Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| 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.
| Cleaning Up η Carinae: Detection of Ammonia in the Homunculus Nebula
We report the first detection of ammonia in the Homunculus Nebula aroundη Carinae, which is also the first detection of emission from apolyatomic molecule in this or any other luminous blue variable (LBV)nebula. Observations of the NH3 (J,K)=(3,3) inversiontransition made with the Australia Telescope Compact Array revealemission at locations where infrared H2 emission had beendetected previously, near the strongest dust emission in the core of theHomunculus. We also detect ammonia emission from the so-called strontiumfilament in the equatorial disk. The presence of NH3 aroundη Car hints that molecular shells around some Wolf-Rayet stars couldhave originated in prior LBV eruptions, rather than in cool redsupergiant winds or the ambient interstellar medium. Combined with thelack of any CO detection, NH3 seems to suggest that theHomunculus is nitrogen-rich like the ionized ejecta around η Car. Italso indicates that the Homunculus is a unique laboratory in which tostudy unusual molecule and dust chemistry, as well as their rapidformation in a nitrogen-rich environment around a hot star. We encouragefuture observations of other transitions like NH 3 (1, 1) and(2, 2), related molecules like N2H+, and renewedattempts to detect CO.
| 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.
| 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.
| 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
| Kinematical Structure of Wolf-Rayet Winds. I.Terminal Wind Velocity
New terminal wind velocities for 164 Wolf-Rayet stars (from the Galaxyand LMC) based on PCyg profiles of lambda1550 CIV resonance line werederived from the archive high and low resolution IUE spectra availableform the INES database. The high resolution data on 59 WR stars (39 fromthe Galaxy and 20 from LMC) were used to calibrate the empiricalrelation lambda_min^Abs- lambda_peak^Emis vs terminal wind velocity,which was then used for determinations of the terminal wind velocitiesfrom the low resolution IUE data. We almost doubled the previous mostextended sample of such measurements. Our new measurements, based onhigh resolution data, are precise within 5-7%. Measurements, based onthe low resolution spectra have the formal errors of approx 40-60%. Acomparison of the present results with other determinations suggestshigher precision of approx 20%. We found that the terminal windvelocities for the Galactic WC and WN stars correlate with the WRspectral subtype. We also found that the LMC WN stars have winds slowerthan their Galactic counterparts, up to two times in the case of the WNEstars. No influence of binarity on terminal wind velocities was found.Our extended set of measurements allowed us to test application of theradiation driven wind theory to the WR stars. We found that, contrary toOB stars, terminal wind velocities of the WR stars correlate only weaklywith stellar temperature. We also note that the terminal to escapevelocity ratio for the WR stars is relatively low: 2.55 pm 1.14 for theGalactic WN stars and 1.78 pm 0.70 for the Galactic WCs. This ratiodecreases with temperature of WR stars, contrary to what is observed inthe case of OB stars. The presented results show complex influence ofchemical composition on the WR winds driving mechanism efficiency. Ourkinematical data on WR winds suggest evolutionary sequence: WNL -->WNE --> WCE --> WCL.
| First Detections of Molecular Gas Associated with the Wolf-Rayet Ring Nebula NGC 3199
This paper presents the first observations of molecular gas associatedwith the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. Thisincludes first observations of the molecules HCN, HCO+, CN,and HNC seen in any Wolf-Rayet ring nebula. Our observations immediatelysuggest the presence of high-density molecular gas (>104cm-3) in the nebula with significant amounts of associatedmolecular gas, which is in the form of clumpy ejecta and/or interstellarmaterial. Molecular CO gas was mapped across the optically brightportion of the nebula and out into the diffuse ionized component usingthe 12CO J=1-->0 line. CO gas is not seen within theoptically bright rim of NGC 3199 but adjacent to it. The opticalemission rim therefore appears to mark regions of photodissociation.Velocity components in the CO data are consistent with those seen inhigh-resolution optical spectra of the Hα line but extend beyondthe visible emission. A prior suggestion of the formation of the nebulavia a bow shock appears unlikely since Hipparcos measurements show theproper motion of WR 18 is almost at right angles to the directionrequired for the bow shock model. Instead, line splitting toward thenorth of the nebula suggests that a possible blowout of the Wolf-Rayetwind through surrounding ejecta may be responsible for some of thevelocity features observed. Preliminary estimates of molecularabundances in the nebula seen toward the central star are significantlyhigher than for the interstellar medium and are similar to those inplanetary nebulae, although CN is distinctly underabundant in comparisonto the very high values found in many planetary nebulae. The abundancesfound are consistent with the idea that at least a portion of themolecular material is associated with ejecta from the central star.Based on observations collected at the Swedish-ESO SubmillimetreTelescope (SEST) at the European Southern Observatory, La Silla, Chile.The Swedish-ESO Submillimetre Telescope is operated jointly by theEuropean Southern Observatory (ESO) and the Swedish National Facilityfor Radio Astronomy, Onsala Space Observatory, at Chalmers University ofTechnology.
| Hubble Space Telescope Imagery and Canada-France-Hawaii Telescope Fabry-Perot Two-dimensional Spectroscopy in Hα of the Ejected Nebula M1-67: Turbulent Status
Bright circumstellar nebulae around massive stars are potentially usefulto derive time-dependent mass-loss rates and hence constrain theevolution of the central stars. A key case in this context is therelatively young ejection-type nebula M1-67 around the runawayPopulation I Wolf-Rayet star WR 124 (=209 BAC), which exhibits a WN 8spectrum. With HST-WFPC2 we have obtained a deep, Hα image ofM1-67. This image shows a wealth of complex detail that was brieflypresented previously by Grosdidier et al. With the interferometer of theUniversité Laval (Québec, Canada), we have obtainedcomplementary Fabry-Perot Hα data using Canada-France-HawaiiTelescope (CFHT) MOS/SIS. From these data M1-67 appears more or less asa spherical (or elliptical, with the major axis along the line ofsight), thick, shell seen almost exactly along its direction of rapidspatial motion away from the observer in the ISM. However, a simplethick shell by itself would not explain the observed multiple radialvelocities along the line of sight. This velocity dispersion leads oneto consider M1-67 as a thick accelerating shell. Given the extremeperturbations of the velocity field in M1-67, it is virtually impossibleto measure any systematic impact of the present WR (or previous LBV)wind on the nebular structure. The irregular nature of the velocityfield is likely due to either large variations in the densitydistribution of the ambient ISM or large variations in the central starmass-loss history. In addition, either from the density field or thevelocity field, we find no clear evidence for a bipolar outflow, as wasclaimed in other studies. On the deep Hα image we have performedcontinuous wavelet transforms to isolate stochastic structures ofdifferent characteristic size and look for scaling laws. Small-scalewavelet coefficients show that the density field of M1-67 is remarkablystructured in chaotically (or possibly radially) oriented filamentseverywhere in the nebula. We draw attention to a short, marginallyinertial range at the smallest scales (6.7-15.0×10-3pc), which can be attributed to turbulence in the nebula, and a strongscale break at larger scales. Examination of the structure functions fordifferent orders shows that the turbulent regime may be intermittent.Using our Fabry-Perot interferograms, we also present an investigationof the statistical properties of fluctuating gas motions using structurefunctions traced by Hα emission-line centroid velocities. We findthat there is a clear correlation at scales 0.02-0.22 pc between themean quadratic differences of radial velocities and distance over thesurface of the nebula. This implies that the velocity field shows aninertial range likely related to turbulence, though not coincident withthe small inertial range detected from the density field. The first- andsecond-order moments of the velocity increments are found to scale as<|Δv(r)|>~r0.5 and<|Δv(r)|2>~r0.9. The former scalinglaw strongly suggests that supersonic, compressible turbulence is atplay in the nebula; on the other hand, the latter scaling law agreesvery well with Larson-type laws for velocity turbulence. Examination ofthe structure functions for different orders shows that the turbulentregime is slightly intermittent and highly multifractal with universalmultifractal indexes α~1.90-1.92 and C1~0.04+/-0.01.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. Also based on observations collected at theCanada-France-Hawaii Telescope (CFHT), which is operated by CNRS ofFrance, NRC of Canada, and the University of Hawaii.
| CO Observations of NGC 2359: The Molecular Clouds Revisited
Based on CO (2-1) observations obtained with the Swedish-ESOSubmillimeter Telescope, the distribution of molecular materialassociated with the Wolf-Rayet ring nebula NGC 2359 has been determined.The angular resolution is 22", and the velocity resolution is 0.33 kms-1. Three molecular components are detected in the directionof the nebula. The bulk of the molecular gas is observed at 54 kms-1 and follows the southeastern border of the nebula.Adopting a distance d=5 kpc for NGC 2359, an H2 mass of about1200+/-500 Msolar appears to be related to the southern partof the nebula, while 140+/-50 Msolar are linked to thefilamentary windblown bubble. The volume density of the molecular gasrelated to the southern bar is >=103 H2molecules cm-3, while lower densities were estimated for thematerial associated with the filamentary windblown bubble. From thepresent data, it is not clear if the molecular gas at 37 kms-1 (~=380+/-120 Msolar, d=5 kpc) is associatedwith the nebula, but the molecular material observed at 67 kms-1 (~=70+/-25 Msolar, d=5 kpc) seems to beunconnected. The comparison between ionized, H I, and moleculardistributions indicates that the H I filament detected with the VeryLarge Array is located at the interface between the ionized andmolecular material, and that the H I filament at 54 km s-1has originated in the photodissociation of the H2. Most ofthe molecular gas associated with the filamentary bubble seems to beinterstellar in origin. The dynamics of the nebula is reanalyzed basedon these new molecular results. It is consistent with either momentumconservation or an intermediate stage between energy and momentumconservation.
| The VIIth catalogue of galactic Wolf-Rayet stars
The VIIth catalogue of galactic PopulationI Wolf-Rayet stars providesimproved coordinates, spectral types and /bv photometry of known WRstars and adds 71 new WR stars to the previous WR catalogue. This censusof galactic WR stars reaches 227 stars, comprising 127 WN stars, 87 WCstars, 10 WN/WC stars and 3 WO stars. This includes 15 WNL and 11 WCLstars within 30 pc of the Galactic Center. We compile and discuss WRspectral classification, variability, periodicity, binarity, terminalwind velocities, correlation with open clusters and OB associations, andcorrelation with Hi bubbles, Hii regions and ring nebulae. Intrinsiccolours and absolute visual magnitudes per subtype are re-assessed for are-determination of optical photometric distances and galacticdistribution of WR stars. In the solar neighbourhood we find projectedon the galactic plane a surface density of 3.3 WR stars perkpc2, with a WC/WN number ratio of 1.5, and a WR binaryfrequency (including probable binaries) of 39%. The galactocentricdistance (RWR) distribution per subtype shows RWRincreasing with decreasing WR subtype, both for the WN and WC subtypes.This RWR distribution allows for the possibility ofWNE-->WCE and WNL-->WCL subtype evolution.
| Wolf-Rayet Stars and Relativistic Objects: Distinctions between the Mass Distributions in Close Binary Systems
The observed properties of Wolf-Rayet stars and relativistic objects inclose binary systems are analyzed. The final masses M CO f for thecarbon-oxygen cores of WR stars in WR + O binaries are calculated takinginto account the radial loss of matter via stellar wind, which dependson the mass of the star. The analysis includes new data on the clumpystructure of WR winds, which appreciably decreases the requiredmass-loss rates for the WR stars. The masses M CO f lie in the range (12)M ȯ (20 44)M ȯ and have a continuous distribution. Themasses of the relativistic objects M x are 1 20M ȯ and have abimodal distribution: the mean masses for neutron stars and black holesare 1.35 ± 0.15M ȯ and 8 10M ȯ, respectively, with agap from 2 4M ȯ in which no neutron stars or black holes areobserved in close binaries. The mean final CO-core mass is &$/line M _{CO}(f) = 7.4 - 10.3M_ ȯ$; , close to the mean mass for the black holes. This suggests that it isnot only the mass of the progenitor that determines the nature of therelativistic object, but other parameters as well-rotation, magneticfield, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binariesthat may have low-mass companions (main-sequence or subgiant M-A stars)are identified; these could be the progenitors of low-mass X-raybinaries with neutron stars and black holes.
| Tailored Analyses of the WN 8 Stars WR 40 and WR 16
We present the results of tailored spectral analyses of two WN 8Wolf-Rayet stars: WR 40 (HD 96548) and WR 16 (HD 86161). These analyseswere carried out utilizing line-blanketed non-LTE model atmospheres withprovisions for a clumped wind. For the first time in WN analyses,stellar parameters are derived by attempting to match the entireobserved spectrum from 900 to 35000 Å, including the complex ironpseudocontinuum between 1200-2100 Å. The inclusion of iron andother metals (O, Ne, Mg, Al, Si, S, Ar, and Ca) in the model atmosphereresults in a decrease in derived stellar radii by ~20% from previous,nitrogen-line analyses, while mass-loss rates are decreased by a factorof ~3 owing to clumping. The abundance of iron and other metals areconstrained with uncertainties of about 50%, demonstrating thatWolf-Rayet stars can be used to determine metallicities and metallicitygradients in other galaxies. We find the presence of metalssignificantly increases the radiative line force the wind experiences,allowing the outer wind to be driven to its theoretical velocitystructure and coming within a factor of 5 of the needed line force inthe inner region (an improvement over earlier radiative transfermodels). Coupled with the substantial reduction in the derivedperformance numbers (factor of ~3), this indicates that radiationpressure alone may be sufficient to drive the winds of WN 8 stars. Ourmodel parameters achieve good fits simultaneously to the H, He, and NIII-IV features of the spectra, a feat that has eluded previous efforts.Comprehensive identification of lines and line complexes are provided,and the effects of the inclusion of metals on the strengths ofdiagnostic lines are discussed.
| Non-thermal emission in Wolf-Rayet stars: are massive companions required?
We examine the radio spectral indices of 23 Wolf-Rayet (WR) stars toidentify the nature of their radio emission. We identify nine systems asnon-thermal emitters. In seven of these systems the non-thermal emissiondominates the radio spectrum, while in the remaining two it is ofcomparable strength to the thermal, stellar wind emission, giving`composite' spectra. Among these nine systems, seven have knownspectroscopic or visual binary companions. The companions are allmassive O or early B-type stars, strongly supporting a connectionbetween the appearance of non-thermal emission in WR stars and thepresence of a massive companion. In three of these binaries, the originof non-thermal emission in a wind-collision region between the stars hasbeen well established in earlier work. The binary systems that exhibitonly thermal emission are all short-period systems where awind-collision zone is deep within the opaque region of the stellar windof the WR star. To detect non-thermal emission in these systems requiresoptically thin lines of sight to the wind-collision region.
| 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.
| X-ray Nova Binary Systems
The physical properties of X-ray novae as close binary systems areanalysed. Observational data in X-ray, UV, optical, IR and radio rangesare summarized. Modern theoretical considerations of the problem ofX-ray novae, taking into account some new ideas and results, aredescribed. Properties of optical stars in X-ray novae are analysed. Dataabout the masses of black holes in X-ray binary systems are summarized.Possible evolutionary links between WR stars in close binary systems andX-ray novae are analysed.
| 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.
| The Structure of Wolf-Rayet Winds. II. Observations of Ionization Stratification in the WN Subtype
Motivated by the question of the importance of ionization stratificationin solving the ``momentum problem'' of Wolf-Rayet stellar winds, we havechosen a sample of 14 WN stars for a systematic study. We performedmeasurements of the emission line widths on ultraviolet, optical, andinfrared spectra to obtain data spanning a large range of ionizationpotentials. We provide extensive tables of these measurements as well asline profile classifications. The presence of ionization stratificationin the wind should result in a correlation between ionization potentialand line width. We find most of the winds to be stratified and discussthe level of stratification found in each star. To test the importanceof ionization stratification to efficient radiation-to-wind momentumtransfer, we compare our empirically measured stratification strengthswith two sets of theoretical performance numbers and give thecorrelation statistics in each case.
| The OB Zoo: A Digital Atlas of Peculiar Spectra
A digital atlas of 20 high-luminosity, peculiar OB spectra in the3800-4900 Å range is presented. The atlas is organized anddiscussed in terms of the following four categories: WN-A or WNL stars,OB Iape or very late WN (WNVL) stars, iron stars, and B-supergiantluminous blue variables (LBVs). Several objects in the earliercategories are also active or quiescent LBVs. Some (but not all) ofthese objects have been well studied, and extensive references areprovided, as are comprehensive spectral-line identifications. Severalnew morphological relationships among the objects have been recognizedthrough this presentation. In particular, attention is drawn to theoccurrence of spatial pairing between nearly identical, unusual spectra,which may have implications for a particular mode of massive-starformation. This small sample includes one or both members of at leastfive such pairs. Physical explanations of these peculiar, likelytransitional spectra and the relationships among them are essential fora complete understanding of massive stellar evolution.
| Wolf-Rayet nebulae as tracers of stellar ionizing fluxes. I. M1-67
We use WR124 (WN8h) and its associated nebula M1-67, to test theoreticalnon-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing fluxdistributions derived from a stellar analysis of WR124, are comparedwith nebular properties via photo-ionization modelling. Our studydemonstrates the significant role that line blanketing plays inaffecting the Lyman ionizing energy distribution of WR stars, ofparticular relevance to the study of H ii regions containing youngstellar populations. We confirm previous results that non-line blanketedWR energy distributions fail to explain the observed nebular propertiesof M1-67, such that the predicted ionizing spectrum is too hard. A lineblanketed analysis of WR124 is carried out using the method of Hillier& Miller (1998), with stellar properties in accord with previousresults, except that the inclusion of clumping in the stellar windreduces its wind performance factor to only ~ 2. The ionizing spectrumof the line blanketed model is much softer than for a comparabletemperature unblanketed case, such that negligible flux is emitted withenergy above the He i lambda 504 edge. Photo-ionization modelling,incorporating the observed radial density distribution for M1-67 revealsexcellent agreement with the observed nebular electron temperature,ionization balance and line strengths. An alternative stellar model ofWR124 is calculated, following the technique of de Koter et al. (1997),augmented to include line blanketing following Schmutz et al. (1991).Good consistency is reached regarding the stellar properties of WR124,but agreement with the nebular properties of M1-67 is somewhat poorerthan for the Hillier & Miller code.
| ICCD Speckle Observations of Binary Stars. XXII. A Survey of Wolf-Rayet Starsfor Close Visual Companions
We present the results of a speckle interferometric survey for closevisual companions, mainly among 29 of the apparently brightestWolf-Rayet (W-R) stars. Only one target, WR 48 = theta Mus, was resolvedas a close astrometric binary (with a separation of 46+/-9 mas). Thissystem is probably a triple comprising a short-period W-R binary plus adistant O supergiant companion. Although our binary detection fractionis low, it is not an unexpected result given the selection effects thatmilitate against easy detection of binaries. New, higher resolutionobservations will almost certainly increase the fraction of binaries.There are four known binaries among the six W-R stars in our sample thathave nonthermal radio emission, and this connection supports the ideathat the nonthermal emission originates in the wind-wind collisionbetween components.
| 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.
| Molecular Gas Associated with the Ejecta Ring Nebula around the Wolf-Rayet Star WR 16
The environments of stars have the potential of providing importantclues to their evolution. The history of a changing stellar output, inthe form of winds and ejecta, is written in the environments ofWolf-Rayet stars. These show that Wolf-Rayet stars are evolved massivestars that are likely to have gone through several stages of mass lossin the forms of both fast and slow winds. At present, the evolutionarypath of massive stars to the Wolf-Rayet stage is not well understood. Aspart of a program to probe the evolution of massive stars through theirenvironments, we have begun a study of the neutral gas around Wolf-Rayetstars. Our intent is to determine information on the mass loss andkinematics of ejected material from Wolf-Rayet stars. In this paper weshow the WN 8 star WR 16 to be almost completely surrounded by a cocoonof molecular gas. Optical spectra of associated ionized gas suggest thatthe whole cocoon is likely to be made of nitrogen-rich, processedmaterial probably ejected in a red supergiant phase. Some tentativeevidence is also given for fast-moving components that may have comefrom an eruption of the star in a prior phase as a luminous bluevariable. The total molecular gas mass is estimated to be in the range5-78 M_solar, depending on the degree of enrichment of heavy elementsand the uncertain distance to the star.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| Quantitative Near-Infrared Spectroscopy of OF and WNL Stars
From new high signal-to-noise ratio (S/N) 1-2.2 μm spectroscopy ofnine extreme early-type stars-including O Iaf, O Iafpe and WN9 types-wedetermine stellar parameters from detailed atmospheric analysis andevaluate results from near-IR analogues of well-known spectraldiagnostics in the optical. We conclude that accurate stellar parameterscan be measured from near-IR spectroscopy alone, an analysis techniqueimportant to studies of luminous stars in the Galactic center and othergalaxies. Derived stellar parameters-mass-loss rates, luminosities,surface abundances, temperatures-show good agreement between optical andnear-IR analyses, provided that IR data are of sufficient spectralresolution (R>2000) and S/N (S/N>30). Wind velocities derived fromHe I 1.0830 μm are consistent with those from ultraviolet P Cygniprofiles. Temperatures 200-1300 K systematically lower are determinedfrom the near-IR diagnostics, a difference not significant indetermining the stellar properties of these objects; which set ofspectral lines provides the more accurate physical parameters-optical orIR-cannot at present be ascertained. The strength of He I 2.0581 μmis very sensitive to the extreme ultraviolet energy distribution whereline blanketing by heavy elements plays an important role; this lineshould not on its own be considered a reliable temperature diagnostic.The three peculiar, extreme emission-line stars-the O Iafpe stars HD152386, HD 152408, and HDE 313846-are more similar in both morphologicaland physical characteristics to WNL-type Wolf-Rayet stars than to normalO Iaf supergiants and should be classified as W-R. Their classificationshould be WN9ha, in which they remain a unique subgroup.
| Observational constraints on the efficiency of acceleration in the optically thin parts of Wolf-Rayet winds
Wolf-Rayet stars have such strong winds that their inner regions areoptically thick, preventing us from seeing the hydrostatic stellarcores. One might expect considerable acceleration of the wind to occurin the optically thick part. However, we show empirically that at least50%, and in some cases up to 100%, of the wind's acceleration occurs inthe optically thin part of the WR wind.
| Wolf-Rayet stars before and after Hipparcos.
Not Available
| Properties of Wolf-Rayet stars from X-ray to radio data
Not Available
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Carena |
| Ascensione retta: | 09h54m52.91s |
| Declinazione: | -57°43'38.3" |
| Magnitudine apparente: | 8.333 |
| Distanza: | 10000000 parsec |
| Moto proprio RA: | -6.8 |
| Moto proprio Dec: | 5 |
| B-T magnitude: | 8.645 |
| V-T magnitude: | 8.359 |
Cataloghi e designazioni:
|
