Contents
Images
Upload your image
DSS Images Other Images
Related articles
A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. I. Characterizing the Variability
We present the results of a systematic search for large-scalespectroscopic variability in apparently single Wolf-Rayet (WR) starsbrighter than v ~ 12.5. In this first paper we characterize the variousforms of variability detected and distinguish several separate groups.For each star in our sample, we obtained 4-5 high-resolution spectrawith signal-to-noise ratio ~100. Our ultimate goal is to identify newcandidates presenting variability that potentially comes from corotatinginteraction regions (CIRs). Out of a sample of 25 stars, 10 were foundto display large-scale changes of which four are of CIR-type (WR 1,WR 115, WR 120, and WR 134). The star WR 134 wasalready known to show such changes from previous studies. Three WN8stars present a different type of large-scale variability and we believedeserve a group of their own. Also, all three WC9d stars in our samplepresent large-scale variability, but it remains to be checked if theseare binaries, as many dust-making WR stars are double. Finally, of theremaining stars, 10 were found to show small-amplitude spectral changes,which we attribute to normal line-profile variability due toinhomogeneities in the wind, and five were found to show no spectralvariability, as far as can be concluded from the data in hand. Follow-upstudies are required to identify potential periods for our candidatesshowing CIR-type changes and eventually estimate a rotation rate forthese WR stars.
| On the Role of the WNH Phase in the Evolution of Very Massive Stars: Enabling the LBV Instability with Feedback
We propose the new designation ``WNH'' for luminous Wolf-Rayet (WR)stars of the nitrogen sequence with hydrogen in their spectra. Thesehave been commonly referred to as WNL stars (WN7h, for example), butthis new shorthand avoids confusion because there are late-type WN starswithout hydrogen and early-type WN stars with hydrogen. Clearlydifferentiating WNH stars from H-poor WN stars is critical whendiscussing them as potential progenitors of Type Ib/c supernovae andgamma-ray bursts-the massive WNH stars are not likely Type Ib/csupernova progenitors, and are distinct from core He burning WR stars.We show that masses of WNH stars are systematically higher than for bonafide H-poor WR stars (both WN and WC), with little overlap. Also,hydrogen mass fractions of the most luminous WNH stars are higher thanthose of luminous blue variables (LBVs). While on the main sequence, astar's mass is reduced due to winds and its luminosity slowly rises, sothe star increases its Eddington factor, which in turn stronglyincreases the mass-loss rate, pushing it even closer to the Eddingtonlimit. Accounting for this feedback, observed properties of WNH starsare a natural and expected outcome for very luminous stars approachingthe end of core H burning. Feedback from the strong WNH wind itselfplays a similar role, enabling the eruptive instability seensubsequently as an LBV. Altogether, for initial masses above 40-60Msolar, we find a strong and self-consistent case thatluminous WNH stars are pre-LBVs rather than post-LBVs (for lower initialmass, the case is less clear). The steady march toward increasedmass-loss rates from feedback also provides a natural explanation forthe continuity in observed spectral traits from O3 V to O3 If* to WNHnoted previously.
| Physical Properties of Wolf-Rayet Stars
The striking broad emission line spectroscopic appearance of Wolf-Rayet(WR) stars has long defied analysis, owing to the extreme physicalconditions within their line- and continuum-forming regions. Recently,model atmosphere studies have advanced sufficiently to enable thedetermination of stellar temperatures, luminosities, abundances,ionizing fluxes, and wind properties. The observed distributions ofnitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and innearby star-forming galaxies are discussed; these imply lower limits toprogenitor masses of ?25, 40, and 75 Mȯ forhydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars,respectively. WR stars in massive star binaries permit studies ofwind-wind interactions and dust formation in WC systems. They also showthat WR stars have typical masses of 10 25 Mȯ, extendingup to 80 Mȯ for H-rich WN stars. Theoretical andobservational evidence that WR winds depend on metallicity is presented,with implications for evolutionary models, ionizing fluxes, and the roleof WR stars within the context of core-collapse supernovae andlong-duration gamma-ray bursts.
| 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.
| The massive eclipsing LMC Wolf-Rayet binary BAT99-129. I. Orbital parameters, hydrogen content and spectroscopic characteristics
BAT99-129 in the LMC is one among a handful ofextra-galactic eclipsing Wolf-Rayet binaries known. We present blue,medium-resolution, phase-dependent NTT-EMMI spectra of this system thatallow us to separate the spectra of the two components of the binary andto obtain a reliable orbital solution for both stars. We assign an O5Vspectral type to the companion, and WN3(h)a to the Wolf-Rayet component.We discuss the spectroscopic characteristics of the system: luminosityratio, radii, rotation velocities. We find a possible oversynchronousrotation velocity for the O star. Surprisingly, the extracted Wolf-Rayetspectrum clearly shows the presence of blueshifted absorption lines,similar to what has been found in all single hot WN stars in the SMC andsome in the LMC. We also discuss the presence of such intrinsic lines inthe context of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binaryevolution and GRB progenitors. Altogether, BAT99 129 is theextragalactic counterpart of the well-known Galactic WR binaryV444 Cygni.
| 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.
| Spectroscopy of the γ-ray burst GRB 021004: a structured jet ploughing through a massive stellar wind
We present spectra of the afterglow of the γ-ray burst GRB 021004taken with the ISIS spectrograph on the William Herschel Telescope (WHT)and with the Focal Reducer/Low Dispersion Spectrograph 1 (FORS1) on theVery Large Telescope (VLT) at three epochs spanning 0.49-6.62 d afterthe burst. We observe strong absorption probably coming from the hostgalaxy, alongside absorption in HI, SiIV and CIV with blueshifts of upto 2900 km s-1 from the explosion centre, which we assumeoriginates close to the progenitor. We find no significant variabilityof these spectral features. We investigate the origin of the outflowingmaterial and evaluate various possible progenitor models. The mostplausible explanation is that these result in the fossil stellar wind ofa highly evolved Wolf-Rayet (WR) star. However, ionization from theburst itself prevents the existence of HI, SiIV and CIV close to theafterglow surface where the fast stellar wind should dominate, and largeamounts of blueshifted hydrogen are not expected in a WR star wind. Wepropose that the WR star wind is enriched by a hydrogen-rich companion,and that the GRB has a structured jet geometry in which the γ-raysemerge in a small opening angle within the wider opening angle of thecone of the afterglow. This scenario is able to explain both thespectral-line features and the irregular light curve of this afterglow.
| 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.
| Hydrogen in the atmosphere of the evolved WN3 Wolf-Rayet star WR 3: defying an evolutionary paradigm?
WR 3 is the brightest very early-type WN star in the sky. Based onseveral years of time-resolved spectroscopy and precision photometry onvarious time-scales, we deduce that WR 3 is most likely a single,weak-lined star of type WN3ha (contrary to its current catalogue-type ofWN3 + O4), with H lines occurring both in emission and absorption in itswind. This conclusion is confirmed and strengthened via detailedmodelling of the spectrum of WR 3. Given the similarity of WR 3 withnumerous H-rich WNE stars in the Large Magellanic Cloud and especiallythe Small Magellanic Cloud, and its location towards the metal-deficientexterior of the Galaxy, we conclude that rotationally induced meridionalcirculation probably led to the apparently unusual formation of this hotGalactic WN star with enhanced hydrogen. Although we cannot completelyrule out the possibility of a binary with a low orbital inclinationand/or long period, we regard this latter possibility as highlyunlikely.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| An Effelsberg HI study of the ISM around WR 126, WR 154 and WR 155
The neutral hydrogen distribution has been studied in the direction ofthree Galactic Wolf-Rayet (WR) stars using the 100 m Effelsberg radiotelescope. Cavities in the HI distribution, regions of low HIemissivity, are observed over a 8-9 km s-1, velocity rangefor WR 126 (≡ST 2), WR 154 (≡HD 213049) and WR 155(≡HD 214419). These minima are interpreted as the observable 21-cmHI line counterpart of interstellar bubbles created by the winds of theWR stars and their progenitors. The HI cavities are elongated structuresdepicting an axial ratio ranging from 1.3 (WR 155) to 3 (WR 126). The WRstars are always eccentric with respect to either the geometric centreof the HI cavity or the absolute minimum inside it. This offset rangesfrom 50% to 80% of the HI hole's minor axis. The major axis of thesestructures range from 13 (WR 155) to 27 pc (WR 126), while the missingHI mass amounts to 45-50 Mȯ (WR 126), 60Mȯ (WR 155) and 85 Mȯ (WR 154).Figures 1, 3, 5 are only available in electronic form athttp://www.edpsciences.org
| Radio observations of interstellar bubbles surrounding massive stars}
We show radio continuum observations of the WR ring nebulae around WR101 and WR 113 obtained using the VLA and HI 21 cm line data of theinterstellar bubble around the O type stars BD +24 deg 3866 and BD+25deg 3952 obtained with the DRAO Synthesis Telescope. We review previousradio continuum and HI line results toward WR and O-type stars.
| The enigmatic WR46: A binary or a pulsator in disguise. II. The spectroscopy
We present spectroscopic monitoring of the Wolf-Rayet (WR) starWR 46 between 1989 and 1998, which has been obtainedsimultaneously with multicolour photometry (Veen et al. \cite{Veen02a},Paper I). The spectroscopic monitoring data show that the radiativefluxes of the optical emission lines (O Vi 3811/34, O Vi 5290, N V 4944,N V 4604/20, He Ii 4686, He Ii 4859, He Ii 5411, He Ii 6560) vary inconcert with the photometric single-wave (sw) frequency f_sw (Paper I),and also the difference of that period between 1989 and 1991. Theline-flux variability does not provide obvious support for a shortsecond period (Paper I). The radial-velocity variations show aremarkable behaviour: usually, they display a coherent single-wave onthe time scale of the double-wave period, while during some nights theradial velocity appears surprisingly to stay constant (see alsoMarchenko et al. \cite{Marchenko00}). These so-called stand-stills maybe related to the observed time-delay effects. A time-delay effectmanifests itself in several phenomena. Firstly, the line flux showssmall, but persistent, time-delays for lines originating from loweroptical depths, the outer-wind lines (N V 4604/20 and He Ii). Secondly,the radial-velocity variations display much larger time-delays than theline fluxes and their behaviour appears less consistent. Assuming thatthe double-wave period controls the radial velocity, the stand-still isobserved to start when the radial motion is in anti-phase with thepresumed orbital motion. Thirdly, the outer-wind lines are observed toenter a stand-still much later than the inner-wind lines. Fourthly, theradial-velocity variations of the peaks of the emission lines precedethe radial-velocity variations of the wings of those lines. In additionto line-flux- and radial-velocity variability, the He Ii 4686 emissionline shows pronounced line-profile changes on a time scale of hours. Ourmonitoring is not sufficient to study this in detail. Furthermore, wediscern a flaring behaviour, i.e., an emission bump appeared on the bluewing of two He Ii-lines (around -1700 km s-1) lasting lessthan 5 min. Finally, the line fluxes follow the observed brightenings,also on a time scale of years. We conclude that the short-term cyclicvariability confirms the WR nature as established from the WR standardmodel analysis by Crowther et al. (\cite{Crowther95}; hereafter referredto as CSH). The various time-delay effects are consistent with theformation of the spectrum in a stratified stellar wind. The outer layerstrail the inner ones. The variability is inconsistent with the formationof the spectrum in a stellar disc as proposed by Niemela et al.(\cite{Niemela95}) and Steiner & Diaz (\cite{Steiner98}). Thelong-term cyclic variability of the brightness and line fluxes isrelated to an increase of the mass-loss-rate, and, possibly, to theperiod changes. The interpretation of the nature of the variability isdeferred to Veen et al. (\cite{Veen02b}, Paper III). Based onobservations collected at the European Southern Observatory (ESO), LaSilla, Chile.
| 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.
| A High Resolution H I Study of the ISM Local to Galactic Wolf-rayet Stars
The neutral matter distribution from the interstellar medium (ISM)located i n the vicinity of the Galactic Wolf-Rayet (WR) stars WR132 andWR140 has been examined by means of the H I 21-cm line observationsobtained with high angular resolution observations. The most interestingdiscoveries are the presence of huge ovoidal H I minimum spanning thevelocity range +13 to +21 km/s (WR132) and -18 to -7 km/s (WR140). Theseminima were created, very likely, by the join t action of the progenitorof both WR stars and the WR star itself. Inside each cavity, two minimaare clearly discernible. The WR star is offset with respect to eitherthe geometrical centre of the main H I void or the inner H I minima. Thedual H I minimum geometry observed inside the main H I cavity, a featurealso seen in the H I distribution od the ISM located close to otherGalactic WR stars, may be a consequence of the interaction processitself.
| New Wolf-Rayet central stars of planetary nebulae identified on the AAO/UKST Hα Survey
The central stars of two of the new planetary nebulae found during scansof the AAO/UKST Hα Survey of the Milky Way have been found toexhibit Wolf-Rayet (WR) emission features. One (PMR 1) is an early-typestar of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC]star which, depending on the classification scheme used, is eitherintermediate in class between [WC9] and [WC10] or the sole member of the[WC10] class. Both stars exhibit unusual spectral features which may beattributed to enhanced nitrogen in their atmospheres and could beindicative of unusual stellar evolution.
| Formation of contact in massive close binaries
We present evolutionary calculations for 74 close binaries systems withinitial primary masses in the range 12...25Msun, and initialsecondary masses between 6 and 24Msun. The initial periodswere chosen such that mass overflow starts during the core hydrogenburning phase of the primary (Case A), or shortly thereafter (Case B).We use a newly developed binary code with up-to-date physics input. Ofparticular relevance is the use of OPAL opacities, and thetime-dependent treatment of semiconvective and thermohaline mixing. Weassume conservative evolution for contact-free systems, i.e., no mass orangular momentum loss from those system except due to stellar winds. Weinvestigate the borderline between contact-free evolution and contact,as a function of the initial system parameters. The fraction of theparameter space where binaries may evolve while avoiding contact - whichwe found already small for the least massive systems considered -becomes even smaller for larger initial primary masses. At the upper endof the considered mass range, no contact-free Case B systems exist.While for primary masses of 16Msun and higher the Case Asystems dominate the contact-free range, at primary masses of12Msun contact-free systems are more frequent for Case B. Weidentify the drop of the exponent x in the main sequence mass-luminosityrelation of the form L~ Mx as the main cause for thisbehaviour. For systems which evolve into contact, we find that this canoccur for distinctively different reasons. While Case A systems areprone to contact due to reverse mass transfer during or after theprimary's main sequence phase, all systems obtain contact for initialmass ratios below ~ 0.65, with a merger as the likely outcome. We alsoinvestigate the effect of the treatment of convection, and found itrelevant for contact and supernova order in Case A systems, particularlyfor the highest considered masses. For Case B systems we find contactfor initial periods above ~ 10 d. However, in that case (and for not toolarge periods) contact occurs only after the mass ratio has beenreversed, due to the increased fraction of the donor's convectiveenvelope. As most of the mass transfer occurs conservatively beforecontact is established, this delayed contact is estimated to yield tothe ejection of only a fraction of the donor star's envelope. Our modelsyield the value of beta , i.e., the fraction of the primaries envelopewhich is accreted by the secondary. We derive the observable propertiesof our systems after the major mass transfer event, where the massgainer is a main sequence or supergiant O or early B type star, and themass loser is a helium star. We point out that the assumption ofconservative evolution for contact-free systems could be tested byfinding helium star companions to O stars. Those are also predicted bynon-conservative models, but with different periods and mass ratios. Wedescribe strategies for increasing the probability to find helium starcompanions in observational search programs.
| 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.
| 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
| A High-Resolution H I Study of the Interstellar Medium Local to HD 193793
A 2° field centered at the optical position of the Wolf-Rayet (WR)star HD 193793 (≡ WR 140) was surveyed in the H I 21 cm line andin the 1420 MHz radio continuum, using the Dominion Radio AstrophysicalObservatory interferometer. The aim of this observations was to look forevidence of an interaction between the WR star and its localinterstellar medium (ISM). Our H I data displays a large oval minimum ata velocity of Vsys~=-12 km s-1, close to theoptical position of HD 193793. We believe this H I void was created byWR 140 and its massive companion (HD 193793 is a member of a well-knownbinary system). The major axis of the H I void is ~11.5 pc and its axialratio, the ratio of major to minor axis, is ~1.4. This structure isobservable over a velocity range of around 11 km s-1. The H Iminimum appears surrounded by a clumpy shell of H I in emission that hasa total mass of neutral hydrogen of MHI~1300 solar masses.The kinematical distance corresponding to a local standard of restvelocity of -12 km s-1 is approximately 5.4 kpc, whichdisagrees with the distance of HD 193793, namely, around 1 kpc.Therefore, it is concluded that the H I bubble was blown in an ISMhaving nonnegligible peculiar motions. Hipparcos proper-motionmeasurements indicate that HD 193793 has a tangential velocity of 28+/-3km s-1 along the major axis of one of the H I minima detectedin the interior of the main H I cavity. To reconcile this motion withthe physical size of the H I structure blown by the winds of the binarysystem, it is concluded that such tangential velocity was acquired bythe system a short while ago, namely, ~1.3×105 yr. Mapsfrom the IRAS database show a large-scale IR feature, with good spatialcorrelation with the H I shell surrounding the main H I void. We believethe emission from the IR shell arises from dust grains heated by theenergetic stellar continuum photons emitted by the stellar objects.
| 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.
| The Puzzle of HD 104994 (WR 46)
Intense coordinated spectroscopic and photometric monitoring of thesuspected Wolf-Rayet binary WR 46 in 1999 reveals clear periodicvariations, P=0.329+/-0.013 days, in the radial velocities of theemission lines of highest ionization potential, O VI and N V, founddeepest in the Wolf-Rayet wind and thus least likely to be perturbed bya companion. These are accompanied by coherent variability in theprofiles of lines with lower ionization/excitation potential and in thecontinuum flux. Most probably originating from orbital motion of theWolf-Rayet component of the binary, this periodic radial velocity signaldisappears from time to time, thus creating a puzzle yet to be solved.We show that the entangled patterns of the line profile variability aremainly governed by transitions between high and low states of thesystem's continuum flux. Based in part on observations obtained at theEuropean Southern Observatory, La Silla, Chile (ESO program 62.H-0110).
| 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.
| 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.
| 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.
| A spectropolarimetric survey of northern hemisphere Wolf-Rayet stars
We present a homogeneous, high signal-to-noise spectropolarimetricsurvey of 16 northern hemisphere Wolf-Rayet (WR) stars. A reduction inpolarization at emission-line wavelengths - the `line effect' - isidentified in four stars: WRs 134, 137, 139, and 141. The magnitude ofthe effect in WR 139 (V444 Cyg) is variable, while WR 136, previouslyreported to show the line effect, does not show it in our data. Assumingthe line effect generally to arise from axisymmetric distortions ofstellar winds, we show that a model in which all WRs have the sameintrinsic (equator-on) polarization, with the observed variations solelya result of inclination effects, is inconsistent with the observations.A model in which the intrinsic polarizations are uniformly distributedis more plausible, but best-fitting results are obtained if thedistribution of polarizations is biased towards small values, with only~20 per cent of stars having intrinsic polarizations greater than ~0.3per cent. Radiative transfer calculations indicate that the observedcontinuum polarizations can be matched by models with equator:poledensity ratios of 2-3. The model spectra have electron-scattering wingsthat are significantly stronger than observed (in both intensity andpolarized flux), confirming that the winds of stars showing intrinsicpolarization must be clumped on small scales as well as being distortedon large scales. We combine the results of our survey with observationsfrom the literature to give a sample of 29 stars which have bothaccurate spectropolarimetric observations and physical parametersderived from standard-model analyses. We find that the line-effect starsare clustered at high M, L in the luminosity-mass-loss rate plane(although they are unexceptional in the terminal velocity-subtype andthe surface-mass-flux-temperature planes). The mass-loss rates derivedfrom radio-continuum observations for these stars are in good accordwith the results of optical emission-line analyses, suggesting that (i)the wind structure of line-effect stars has a density contrast which iseffectively constant with radius, and (ii) the high M values may beartefacts of large-scale wind structure. Assuming that observedspectroscopic and photometric variability of the line-effect stars isrelated to the WR rotation period, we compute equatorial rotationvelocities. These velocities correspond to ~10 per cent of the corebreakup rates, and may be large enough to produce significantwind-compression effects according to the models of Ignace, Cassinelli& Bjorkman.
| The relationship between the WR classification and stellar models. II. The WN stars without hydrogen
We consider the relationships between the classification parameters ofWN stars in the new 3-dimensional classification of Smith et al. (1996)and the corresponding and related parameters that define stellaratmosphere models. Specifically, we consider: FWHM of HeII 4686 vs.v_infty ; hydrogen content by direct inspection vs. hydrogen content bymodelling and vs. colour (b-v)_0; ionisation subclass and M_v; vs.effective temperature. From these data we argue that the WN b and onlythe WN b stars (i.e. stars with EW 5411 > 40 Angstroms \ or FWHM 4686> 30 Angstroms) are entirely free of hydrogen. For the WN b stars, weconsider the relationships of EW 5411 and FWHM 4686 to the derivedtemperature T_*; the mass loss rate; and the surface mass flux. Itappears that, to first approximation, the stars are a one-parameterfamily and the spectral classification criteria are sufficient to givean indication of the intrinsic colour, absolute magnitude (not veryaccurately), effective temperature T_* and terminal velocity.Theoretical models suggest that the critical parameter defining most ofthe properties of a WN b star is its present mass. However, thebehaviour of FWHM 4686 suggests the presence of a second parameter thataffects the mass loss rate and terminal velocity of the wind. We suggestthat the second parameter may be either (or a combination of) theinternal mean molecular weight or the rotation rate of the star. Wefurther compare the relationships predicted by evolutionary models withthose found for observed stars (using atmosphere models), highlightingthe present difficulties in these comparisons.
| The nitrogen spectra of Wolf-Rayet stars. A grid of models and its application to the Galactic WN sample
Adopting the ``standard model'' for Wolf-Rayet atmospheres, non-LTEradiation transfer calculations are performed which account for heliumand nitrogen. Grids of theoretical models are presented for the wholerelevant parameter range. The WN classification criteria are employed inorder to identify the subtype domains, and inconsistencies arediscussed. The (almost complete) sample of known Galactic WN stars isanalyzed by comparing the observed spectra with the synthetic spectra ofthe grid models. This is the first time that nitrogen line analyses areperformed for the whole WN sample, while previous comprehensive studieswere restricted to helium models. The obtained parameters roughlyconfirm the results from the previous helium analyses, as far as latesubtypes (WNL) and early subtypes with strong lines (WNE-s) areconcerned. For early subtypes with weak lines (WNE-w), however, theparameters are substantially revised. The hottest WN star, with astellar (effective) temperature of 140kK, is WR2, which could not beanalyzed previously from its helium lines due to the lack of He I. Theother members of the WNE-w subgroup have stellar temperatures between 40and 90kK, thus populating the same temperature range as the strong-linedWNE-w, but with less dense winds. The luminosities are revised accordingto the new parameters. Moreover, reddening corrections are newlydetermined form comparing IUE data with the UV model fluxes. The averageluminosity is now log L/Lsun = 5.5 for WNE stars (both,strong and weak lined), and log L/Lsun = 5.9 for WNL (notsignificantly revised). The empirical minimum WN luminosity is10(5.0}L_{sun) , reducing former incompatibilities with predictions fromevolutionary calculations. The ratio between mechanical and radiativemomentum flow is slightly affected by the revisions, but remains muchhigher than unity: 9, 9 and 29 for the WNL, WNE-w and WNE-s subclass,respectively. Partly based on observations collected at the EuropeanSouthern Observatory (ESO), La Silla, Chile, and on archival data fromthe International Ultraviolet Explorer (IUE)
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Cassiopée |
| Right ascension: | 01h38m55.64s |
| Declination: | +58°09'22.6" |
| Apparent magnitude: | 10.625 |
| Proper motion RA: | -0.7 |
| Proper motion Dec: | -2.6 |
| B-T magnitude: | 10.616 |
| V-T magnitude: | 10.625 |
Catalogs and designations:
|
