Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Triggered massive star formation in the vicinity of WR 48a
We utilise Midcourse Space Experiment mid-IR imaging and published datato discuss the (massive) star formation region at galactic longitude305o, apparently associated with the Wolf RayetWR 48a and the attendant clusters Danks 1and 2. A spectacular three lobed wind blown bubble surroundsthe aforementioned sources, for which we may infer a minimum age of3 Myr from the presence of the WCL star. Near IR data reveals thepresence of numerous embedded sources on the periphery of the wind blownbubble. The presence of coincindent H2O, OH and methanolmaser emission is suggestive of ongoing massive star formation, which issuppported by the fluxes of the associated IR sources, and the requisiteLyC flux required to support the emission from the subset that haveassociated ucH II regions. Consideration of the integrated radio flux ofthe complex implies that a minimum of 31 O7V stars must be present,under the assumption of no photon leakage. Given the age and morphologyof the complex and in particular the observation that the centralexciting clusters have entirely cleared their natal material, we expectthis assumption will be violated, and hence that the true population ofmassive stars is likely to be significantly larger. If confirmed, theG305 complex represents one of the most massive regions of ongoingtriggered star formation currently identified in the galaxy.
| 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.
| 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.
| 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.
| On the 4640 Angstrom feature in Wolf-Rayet galaxies
Several Wolf-Rayet galaxies exhibit broad emission features around 4640Angstrom in their optical spectra. These features are usually identifiedas N III and C III/C IV emission lines from WR stars. In a few WRgalaxies the flux in this feature has been measured to be as large orlarger than that in the corresponding He II lambda4686 stellar emissionline. We demonstrate that a 4640/4686 flux ratio larger than unitycannot be produced by any known population of WR stars. In particular,we show that the enhanced ratio cannot be explained by the presence ofcarbon lines from WC stars. We examine the possible origins of the broadlambda4640 feature and offer several possible explanations for theenhanced strength in this emission feature. The most plausibleexplanations involve the presence of large numbers of Of stars in thestarburst regions and/or the contamination of the stellar lines bynebular emission features. We discuss the implications that bothpossibilities have for the interpretation of the star formationhistories in WR galaxies, as derived from their massive star content. Wefind that the instantaneous burst scenario cannot be correct for anymetal-rich region whose optical spectrum exhibits both an enhanced4640/4686 flux ratio and C IV 5808 emission from WC stars. These regionsmust have experienced a "multiple-burst" star-formation event, composedof several instantaneous bursts separated by short time intervals (a fewMyr).
| 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
| The gamma Velorum binary system. I. O star parameters and light ratio
In this paper we demonstrate how previous determinations of the lightratio between the O and Wolf-Rayet stellar components of the gamma Velsystem are affected by large uncertainties. This is due, amongst otherthings, to the difficulty of measuring the equivalent widths of emissionand absorption lines. We then present a new technique to de-blend andmeasure spectral lines, in which we compensate for the observedabsorption features with synthetic profiles. From the new values of thediagnostic line strengths we determine a hotter spectral type for the Ostar companion (O7.5) than previously published. The light ratio is thendetermined, together with the stellar parameters, via a spectroscopicanalysis. We obtain Delta M_V=1.47+/-0.13 mag. From the light ratio andthe system's luminosity we find M_V(O) = -5.14 mag and M_V(WR) = -3.67mag. Simultaneously we determine ifmmode T_eff else T_efffi(O) = 35 000K, L(O) = 2.1x10(5) Lsun and cal M(O) = 30 Msun.An age of 3.59x10(6) yr is derived from these parameters andevolutionary tracks. We find that the H/He abundance ratio is solar.From a hydro-dynamical calculation of the radiation-driven wind weobtain dot{M}(O) = 1.8x10(-7) Msun yr(-1) and vinfty(O) =2500 km s(-1) . From the O star mass derived here and the mass ratiofrom the literature we derive the mass of the Wolf-Rayet star, cal M(WR)= 9 Msun. The mass-luminosity relation for Wolf-Rayet starsthen leads to L(WR) = 1.5x10(5) Lsun. We finally present thegamma Vel Wolf-Rayet spectrum de-convolved from the O star in the range3800-6700 Angstroms.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A Photometric System for Detection of Embedded Wolf-Rayet Stars
We describe a photometric system designed for detection of WR stars inthe near infrared region and based on a combination of narrow bandfilters and a CCD detector. The system is sensitive to Of and faintlined WN and WC stars and suitable for obscured regions of starformation in our Galaxy. (SECTION: Stars)
| A Survey of Nebulae around Galactic Wolf-Rayet Stars in the Southern Sky. III. Survey Completion and Conclusions
We present the conclusion of a narrow-band optical CCD survey ofWolf-Rayet stars in the southern portion of the Milky Way. In this partof the survey we complete our survey of the southern Galaxy and reportthe detection of 10 new optical nebulae associated with Wolf-Rayetstars. This brings the final survey total to 40 Wolf-Rayet stars withassociated nebulae in 114 southern Galactic fields for a 35% detectionrate. Our results suggest that the Galactic environment has littleapparent effect on the detection rate of nebulae associated withWolf-Rayet stars. Indeed, a more important role in the production ofnebulae is likely to be played by the evolution of the central star. Thesurvey results also suggest a slightly higher incidence of nebuladetection around WN stars over WC stars, although nebulae associatedwith WC stars are noted as being generally larger and some may have beenmissed through being larger than the CCD array used. Indeed, theincreased rate of nebula detection compared to that of a northernGalactic survey can be accounted for solely through the fact that alarger region of sky around the Wolf-Rayet stars was imaged in oursouthern survey as compared to the northern survey. Larger nebulaeexisting around WC as opposed to WN stars are also consistent with thecurrent theory of the evolution of Wolf-Rayet stars from WN to WC.
| UBV Photometry of Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2855R&db_key=AST
| Large IRAS Shells Around Galactic Wolf-Rayet Stars and the O Star Phase of Wolf-Rayet Evolution
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....112.2828M
| An IRAS-based Search for New Dusty Late-Type WC Wolf-Rayet Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS..100..413C&db_key=AST
| Spectral analyses of 25 Galactic Wolf-Rayet stars of the carbon sequence.
We present a grid of helium-carbon models for Wolf-Rayet (WR) stars ofthe carbon sequence (WC) with β_ C_=0.2 (carbon mass fraction),thus extending our previously released grid with β_C_=0.6 to adifferent chemical composition. The WR model atmospheres are based onthe so-called standard assumptions. The calculations account for non-LTEradiation transfer in spherically expanding atmospheres. Helium andcarbon are represented by detailed model atoms, especially concerningthe ions Ciii and Civ. Using the model grids 25 Galactic WC stars ofintermediate subtype (WC5 to WC8) are analyzed. Subsequently we performfine analyses by calculating several individual models for each of theprogram stars. Temperatures, radii, mass-loss rates and terminalvelocities are determined together with the carbon to helium ratio. Theanalyzed WC stars are found to form two groups, which can bedistinguished by the strength of their emission lines. Stars with weaklines (WC-w) have effective temperatures close to 50kK and their windsare relatively thin, forming the continuous spectrum in regions withsmall expansion velocities. WC stars with strong lines (WC-s) havehigher effective temperatures (60 to 100kK, referring to the coreradius) and thick winds. Thus there is a strong analogy to thedistribution of the early-type WN stars (WNE-w and WNE-s, respectively).For the WC stars we determine luminosities between 10^4.7^ and10^5.5^Lsun_ and mass-loss rates from 10^-4.8^ to10^-3.9^Mȯ/yr. The carbon mass fraction varies from 0.2 to 0.6. Nocorrelation is found between the carbon abundance and any of the stellarparameters (e.g. temperature, luminosity) or the spectral subtype. Theevolution of WR stars is discussed by comparing the results of ouranalyses with evolutionary tracks.
| A Large Bubble External to the Wolf-Rayet Ring Nebula NGC 6888
Abstract image available at:http://adsabs.harvard.edu/abs/1995AJ....109.2257M
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| Terminal Velocities of Wolf-Rayet Star Winds from Low Resolution IUE Spectra
Attracted by the simplicity of the recently published by Prinja (1994)method of determination of terminal wind velocities in hot stars fromlow resolution IUE spectra we investigate its application to WR stars.With a large sample of low resolution IUE spectra of WR stars we foundeven simpler, that is linear instead of square, empirical relationbetween Delta lambda as defined by Prinja (1994) and terminal windvelocity -- vinfty. Using this new empirical relation wepresent vinfty for a sample of 85 galactic and LMC stars, 19of them determined for the first time. We almost tripled the number ofterminal velocity determinations for LMC WR stars. The comparison withother determinations shows that this simple method is accurate to within10-20%. We confirm the correlation between terminal velocity and WCsubtype. We also show that terminal velocities of WN stars are lowerthan that of WCE. A comparison between galactic and LMC stars shows thatthe LMC WN stars have slower winds in most of WN subtypes.
| The ROSAT PSPC survey of the Wolf-Rayet stars
Not Available
| The interstellar medium around Wolf-Rayet stars: clues to evolution (Invited)
Not Available
| Low resolution IUE spectra of Wolf-Rayet stars.
We present uniformly reduced and measured equivalent widths, FWHM andobserved line fluxes for 94 "single" WR stars (34 galactic WN, 22galactic WC, 31 LMC WN and 7 LMC WC) based on the archive IUE spectra ofWR stars gathered from different observational runs and from differentepochs. The spectra are used for spectral classification in theultraviolet region and for searching correlations among the strength andwidths of emission lines of different ions. Some correlations withoptical and near IR lines observed by other authors are given as well.The set of spectra we use is almost complete to 12 magnitude andrepresentative according to spectral subtype of WR stars.
| A survey of nebulae around galactic wolf-rayet stars in the southern sky, 2.
We present the second half of a charge coupled device (CCD) narrow-bandimaging survey of galactic Wolf-Rayet stars located in the southernhemisphere as listed by van der Hucht et al. (1981). Images of 50Wolf-Rayet stars were taken using a wide-field CCD and narrowbandinterference filters centered on H alpha and (O III) 5007 A wavelengths.The first half of the survey (Marston, Chu, & Garcia-Segura 1993,hereafter Paper I) revealed six new ring nebulae residing aroundWolf-Rayet stars. Here we reveal a possible 11 new rings and theexistence of multiple rings associated with two previously known nebula,RCW 118 and G2.4+1.4 and around the stars WR 16 and WR 43. Combining ourresults with those of Miller & Chu (1993) and Paper I, 92% of thevan der Hucht catalog of Wolf-Rayet stars have now been surveyed. Of the38 possible ring nebulae found in our surveys to date, 22 reside aroundWN subtype Wolf-Rayet stars, 13 around WC stars, one around a triplet ofWolf-Rayet stars and one around a WO star (WR 102). One ring existsaround a WN/WC star (WR 98). A bias toward rings being observed aroundW-R + OB binaries is noted. Such pairings are generally bright, and thedetection of a ring around them may merely be a function of theircombined luminosity. Several Wolf-Rayet stars are shown to be surroundedby multiple rings (two or three) which suggests that a number ofejections of stellar material have taken place during their evolution.
| The origin of extended interstellar shells around Wolf-Rayet stars having bright optical ring nebulae
Investigations of the interstellar environment around Wolf-Rayet (WR)stars have lead to the discovery of extended shells of gas and dust50-100 pc in diameter in the lines of sight toward three WR stars. Inthis paper, several origins for these extended shells are discussed.While positional coincidences cannot be excluded, the locations of theWR stars near the projected centers of the shells, the detection of onlyshortward-shifted, high-velocity UV absorption line components in theirIUE spectra, plus commonality of some WR star properties which are rarein the general WR star population suggest some casual connectionsbetween the WR stars and formation of interstellar shells. To accesswhether the high-velocity UV interstellar absorption lines are afrequent phenomenon related to WR stellar winds, we present a survey ofsuch features in all WR stars observed with IUE through 1991. Of 35stars studied, only four are found to have components with velocitydisplacements greater than 45 km/s which are not attributable topreviously identified OB association superbubbles. The means asurprising 82% of non-OB association WR stars show no evidence ofhigh-velocity gas in their lines of sight at IUE's spectral resolution,suggesting that high-velocity interstellar absorption lines are not acommon consequence of Wolf-Rayet star stellar winds alone. We review theproperties of three WR stars (HD 50896, HD 96548, and HD 192163) whichmay reside inside extended interstellar shells and find that they aresimilar in terms of spectral class (WN5-8), presence of an optical ringnebula, and reported photometric variability. Evaluation of possibleorigins of the extended shells suggests these three stars are in a postX-ray binary stage of high-mass binary star evolution. If this iscorrect, then the large interstellar shells detected might be evidenceof either supernova remnant shells generated by the explosion of thebinary's primary star, or non-conservative mass transfer during a RocheLobe overflow stage of the binary after the supernova explosion. Ineither of these cases the bright optical ring nebulae associated withthese three WR stars may signify recent Roche Lobe overflows consistentwith spectroscopic abundance analysis.
| The Episodic Dust-Maker WR:125 - Part Two - Spectroscopy and Photometry during Infrared Maximum
Abstract image available at:http://adsabs.harvard.edu/abs/1994MNRAS.266..247W
| Not Available
Not Available
| Synthetic UV Lines of Si IV, C IV, and He II from a Population of Massive Stars in Starburst Galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...418..749R&db_key=AST
| Spectrophotometry of Wolf-Rayet stars. I - Continuum energy distributions
All available low-resolution IUE spectra are assembled for Galactic,LMC, and SMC W-R stars and are merged with ground-based optical and NIRspectra in order to collate in a systematic fashion the shapes of theseenergy distributions over the wavelength range 0.1-1 micron. They can beconsistently fitted by a power law of the form F(lambda) isapproximately equal to lambda exp -alpha over the range 1500-9000 A toderive color excesses E(B-V) and spectral indices by removing the 2175-Ainterstellar absorption feature. The WN star color excesses derived arefound to be in good agreement with those of Schmutz and Vacca (1991) andKoesterke et al. (1991). Significant heterogeneity in spectral indexvalues was generally seen with any given subtype, but the groupsconsisting of the combined set of Galactic and LMC W-R stars, theseparate WN and WC sequences, and the Galactic and LMC W-R stars allshowed a striking and consistent Gaussian-like frequency distribution ofvalues.
| Search for H I bubbles around Wolf-Rayet stars between L = 302 deg and 312 deg
The results of a search for H I bubbles around the 14 WR stars known inthe section of the Galaxy between l = 302 deg and l = 312 deg arepresented. The results disclose four additional H I bubbles, all aroundWR stars at Galactic latitudes higher than absolute value of b = 2 deg.This increases to eight the number of known neutral gas bubblessurrounding WR stars. The new H I voids are associated with the WR stars52, 54, 57, and 61. The fact that no H I bubbles were found around WRstars close to the Galactic midplane is probably a selection effect.
| Terminal velocities for a large sample of O stars, B supergiants, and Wolf-Rayet stars
It is argued that easily measured, reliable estimates of terminalvelocities for early-type stars are provided by the central velocityasymptotically approached by narrow absorption features and by theviolet limit of zero residual intensity in saturated P Cygni profiles.These estimators are used to determine terminal velocities, v(infinity),for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OBstars, the values are typically 15-20 percent smaller than the extremeviolet edge velocities, v(edge), while for WR stars v(infinity) = 0.76v(edge) on average. New mass-loss rates for WR stars which are thermalradio emitters are given, taking into account the new terminalvelocities and recent revisions to estimates of distances and to themean nuclear mass per electron. The relationships between v(infinity),the surface escape velocities, and effective temperatures are examined.
| Color excesses, intrinsic colors, and absolute magnitudes of Galactic and Large Magellanic Cloud Wolf-Rayet stars
A new method of determining the color excesses of WR stars in the Galaxyand the LMC has been developed and is used to determine the excesses for44 Galactic and 32 LMC WR stars. The excesses are combined withline-free, narrow-band spectrophotometry to derive intrinsic colors ofthe WR stars of nearly all spectral subtypes. No correlation of UVspectral index or intrinsic colors with spectral subtype is found forthe samples of single WN or WC stars. There is evidence that early WNstars in the LMC have flatter UV continua and redder intrinsic colorsthan early WN stars in the Galaxy. No separation is found between thevalues derived for Galactic WC stars and those obtained for LMC WCstars. The intrinsic colors are compared with those calculated frommodel atmospheres of WR stars and generally good agreement is found.Absolute magnitudes are derived for WR stars in the LMC and for thoseGalactic WR stars located in clusters and associations for which thereare reliable distance estimates.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Mosca |
| Ascensione retta: | 13h43m16.35s |
| Declinazione: | -67°24'05.0" |
| Magnitudine apparente: | 9.656 |
| Moto proprio RA: | -6.5 |
| Moto proprio Dec: | -7.6 |
| B-T magnitude: | 9.801 |
| V-T magnitude: | 9.668 |
Cataloghi e designazioni:
|
