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The Wolf-Rayet Stars HD 4004 and HD 50896: Two of a Kind
We present the results of the analysis of 151 spectra of the Wolf-Rayetstar HD 4004 (WR1) obtained in 1999 and in 2005. The line-profilevariability is found to be periodic, with P=7.684 days, and to havecharacteristics that are very similar to those of another variable WRstar, HD 50896 (WR6). The similarities point toward a common physicalphenomenon in both systems. Of the scenarios that can explain theobservations, such as colliding winds in two stars with similar windmomenta and the ejection of streams or jets from two opposite locationson the stellar surface, the latter seems more likely due to the lack ofobservational evidence for a strong wind-bearing companion.
| 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
| The Galactic WN stars. Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
Context: .Very massive stars pass through the Wolf-Rayet (WR) stagebefore they finally explode. Details of their evolution have not yetbeen safely established, and their physics are not well understood.Their spectral analysis requires adequate model atmospheres, which havebeen developed step by step during the past decades and account in theirrecent version for line blanketing by the millions of lines from ironand iron-group elements. However, only very few WN stars have beenre-analyzed by means of line-blanketed models yet. Aims: .Thequantitative spectral analysis of a large sample of Galactic WN starswith the most advanced generation of model atmospheres should provide anempirical basis for various studies about the origin, evolution, andphysics of the Wolf-Rayet stars and their powerful winds. Methods:.We analyze a large sample of Galactic WN stars by means of the PotsdamWolf-Rayet (PoWR) model atmospheres, which account for iron lineblanketing and clumping. The results are compared with a syntheticpopulation, generated from the Geneva tracks for massive starevolution. Results: .We obtain a homogeneous set of stellar andatmospheric parameters for the Galactic WN stars, partly revisingearlier results. Conclusions: .Comparing the results of ourspectral analyses of the Galactic WN stars with the predictions of theGeneva evolutionary calculations, we conclude that there is roughqualitative agreement. However, the quantitative discrepancies are stillsevere, and there is no preference for the tracks that account for theeffects of rotation. It seems that the evolution of massive stars isstill not satisfactorily understood.
| Multiwavelength studies of WR 21a and its surroundings
We present results of high-resolution radio continuum observationstowards the binary star WR 21a (Wack 2134) obtained with the AustraliaTelescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected thesystem at 4.8 GHz (6 cm) with a flux density of 0.25±0.06 mJy andset an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectralindex of α < 0.3 (Sν ∝να) suggests the presence of non-thermal emission,probably originating in a colliding-wind region. A second, unrelatedradio source was detected ~10 arcsec north of WR 21a at (RA,Dec)J2000=(10h25m56.49s, -57°48arcmin34.4 arcsec), with flux densities of 0.36 and 0.55 mJy at 4.8 and8.64 GHz, respectively, resulting in α = 0.72. H i observations inthe area are dominated by absorption against the prominent H ii regionRCW 49. Analysis of a complete set of archived X-ray observations of WR21a confirms its strong variability but throws into doubt previoussuggestions by Reig (1999) of a period of years for the system. Finally,we comment on the association with the nearby EGRET source 3EGJ1027-5817.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| An XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| 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.
| Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data
In this paper, we present new results from an extensive set of HubbleSpace Telescope (HST) and ground-based observations of the Seyfertgalaxy NGC 2110. The HST data sets include Wide Field Planetary Camera 2(WFPC2) observations as well as the first high-spatial resolutionspectroscopy ([OI], [NII], Hα and [SII] lines) of this galaxyobtained using the Space Telescope Imaging Spectrograph (STIS). Theground-based data are three-dimensional (x, y, λ) spectrographicobservations obtained using the integral field spectrograph OASIS on theCanada-France-Hawaii Telescope, complemented by near-infrared long-slitspectroscopy obtained using NIRSPEC on the Keck-II telescope. The OASISobservations cover regions containing both stellar absorption lines andmajor optical emission lines. The NIRSPEC observations cover the H and Kbands.Combined with archival HST observations, the WFPC2 data provide us witha high-spatial resolution extinction map. The OASIS data allowedbidimensional mapping of the stellar and gaseous kinematics, as well asof the spectral properties of the ionized gas. These results arecompared to those obtained in the near-infrared with NIRSPEC/Keck. Last,we used the STIS data to probe the ionized gas kinematics and propertiesin the inner 4arcsec along PA = 156° at unprecedented spatialresolution.Our two-dimensional (2D) map of the stellar velocity field and thenear-infrared stellar velocity profile are symmetric about the nucleus,confirming the results of previous long-slit observations. The asymmetryof the velocity field of the ionized gas is present at the same levelfor visible and near-infrared lines, indicating this is not a reddeningeffect. Multiple Gaussian fitting of the emission-line profile allowedthe contributions of the broad and a narrow components to bedisentangled. The intensity peak of the [OIII] narrow component islocated north of the nucleus, indicating that the bulk of the narrow[OIII] emission comes from the jet-like structure (Mulchaey et al.) andnot from the nucleus itself. We suggest that the northern arm is theanomalous one, contrary to what has been claimed earlier. Last, we alsoshow that the elongated region of high gas velocity dispersion locatedclose to the nucleus and discovered by Gonzàlez Delgado et al. isintrinsic to the narrow component.
| XMM-Newton observations of the nitrogen-rich Wolf-Rayet star WR 1
We present XMM-Newton results for the X-ray spectrum from the N-richWolf-Rayet (WR) star WR 1. The EPIC instrument was used to obtain amedium-resolution spectrum. The following features characterize thisspectrum: (a) significant emission ``bumps'' appear that are coincidentwith the wavelengths of typical strong lines, such as MgXI, SiXIII, andSXV; (b) little emission is detected above 4 keV, in contrast to recentreports of a hard component in the stars WR 6 and WR 110 which are ofsimilar subtype; and (c) evidence for sulfur K-edge absorption at about2.6 keV, which could only arise from absorption of X-rays by the ambientstellar wind. The lack of hard emission in our dataset is suggestivethat WR 1 may truly be a single star, thus representing the firstdetailed X-ray spectrum that isolates the WR wind alone (in contrast tocolliding wind zones). Although the properties of the S-edge are notwell-constrained by our data, it does appear to be real, and itsdetection indicates that at least some of the hot gas in WR 1 mustreside interior to the radius of optical depth unity for the totalabsorptive opacity at the energy of the edge.
| 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.
| 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.
| X-rays from Wolf-Rayet Stars
We describe observations of several apparently single Wolf-Rayet starsthat have now been observed with the XMM-Newton instrument. Prior tothis, the X-ray data of Wolf-Rayet stars consisted primarily of ROSATpassband detections (many of which were marginal). Data for thenitrogen-rich stars WR 6 (EZ CMa; HD50896) and WR 110 have beendescribed by Skinner etal, who unexpectedly found a hard component inthe spectra. We report on a third WN star, WR 1, which also shows thehint of a hard tail. In our program we also observed the carbon-richstar WR 114. Even after 19000 seconds of integration, we were unable todetect this star, which places a 1-sigma upper limit to the ratio ofX-ray to Bolometric luminosity of 10-8.4, about 25 timessmaller than typical O stars. (This research has been supported by NASAgrant NAG5-12557.)
| XMM-Newton and Very Large Array Observations of the Variable Wolf-Rayet Star EZ Canis Majoris: Evidence for a Close Companion?
We present new X-ray and radio observations of the Wolf-Rayet star EZCMa (HD 50896) obtained with XMM-Newton and the Very Large Array (VLA).This WN4 star exhibits optical and UV variability at a period of 3.765days whose cause is unknown. Binarity may be responsible, but theexistence of a companion has not been proven. The radio spectral energydistribution of EZ CMa determined from VLA observations at fivefrequencies is in excellent agreement with predictions for free-freewind emission, and the ionized mass-loss rate allowing for distanceuncertainties is M=3.8(+/-2.6)×10-5 Msolaryr-1. The CCD X-ray spectra show prominent Si XIII and S XVemission lines and can be acceptably modeled as an absorbedmultitemperature optically thin plasma, confirming earlier ASCA results.Nonsolar abundances are inferred with Fe notably deficient. The X-rayemission is dominated by cooler plasma at a temperaturekTcool~0.6 keV, but a harder component is also detected, andthe derived temperature is kThot~3.0-4.2 keV if the emissionis thermal. This is too high to be explained by radiative wind shockmodels, and the X-ray luminosity of the hard component is 3 orders ofmagnitude lower than expected for accretion onto a neutron starcompanion. We show that the hard emission could be produced by theWolf-Rayet wind shocking onto a normal (nondegenerate) stellar companionat close separation. Finally, using comparable data sets we demonstratethat the X-ray and radio properties of EZ CMa are strikingly similar tothose of the WN5-6 star WR 110. This similarity points to common X-rayand radio emission processes in WN stars and discredits the idea that EZCMa is anomalous within its class.
| 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.
| Terminal Speeds and Ion Fractions from [Ca IV] 3.207 μm in Three Single WN Stars
We report on the forbidden emission lines of [Ca IV] at 3.207 μm and[Ne III] at 15.56 μm based on Infrared Space Observatoryobservations. The data set consists of three single Wolf-Rayet stars (WR1, WR 134, and WR 136) and three binary systems (WR 11, WR 146, and WR147). For the single stars, only the [Ca IV] line was observed. Ourprimary objectives are to determine the wind terminal speed from theemission-line widths, derive ion number fractions, and discuss therelation between emission profile shapes and wind flow geometry.Compared to previous determinations of the terminal speed, we findvalues that are similar or somewhat smaller. Interestingly, the linewidth at the continuum level is typically 10%-30% broader than at thehalf-maximum level. The extra broadness suggests the effect ofturbulence in the wind. For the ion fraction, we assume that calcium isnot enriched in any of the W-R winds. Ion fractions of Ca+3and Ne+2 are derived for the binary systems and found to beconsistent with previously published values. For the new observations ofthe single WN stars, all of the ion fractions for Ca+3 havesimilar values, which are close to the maximum expected value, thussuggesting that Ca+3 is the dominant ion at large radii inthe wind. Finally, the line profile shapes in the binaries WR 11, WR146, and WR 147 appear asymmetric, as might be expected from suchsystems. For WR 136, the [Ca IV] profile is flat-topped, consistent witha spherical flow. For WR 1 and WR 134, the [Ca IV] profiles are too weakto draw conclusions about the wind flow geometry. Based on observationswith ISO, an ESA project with instruments funded by ESA member states(especially the PI countries: France, Germany, the Netherlands, and theUnited Kingdom) and with the participation of ISAS and NASA.
| The 76th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1406 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| Modelling X-ray variability in the structured atmospheres of hot stars
We describe X-ray production in the atmospheres of hot, early-type starsin the framework of a ``stochastic shock model''. The extended envelopeof a star is assumed to possess numerous X-ray emitting ``hot'' zonesthat are produced by shocks and embedded in the ambient ``cold'' mediumin dynamical equilibrium. It is shown that the apparent lack of X-rayvariability on short ( ~ hours) timescales do not contradict a shockmodel for X-ray production. The character of the X-ray variability isfound to depend on the frequency with which hot zones are generated, thecool wind opacity to X-rays, and the wind flow parameters, such as massloss rate and terminal speed.
| A Mid-Infrared Spectral Survey of Galactic Wolf-Rayet Stars
We present 8-13 μm spectra at resolution R~600 of 29 northernGalactic Wolf-Rayet stars, including the first ever reportedmid-infrared (MIR) spectrum for many. Among the subtypes of the starsstudied were 14 WC, 13 WN, 1 WN/WC, and an additional reclassified WN.Lines of He I and He II, along with fine-structure lines of Ne II and SIV, are strongly present in 22 of the sources observed, while six of thesources exhibit the powerful emission of heated circumstellar carbondust. We point out similarities between our spectra and Infrared SpaceObservatory (ISO) observations of several of the same sources and notean unresolved discrepancy between the two data sets for the WC6 star WR146. We investigate the diagnostic power of MIR He I and He II lines forsubtype discrimination and find the line ratio Wλ(9.7μm He II)/Wλ(11.3 μm He I+He II) can providemoderate discrimination within the WN and WC types, though the smallnumber of stars with corresponding line pairs detected made suchassessment difficult.
| The VIIth catalogue of galactic Wolf-Rayet stars
The VIIth catalogue of galactic PopulationI Wolf-Rayet stars providesimproved coordinates, spectral types and /bv photometry of known WRstars and adds 71 new WR stars to the previous WR catalogue. This censusof galactic WR stars reaches 227 stars, comprising 127 WN stars, 87 WCstars, 10 WN/WC stars and 3 WO stars. This includes 15 WNL and 11 WCLstars within 30 pc of the Galactic Center. We compile and discuss WRspectral classification, variability, periodicity, binarity, terminalwind velocities, correlation with open clusters and OB associations, andcorrelation with Hi bubbles, Hii regions and ring nebulae. Intrinsiccolours and absolute visual magnitudes per subtype are re-assessed for are-determination of optical photometric distances and galacticdistribution of WR stars. In the solar neighbourhood we find projectedon the galactic plane a surface density of 3.3 WR stars perkpc2, with a WC/WN number ratio of 1.5, and a WR binaryfrequency (including probable binaries) of 39%. The galactocentricdistance (RWR) distribution per subtype shows RWRincreasing with decreasing WR subtype, both for the WN and WC subtypes.This RWR distribution allows for the possibility ofWNE-->WCE and WNL-->WCL subtype evolution.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| 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.
| Spectroscopy of HD 4004. Correlated profile variations?
We present results of detailed analysis of spectral variability observedin the optical spectrum (lambda lambda 4350-5700 Å) of HD 4004 (WR1). Basing on data obtained during spectroscopic monitoring of thisobject in three nights in September 1994, and six nights in November1995 we show that the variations observed in He Ii lines lambda 4542,4686, 4860 and 5411 are very similar and well correlated. We also showthat the amount of variability in these lines is identical. We provideevidence, however, that the variations as observed in those He Ii showalso noticeable differences. The projected velocities of perturbationsexposed on He Ii emission line profiles do not scale with line width(FWHM). This means that the factor responsible for the observed profilevariations is not coupled with the wind and its velocity law. Based onour results we argue for two-component (fast-slow) wind model of HD4004. Based on observations obtained with the 1.22 ``Galileo'' telescopeof the Padova University in Asiago (Italy).
| Blobs in Wolf-Rayet Winds: Random Photometric and Polarimetric Variability
Some isolated Wolf-Rayet stars present random variability in theiroptical flux and polarization. We make the assumption that suchvariability is caused by the presence of regions of enhanceddensity-i.e., blobs-in their envelopes. In order to find the physicalcharacteristics of such regions, we have modeled the stellar emissionusing a Monte Carlo code to treat the radiative transfer in aninhomogeneous electron scattering envelope. We are able to treatmultiple scattering in the regions of enhanced density as well as in theenvelope itself. The finite sizes of the source and structures in thewind are also taken into account. Most of the results presented here arebased on a parameter study of models with a single blob. The effectscaused by multiple blobs in the envelope are considered to a morelimited extent. Our simulations indicate that the density enhancementsmust have a large geometric cross section in order to produce theobserved photopolarimetric variability. The sizes must be of the orderof 1 stellar radius, and the blobs must be located near the base of theenvelope. These sizes are the same inferred from the widths of thesubpeaks in optical emission lines of Wolf-Rayet stars. Other early-typestars show random polarimetric fluctuations with characteristics similarto those observed in Wolf-Rayet stars, which may also be interpreted interms of a clumpy wind. Although the origin of such structures is stillunclear, the same mechanism may be working in different types of hotstar envelopes to produce such inhomogeneities.
| 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.
| Spectroscopy of HD 4004. Search for fast spectral variability
We present results of a detailed analysis of spectral variabilityobserved in the optical spectrum of HD 4004 (WR1). On the basis of dataobtained during monitoring of this object on 3 consecutive nights in Dec1993, and two additional nights in Dec 1994 we show that the variabilityin He Ii lambda 5411, C Iv lambda 5808 and He I lambda 5876 may reach asmuch as 50% in equivalent width. This variability shows a systematic,gradual character, which justifies a search for periodicity. We showthat the observed spectral-line-profile changes may mimic radialvelocity variations but that any real radial velocity variations in theobserved lines of HD 4004 are small. Our search for periodicity in thedata was unsuccessful: the data do not show periodicities within thetime covered by the observations ( ~ 2 days). Our data suggest, however,the existence of a longer-term variability pattern. We show that theobserved pattern of the Temporal Variance Spectrum in different linescan be qualitatively understood in terms of stratification. Based onobservations obtained with the 1.82 ``Copernicus'' telescope of thePadova Astronomical Observatory in Mt. Ekar (Italy).
| Book Review: Stars / Freeman, 1998
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| An investigation of the large-scale variability of the apparently single Wolf-Rayet star WR 1
In recent years, much studies have focused on determining the origin ofthe large-scale line-profile and/or photometric patterns of variabilitydisplayed by some apparently single Wolf-Rayet stars, with the existenceof an unseen (collapsed?) companion or of spatially extended windstructures as potential candidates. We present observations ofWR 1 which highlight the unusual character of thevariations in this object. Our narrowband photometric observationsreveal a gradual increase of the stellar continuum flux amounting toDelta v ~ 0.09 mag followed by a decline on about the same timescale(3-4 days). Only marginal evidence for variability is found during the11 following nights. Strong, daily line-profile variations are alsoobserved but they cannot be easily linked to the photometric variations.Similarly to the continuum flux variations, coherent time-dependentchanges are observed in 1996 in the centroid, equivalent width, andskewness of He Ii lambda 4686. Despite the generally coherent nature ofthe variations, we do not find evidence in our data for the periodsclaimed in previous studies. While the issue of a cyclical pattern ofvariability in WR 1 is still controversial, it isclear that this object might constitute in the future a cornerstone forour understanding of the mechanisms leading to the formation of largelyanisotropic outflows in Wolf-Rayet stars.
| Wind Inhomogeneities in Wolf-Rayet Stars. III. Unusual Emission-Line Profile Variations in gamma^2 Velorum
We present very high resolution, very high signal-to-noise ratio spectrafrom the 3.6 m Canada-France-Hawaii telescope for the strong C IIIlambda5696 emission line in gamma^2 Velorum, the brightest Wolf-Rayet(WR) star in the sky. From two nights of spectroscopic monitoring, wehave detected line profile variations (LPVs) in the form of movingemission features (subpeaks) on the broad, ``flat-topped'' underlyingprofile. We use the working model from the previous paper in this seriesto study these LPVs in terms of inhomogeneities in the radiallyexpanding stellar wind. A comparison between simulations and the datasuggests the variable subpeaks to consist of a superposition of twodistinct components: (1) narrow-moving subpeaks with a meanline-of-sight velocity dispersion sigma_xi~=80 km s^-1, and (2)broad-moving subpeaks with sigma_xi~=200 km s^-1. Both narrow and broadsubpeak components are seen to move systematically in a direction fromthe line center (xi=0) toward the line edges. This motion is found to beconsistent with a radial wind expansion at a mean acceleration ratea_r=13+/-3 m s^-2. The narrow subpeaks are found to be similar to thestochastic subpeaks seen in the LPVs from several single and long-periodbinary WR stars. On the other hand, the broad features are reminiscentof the recurrent subpeaks observed in the LPVs from a few peculiar WRstars (HD 4004, HD 191765, and EZ CMa), although in gamma^2 Vel, we findno evidence for a recurrent behavior. We investigate the possibilitythat, in gamma^2 Vel, these broad variable subpeaks arise from theshock-cone region, at the interface where the wind from the WR starcollides with that of its O companion. We find no convincing evidence tosupport this hypothesis, although we suspect that there could be arelation between the existence of broad LPV subpeaks and the presence ofthe more massive O companion.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| The Optical Spectrum of HD 4004 (WN4b). Evidence of Variability
We describe the optical spectrum of Wolf-Rayet star HD 4004 (WN4b) inthe wavelength range from 3900 to 7000 A. Present observations revealthe stratification effects in the envelope of this star. Also evidencefor variability of HeII line lambda 5411 is presented. Since no radialvelocity variations are observed in the optical spectrum of this star weconclude that HD 4004 is a single Wolf-Rayet star and the observedemission line profile changes are of intrinsic nature.
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