreticalsimulations of Wolf-Rayet (WR) type spectral features in starbursts. Wefocus on the evolution of the nebular Hbeta line instarburst in general, on the intensity ratios I(nebular He II;lambda4686)/I(Hbeta ), I(blue bump)/I(Hbeta ), and I(redbump)/I(Hbeta ) as a function of the equivalent width ofHbeta of WR galaxies in particular. The binary evolutionaryprocesses that dominate the evolution of the considered spectralfeatures are the Roche lobe overflow in case Br systems, the masstransfer efficiency, and the merger rate. We show that the predictionson nebular He II depend critically on the uncertainties in the theory ofWR atmospheres and particularly on uncertainties in the treatment of thesubsonic velocity region of the WR wind. The observations of lowmetallicity starbursts are best reproduced by a theoretical model with asignificant number of binaries and with a metallicity-dependent WR wind.
| Optical line emission from the supernova remnant G 73.9+0.9
Flux calibrated images of the field around the known supernova remnant G73.9+0.9 in the Hiα+[N Ii], [S Ii], [O Ii], and [O Iii] emissionlines are presented. The low ionization images are characterized bydiffuse emission both within and outside the extent of the remnant. Theflux calibrated images revealed a few small scale structures in the eastareas of G 73.9+0.9. The long-slit spectra identify the emission fromone of them as emission from shock heated gas. This patchy structure is 3 arcmin long and emits Hα flux at a level of 33× 10-17 erg s-1 cm-2arcsec-2. The bright diffuse arc-like structure in the centerof the field seems to be associated to G 73.9+0.9 given its spectralsignature and positional relation with the non-thermal radio emission. A8 arcmin long filamentary structure with an absolute Hα fluxof 9 × 10-17 erg s-1 cm-2arcsec-2 is detected in the [O Iii] emission line to thesouth of G 73.9+0.9 but is probably unrelated. The deep long-slitspectra suggest complete recombination zones, shock velocities below 90km s-1, low electron densities (<50 cm-3) andnon-negligible magnetic field strengths.
| Wind inhomogeneities in [WC] central stars of planetary nebulae
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| MASSIVE STARS IN THE LOCAL GROUP: Implications for Stellar Evolution and Star Formation
The galaxies of the Local Group serve as important laboratories forunderstanding the physics of massive stars. Here I discuss what isinvolved in identifying various kinds of massive stars in nearbygalaxies: the hydrogen-burning O-type stars and their evolved He-burningevolutionary descendants, the luminous blue variables, red supergiants,and Wolf-Rayet stars. Primarily I review what our knowledge of themassive star population in nearby galaxies has taught us about stellarevolution and star formation. I show that the current generation ofstellar evolutionary models do well at matching some of the observedfeatures and provide a look at the sort of new observational data thatwill provide a benchmark against which new models can be evaluated.
| A hidden population of Wolf-Rayet stars in the massive galactic cluster Westerlund 1
We report the discovery of a hitherto undetected population ofWolf-Rayet stars in the young galactic open cluster Westerlund1. Optical spectroscopy of the cluster identified 11 suchobjects; provisional classification suggests that 6 are nitrogen rich(WN) and 5 carbon rich (WC). Including the previously identified Blue,Yellow & Red Super- & Hypergiants, Westerlund1 clearly has a very rich population of massive post-MainSequence objects. To date, the post-MS population of Westerlund1 is significantly larger than that of any other galacticyoung open cluster - with the possible exception of theArches - implying that it is potentially amongst themost massive young clusters yet identified in the Local Group. Based onobservations collected at the European Southern Observatory, La Silla,Chile (ESO 67.D-0211).
| A new Wolf-Rayet star in Cygnus
We report the discovery of a new Wolf-Rayet star in the direction ofCygnus. The star is strongly reddened but quite bright in the infrared,with J = 9.22, H = 8.08 and KS = 7.09 (2MASS). On the basisof its H + K spectrum, we have classified WR 142a a WC8 star. We haveestimated its properties using as a reference those of other WC8 starsin the solar neighbourhood as well as those of WR 135, whosenear-infrared spectrum is remarkably similar. We thus obtain aforeground reddening of AV =~ 8.1 mag, MJ =~ -4.3,log (L/Lsun) ~ 5.0-5.2, R = 0.8 Rsun, T =~ 125 000K, M = 7.9-9.7 Msun, {dot M} = (1.4-2.3) x 10-5Msun yr-1. The derived distance modulus, DM = 11.2+/- 0.7 mag, places it in a region occupied by several OB associationsin the Cygnus arm, and particularly in the outskirts of both Cygnus OB2and Cygnus OB9. The position in the sky alone does not allow us tounambiguously assign the star to either association, but based on themuch richer massive star content of Cygnus OB2 membership in this latterassociation appears to be more likely. Based on observations collectedat the German-Spanish Astronomical Centre, Calar Alto, operated by theMax-Planck-Institut für Astronomie, Heidelberg, jointly with theSpanish National Commission for Astronomy.
| Modelling the colliding-winds spectra of the 19-d WR + OB binary in the massive triple system θ Muscae
High signal-to-noise ratio, moderate-resolution spectra, providing fullphase coverage of the 19-d WC6 + OB binary θ Mus (WR 48, HD113904), have been obtained and show dramatic variations of the CIIIλ5696 emission-line profile. We have modelled these lineprofile variations using a purely geometrical model which assumes thatthe emission arises from two regions, an optically thin spherical shellaround the WR star and a cone-shaped region that partially wraps aroundthe OB star. The cone-shaped region represents the shock front arisingfrom the collision between the winds of the two stars. This work buildsupon our earlier study of WR 42 and WR 79, and uses a completely newcode for the modelling, which includes the effects of turbulence. We nowfind much better agreement between the orbital inclination angles foundfor these stars with those determined using other methods. The fittingparameters found via modelling the C IIIλ5696 profile variationsof θ Mus are used to infer that the OB companion most likely hasa spectral type of O6V or O7V. The modelling presented here continues toshow the exciting promise of a better understanding of WR starfundamental parameters.
| 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.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| 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
| 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.
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Observation and Astrometry data
| Constellation: | Schwan |
| Right ascension: | 20h11m53.53s |
| Declination: | +36°11'50.5" |
| Apparent magnitude: | 8.104 |
| Distance: | 2173.913 parsecs |
| Proper motion RA: | -1.7 |
| Proper motion Dec: | -4.2 |
| B-T magnitude: | 8.33 |
| V-T magnitude: | 8.123 |
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INHABITED SKY
