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Resim Yükleyin
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Ýlgili Makaleler
Discovery of Hot Supergiant Stars near the Galactic Center
We report new results of a campaign to find Wolf-Rayet and O (WR/O)stars and high-mass X-ray binaries (HMXBs) in the Galactic center. Wesearched for candidates by cross-correlating the Two Micron All SkySurvey with a deep Chandra X-Ray Observatory catalog of point sources inthe Radio Arches region. Following up with K-band spectroscopy, we foundtwo massive stellar counterparts to CXOGC J174555.3-285126 and CXOGCJ174617.0-285131, which we classify as a broad-lined WR star of subtypeWN6b and an O Ia supergiant, respectively. Their X-ray properties aremost consistent with those of known colliding-wind binaries in theGalaxy and the Large Magellanic Cloud, although a scenario involvinglow-rate accretion onto a compact object is also possible. The O Ia starlies 4.4 pc in projection from the Quintuplet cluster and has a radialvelocity consistent with that of the Quintuplet, suggesting that thisstar might have escaped from the cluster. We also present the discoveryof a B2 Ia supergiant, which we identified as a candidate massive starusing 8 μm Spitzer maps of the Galactic center in a region near theknown massive X-ray-emitting star CXOGC J174516.1-290315. We discuss theorigin of these stars in the context of evolving stellar clusters in theGalactic center.
| 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.
| 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.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Massive Stars in the Arches Cluster
We present and use new spectra and narrowband images, along withpreviously published broadband images, of stars in the Arches cluster toextract photometry, astrometry, equivalent width, and velocityinformation. The data are interpreted with a wind/atmosphere code todetermine stellar temperatures, luminosities, mass-loss rates, andabundances. We have doubled the number of known emission-line stars, andwe have also made the first spectroscopic identification of the mainsequence for any population in the Galactic center. We conclude that themost massive stars are bona fide Wolf-Rayet (W-R) stars; are some of themost massive stars known, havingMinit>100Msolar and have prodigious winds,M>10-5 Msolar yr-1, that areenriched with helium and nitrogen; with these identifications, theArches cluster contains about 5% of all known W-R stars in the Galaxy.We find an upper limit to the velocity dispersion of 22 kms-1, implying an upper limit to the cluster mass of7×104 Msolar within a radius of 0.23 pc; wealso estimate the bulk heliocentric velocity of the cluster to bevcluster,solar~+95 km s-1. Taken together, theseresults suggest that the Arches cluster was formed in a short, butmassive, burst of star formation about 2.5+/-0.5 Myr ago, from amolecular cloud that is no longer present. The cluster happens to beapproaching and ionizing the surface of a background molecular cloud,thus producing the thermal arched filaments. We estimate that thecluster produces 4×1051 ionizing photonss-1, more than enough to account for the observed thermalradio flux from the nearby cloud, 3×1049 ionizingphotons s-1. Commensurately, it produces 107.8Lsolar in total luminosity, providing the heating source forthe nearby molecular cloud, Lcloud~107Lsolar. These interactions between a cluster of hot stars anda wayward molecular cloud are similar to those seen in the``Quintuplet/Sickle'' region. The small spread of formation times forthe known young clusters in the Galactic center and the relative lack ofintermediate-age stars(τage=107.0-107.3 yr) suggest thatthe Galactic center has recently been host to a burst of star formation.Finally, we have made new identifications of near-infrared sources thatare counterparts to recently identified X-ray and radio sources. Basedon observations with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| The evolved early-type binary HDE 228766 revisited
We use an extensive set of spectroscopic observations to reinvestigatethe properties of the massive binary HDE 228766. Conventionalclassification criteria suggest that HDE 228766 consists of an O7primary and an Of+ secondary. However, several spectralfeatures of the secondary, such as the simultaneous presence of N III, NIV and N V emissions, make it a rather unusual object. We find that theorbital motion of the secondary is probably best described by the radialvelocities of the narrow N III emission lines. Our orbital solutionyields m1 sin 3i = 31.7 and m2 sin3i = 25.5 Msun for the primary and secondaryrespectively. The He II lambda 4542 absorption in the secondary'sspectrum appears considerably blue-shifted with respect to the narrowemission lines, indicating that the absorption is probably formed in theaccelerating part of the secondary's wind. We use a tomographictechnique to investigate the profile variability of the broader emissionlines. In addition to a strong emission from the secondary, the Hαline displays a weak emission feature that is probably associated with awind interaction region located near the surface of the primary star.Finally, our analysis of the spectrum with a non-LTE code indicates thatthe secondary is an evolved object that exhibits some CNO processedmaterial in its atmosphere and has a large mass loss rate. Assuming adistance of 3.5 kpc (which follows from adopting MV, s = -6for the secondary) we infer dot {M} =~ 10-5 Msunyr-1 for the secondary. Our results suggest that HDE 228766could be in a post-Roche lobe overflow stage. The secondary may beclassified as WN8ha and is currently in a transition stage between a``normal'' Of star and a WNL-type Wolf-Rayet star. Based on observationscollected at the Observatoire de Haute Provence (France) and theObservatorio Astronómico Nacional of San Pedro Mártir(Mexico).
| 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 behavior of the NIV 5203.2 Å line in stars with Wolf-Rayet and early O-type spectra
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| 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.
| 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 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.
| H II Shells Surrounding Wolf-Rayet Stars in M31
We present the results of an ongoing investigation to provide a detailedview of the processes by which massive stars shape the surroundinginterstellar medium (ISM), from parsec to kiloparsec scales. In thispaper, we focus on studying the environments of Wolf-Rayet (W-R) starsin M31 to find evidence for W-R wind-ISM interactions, through imagingionized hydrogen nebulae surrounding these stars. We have conducted asystematic survey for H II shells surrounding 48 of the 49 known W-Rstars in M31. There are 17 W-R stars surrounded by single shells, orshell fragments, seven stars surrounded by concentric, limb-brightenedshells, 20 stars where there is no clear physical association of thestar with nearby Hα emission, and four stars that lack nearbyHα emission. Of the 17+7 shells above, there are 12 that containone or two massive stars (including a W-R star) and that are <=40 pcin radius. These 12 shells may be classical W-R ejecta or windblownshells. Further, there may be excess Hα point-source emissionassociated with one of the 12 W-R stars surrounded by putative ejecta orwindblown shells. There is also evidence for excess point-sourceemission associated with 11 other W-R stars. The excess emission mayarise from unresolved circumstellar shells or within the extended outerenvelopes of the stars themselves. In a few cases, we find clearmorphological evidence for W-R shells interacting with each other. Inseveral Hα images, we see W-R winds disrupting, or punchingthrough, the walls of limb-brightened H II shells.
| NaSt1: a Wolf-Rayet star cloaked by an eta Car-like nebula?
We present a study of the peculiar Galactic emission-line object NaSt1(WR122, IRAS 18497+0056), which has previously been classified as aWolf-Rayet (WR) star. Our spectroscopic data set comprises Keck I-HIRES,WHT-ISIS and UKIRT-CGS4 observations, which show that NaSt1 has a highlyreddened nebular spectrum with extremely strong permitted and forbiddenlines covering a wide range in excitation (Hi, Hei-ii, Ni-iii, [Nii],[Neiii-iv], Mgi-ii, Siii, [Sii-iii], [Ariii-v], [Cav-vii], [Feii-vii],[Niii-iii]). [Oii-iii] is unusually weak, with Hei-ii and [Nii] verystrong, and carbon absent, suggestive of chemical peculiarities.Narrow-band WHT imaging reveals an elliptical nebula with an averagediameter of 6.8arcsec. We measure an interstellar extinction ofE(B-V)~2.1mag and estimate a distance of 1-3kpc, suggesting that NaSt1is a luminous object, with 4<=log(L/L_solar)<=6.5. We determinethe physical parameters and abundances from the nebular forbidden lines.For T_e=13000K and N_e=3x10^6cm^-3, we obtain He/H>0.64, N enhancedby a factor of 20, O deficient by a factor of 140, while Ne, Ar and Sare normal compared with average Hii region abundances. This unusualabundance pattern suggests that the nebula consists of fullyCNO-processed material. We compare the spectral appearance of NaSt1 withother luminous emission-line objects, and conclude that it is not anOfpe/WN9, B[e] star or symbiotic nova, although it does share severalcharacteristics of these systems. We suggest instead that NaSt1 containsa massive evolved star that ejected its heavily CNO-processed outerlayers a few thousand years ago. Although the stellar remnant iscompletely hidden from view by the dense nebula, we argue that the starmust be an early-type WR star. The only object that shares some of thepeculiarities of NaSt1 is eta Carinae. Whatever its true nature, NaSt1should no longer be considered as a late-WN classification standard inthe near-infrared.
| 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).
| Massive Stars in the Quintuplet Cluster
We present near-infrared photometry and K-band spectra of newlyidentified massive stars in the Quintuplet cluster, one of the threemassive clusters projected within 50 pc of the Galactic center. We findthat the cluster contains a variety of massive stars, including moreunambiguously identified Wolf-Rayet stars than any cluster in theGalaxy, and over a dozen stars in earlier stages of evolution, i.e.,luminous blue variables (LBVs), Ofpe/WN9, and OB supergiants. One newlyidentified star is the second luminous blue variable in the cluster,after the ``Pistol star.'' Although we are unable to provide certainspectral classifications for the five enigmatic Quintuplet-propermembers, we tentatively propose that they are extremely dusty versionsof the WC stars found elsewhere in the cluster and similar to the dozenor so known examples in the Galaxy. Although the cluster parameters areuncertain because of photometric errors and uncertainties in stellarmodels, i.e., extrapolating initial masses and estimating ionizingfluxes, we have the following conclusions. Given the evolutionary stagesof the identified stars, the cluster appears to be about 4+/-1 Myr old,assuming coeval formation. The total mass in observed stars is ~10^3M_solar, and the implied mass is ~10^4 M_solar, assuming a lower masscutoff of 1 M_solar and a Salpeter initial mass function. The impliedmass density in stars is greater than or similar to a few thousandM_solar pc^-3. The newly identified stars increase the estimatedionizing flux from this cluster by about an order of magnitude withrespect to earlier estimates, to 10^50.9 photons s^-1, or roughly whatis required to ionize the nearby ``Sickle'' H II region (G0.18-0.04).The total luminosity from the massive cluster stars is ~10^7.5 L_solar,enough to account for the heating of the nearby molecular cloud,M0.20-0.033. We propose a picture that integrates most of the majorfeatures in this part of the sky, excepting the nonthermal filaments. Wecompare the cluster to other young massive clusters and globularclusters, finding that it is unique in stellar content and age, except,perhaps, for the young cluster in the central parsec of the Galaxy. Inaddition, we find that the cluster is comparable to small ``super starclusters.''
| 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.
| 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.
| 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 K-Band Spectral Atlas of Wolf-Rayet Stars
We present K-band spectra (R ~ 525) of 38 northern Galactic WR Stars, ofwhich 16 are WC, 19 are WN, two are WN/WC, and one is WO. The spectrahave the expected trend of stronger lines for higher ionization specieswith earlier spectral subtype. Spectra for the late WC stars can appearto have weak emission lines, an effect due to different amounts of dustdilution in the individual stars. There are also differences in spectralmorphology for stars within other subtypes. In general, the spectra forall WC stars earlier than WC9 tend to be quite similar, while thespectra for WN subtypes are more easily differentiated. Severalpreviously unidentified emission lines are seen in the spectra, mostnotably, features near 2.247 and 2.368 mu m in late-type WN stars, andone near 2.222 mu m in late-type WC stars. We attribute the 2.247 mu mline to a N III transition and argue that it might provide the bestmethod for discriminating between WNL and OIf+ stars in the K band. Weinvestigate the behavior of the 2.11 mu m (He I + N III), 2.166 mu m (HI + He I + He II), and 2.189 mu m (He II) emission lines in WN types andfind that these lines provide for accurate discrimination within thesample to within 1 subtype. From this investigation, it appears that theratio of W2.189 mu m/W2.11 mu m is sensitive to subtype and shows theleast dispersion within subtypes. In addition, we find that the W2.189mu m/W2.166 mu m ratio also scales with subtype in a well-behaved manneronce it is corrected for contamination of the 2.166 mu m line by He II14--8 and for the presence of an O star companion in binary systems. Wealso investigate the behavior of the 2.058 mu m (He I), 2.08 mu m (CIV), and the 2.11 mu m (C III + He I) emission lines in WC types. Theratio of W2.08 mu m/W2.11 mu m correlates with subtype; however, it isnot easy to distinguish between individual subtypes earlier than WC8 byjust using this quantity. The dominance of the 2.058 mu m line in WC9types distinguishes this subtype from all other WC subtypes. Two WC9stars in our sample have nearly featureless spectra due to dustdilution. It is possible to classify a WR star to within one subtype inthe WN sequence based upon the sample in this atlas. The similarity ofWC spectra makes it difficult to distinguish among individual subtypesearlier than WC8.
| Fundamental parameters of Wolf-Rayet stars. VI. Large Magellanic Cloud WNL stars.
We present a detailed, quantitative study of late WN (WNL) stars in theLMC, based on new optical spectroscopy (AAT, MSO) and the Hillier(1990A&A...231..111H) atmospheric model. In a previous paper(Crowther et al. 1995A&A...293..172C), we showed that 4 out of the10 known LMC Ofpe/WN9 stars should be re-classified WN9-10. We nowpresent observations of the remaining stars (except the LBV R127), andshow that they are also WNL (WN9-11) stars, with the exception of R99.Our total sample consists of 17 stars, and represents all but one of thesingle LMC WN6-11 population and allows a direct comparison with thestellar parameters and chemical abundances of Galactic WNL stars(Crowther et al. 1995A&A...293..403C; Hamann et al.1995A&A...299..151H). Previously unpublished ultraviolet (HST-FOS,IUE-HIRES) spectroscopy are presented for a subset of our programmestars. We find observational evidence for lower metallicities in LMC WNLstars compared to the Galaxy, though this is not reflected in theirstellar properties. For Galactic and LMC stars we find: (i) a similarrange in temperature and luminosity, in contrast to evolutionarypredictions; (ii) comparable wind performance values(˙(M)vinfinity_/[L/c]) and hydrogen composition, with abroad correlation between increasing helium content and wind performancenumber; (iii) a general trend to lower wind velocities at lower stellartemperature, with possibly slower winds for LMC WN9-11 stars. Some 30Dor WNL stars show exceptional properties: Brey 89 (HD38282, WN6h) hasthe highest luminosity (log (L/Lsun_)~6.25) and mass-lossrate (log(˙(M)/(Msun_/yr))~-3.6) known for any WR star,while Brey 80 (R135, WN7h) has an enormous wind performance number of50. The observed physical properties of our sample of LMC WNL starssupports the Crowther et al. (1995A&A...293..427C) evolutionaryscheme for Galactic stars, in that the most massive O stars, exclusiveto 30 Dor, evolve directly to O3If/WN6 and subsequently WN6-7 stars(e.g. Brey 89), without passing through an intermediate LBV phase. Incontrast, lower initial mass stars evolve through a LBV phase,encompassing a WN9-11 stage (e.g. BE 294), with WN8 stars being theirimmediate successors.
| O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from Hubble Space Telescope Ultraviolet Observations
We present new HST/FOS ultraviolet spectroscopic observations of sevenLMC Ofpe/WN9 stars. We find that Ofpe/WN9 stars have slow winds withterminal velocities of about 400 km s-1 and high mass-loss rates of theorder of 2--5 x 10-5 Mȯ yr-1. Ofpe/WN9 stellar temperatures andradii are in the range 30,000--39,000 K, and 19--39 Rȯ,respectively. Stellar luminosities are between log (L/Lȯ) = 5.6 and6.3. We study the Ofpe/WN9 stars winds and examine their evolutionarystatus. We find that Ofpe/WN9 stars are intermediate between O and W-Rstars in terms of the wind momentum flux. We also find that the stellarproperties and wind momentum of the Ofpe/WN9 sample place them in theevolutionary sequence: O --> Of --> H-rich WNL --> Ofpe/WN9,for initial stellar masses less than ~100 Mȯ. In view of persistingdiscrepancies of standard massive star models with observations, wecompute massive main-sequence models according to three differentevolutionary scenarios. We find that both higher mass-loss rate andenhanced mixing between core and envelope are required in order to yieldmodels compatible with the derived stellar and wind properties ofOfpe/WN9 stars. The emerging picture may be consistent with earlierevidence of Ofpe/WN9 stars being quiescent luminous blue variables(LBVs). This idea is further strengthened by the highly reduced surfaceH mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5 to 0.3, whichstill excludes Ofpe/WN9 stars from being core He-burning objects, but isalmost identical to the Xs values recently measured in LBVs.
| The distinction between OIafpe and WNLha stars. A spectral analysis of HD 151804, HD 152408 and HDE 313846.
From detailed atmospheric analysis of the Galactic late O supergiants,HD 151804 (O8Iaf) and HD 152408 (O8:Iafpe) and the morphologicallyrelated Wolf-Rayet star, HDE 313846 (WR108; WN9ha) we conclude thatlittle distinguishes the overall structure and composition of thestellar atmosphere of HD 152408 from HDE 313846, such that HD 152408could be considered to be a W-R star equivalent to HDE 313846. Ourquantitative spectroscopic analysis uses ultraviolet (IUE), optical(AAT, Mt. Stromlo) and infrared (UKIRT, CTIO) spectroscopy and is basedon model atmospheres obtained with the iterative scheme of Hillier(1990A&A...231..116H). Stellar properties for all three stars arequite similar: log L_*_/Lsun_=~5.8,R_*_/Rsun_=~35,-4.9<=log˙(M)/Msun_/yr<=-4.5,955<=vinfinity_<=1445km/s. Spectroscopic mass-lossrates are in excellent agreement with radio methods and are morerepresentative of late WN (WNL) stars than O-type supergiants. From thiscomparison we find that the O and WNL spectral classifications aredistinguished principally by surface mass flux (˙(M)/4πR_*_^2^),with stellar temperature and surface helium content secondary effects.Since surface chemical abundances do not necessarily define the coreenergy production mechanism - a commonly used distinction between O andWR types - we argue caution when comparing observed O:WNL ratios withtheoretical predictions and inferring stellar evolution in variousgalaxies. The increasing helium content of the programme stars - O8Iaf(y=N(He)/[N(H)+N(He)]=0.20), O8:Iafpe (y=0.40) and WN9ha (y=0.45) - isstrongly indicative of a direct evolutionary connection fromO8If=>WN9ha without passage through an intermediate Luminous BlueVariable phase, contrary to current evolutionary theory, and suggestiveof additional processes (e.g. rotation) bringing processed material tothe surface. The high helium content of HD 151804 confirms previousfindings that Of stars are helium enriched by core processed material.
| Infrared Spectra of Massive Stars in Transition: WNL, Of, Of/WN, Be, B[e], and Luminous Blue Variable Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...470..597M
| A three-dimensional classification for WN stars
A three-dimensional classification for WN stars is presented using (1)the HeII 5411/HeI 5875 ratio as a primary indicator of ionization, (2)FWHM 4686 and EW 5411 as indicators of line width and strength, and (3)an oscillating Pickering decrement as an indicator of the presence ofhydrogen. All WN stars in the Galaxy and two-thirds of the LMC stars areclassified on the new system. Almost all spectra inspected fall smoothlyinto categories within which the spectra are very similar. Allionization subclasses show a tight correlation between line strength andwidth, with stars containing hydrogen at the weak, narrow end, and WN/Cstars near the strong, broad end. H^+/He^++ correlates with strength andwidth with a cut-off for the presence of hydrogen, which is slightlydependent on ionization subclass, at about FWHM 4686=30A and EW5411=25A. The correlations found indicate that high (initial) mass starsevolve as narrow-line stars from late to early ionization subclass.Lower (initial) mass stars evolve with increasing line strength andwidth, probably to earlier ionization subclass. The HeII 4686/NV,III4604-40 ratio shows a clear correlation with Galactocentric radius,presumably an effect of the Z gradient. CIV 5808/HeII 5411 shows no suchcorrelation. LMC WN stars can be classified without difficulty by thecriteria established for Galactic WN stars. While individual spectra ofa given subtype are similar in the two galaxies, the frequencydistributions over ionization subclass, over EW and FWHM in subclassesWN4 and WN5, and hydrogen content in subclasses WN6-8 are different. Theeffects are presumably due to metallicity, but the causal connection isunclear.
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| The Discovery of Hot Stars near the Galactic Center Thermal Radio Filaments
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...461..750C&db_key=AST
| Wolf-Rayet Stars. Physical, Chemical and Mass Loss Properties and Evolutionary Status
This paper reviews the current status of knowledge regarding the basicphysical and chemical properties of Wolf-Rayet stars; their overall massloss and stellar wind characteristics and current ideas about theirevolutionary status. WR stars are believed to be the evolved descendentsof massive O-type stars, in which extensive mass loss reveals successivestages of nuclear processed material: WN stars the products of interiorCNO-cycle hydrogen burning, and WC and WO stars the products of interiorhelium burning. Recent stellar evolution models, particularly thoseincorporating internal mixing, predict results which are in good accordwith the different chemical compositions observationally inferred forWN, WC and WO stars. WR stars exhibit the highest levels of mass lossamongst earlytype stars: mass loss rates, typically, lie in the range [110]×10‑5 Mȯyr‑1. Radiation pressure-driven windsincorporating multi-scattering in high ionisation-stratified winds maycause these levels, but additional mechanisms may also be needed.
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