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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.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| The Galactic WN stars. Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
Context: .Very massive stars pass through the Wolf-Rayet (WR) stagebefore they finally explode. Details of their evolution have not yetbeen safely established, and their physics are not well understood.Their spectral analysis requires adequate model atmospheres, which havebeen developed step by step during the past decades and account in theirrecent version for line blanketing by the millions of lines from ironand iron-group elements. However, only very few WN stars have beenre-analyzed by means of line-blanketed models yet. Aims: .Thequantitative spectral analysis of a large sample of Galactic WN starswith the most advanced generation of model atmospheres should provide anempirical basis for various studies about the origin, evolution, andphysics of the Wolf-Rayet stars and their powerful winds. Methods:.We analyze a large sample of Galactic WN stars by means of the PotsdamWolf-Rayet (PoWR) model atmospheres, which account for iron lineblanketing and clumping. The results are compared with a syntheticpopulation, generated from the Geneva tracks for massive starevolution. Results: .We obtain a homogeneous set of stellar andatmospheric parameters for the Galactic WN stars, partly revisingearlier results. Conclusions: .Comparing the results of ourspectral analyses of the Galactic WN stars with the predictions of theGeneva evolutionary calculations, we conclude that there is roughqualitative agreement. However, the quantitative discrepancies are stillsevere, and there is no preference for the tracks that account for theeffects of rotation. It seems that the evolution of massive stars isstill not satisfactorily understood.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| 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.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| 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.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations
In a previous paper on the Magellanic Clouds, we demonstrated thatcoeval clusters provide a powerful tool for probing the progenitormasses of Wolf-Rayet (W-R) stars and luminous blue variables (LBVs).Here we extend this work to the higher metallicity regions of the MilkyWay, studying 12 Galactic clusters. We present new spectral types forthe unevolved stars and use these, plus data from the literature, toconstruct H-R diagrams. We find that all but two of the clusters arehighly coeval, with the highest mass stars having formed over a periodof less than 1 Myr. The turnoff masses show that at Milky Waymetallicities some W-R stars (of early WN type) come from stars withmasses as low as 20-25 Msolar. Other early-type WN starsappears to have evolved from high masses, suggesting that a large rangeof masses evolve through an early WN stage. On the other hand, WN7 starsare found only in clusters with very high turnoff masses, over 120Msolar. Similarly, the LBVs are only found in clusters withthe highest turnoff masses, as we found in the Magellanic Clouds,providing very strong evidence that LBVs are a normal stage in theevolution of the most massive stars. Although clusters containing WN7stars and LBVs can be as young as 1 Myr, we argue that these objects areevolved, and that the young age simply reflects the very high massesthat characterize the progenitors of such stars. In particular, we showthat the LBV η Car appears to be coeval with the rest of theTrumpler 14/16 complex. Although the WC stars in the Magellanic Cloudswere found in clusters with turnoff masses as low as 45Msolar, the three Galactic WC stars in our sample are allfound in clusters with high turnoff masses (>70 Msolar)whether this difference is significant or due to small number statisticsremains to be seen. The bolometric corrections of Galactic W-R stars arehard to establish using the cluster turnoff method but are consistentwith the ``standard model'' of Hillier.
| Filamentary Shell Structures from the AAO/UKST Hα Survey
Here we present the first results of a search for new optical supernovaremnant candidates and other filamentary objects on films produced bythe Anglo-Australian Observatory/UK Schmidt Telescope Hα Survey.Sixty-one fields, or 26% of the Galactic plane survey fields, have beenvisually examined. This has resulted in the detection of four newlargediameter filamentary structures, and the discovery of extensive newoptical emission in two previously known optical supernova remnantcandidates.
| 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.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Mass-loss rates of Wolf-Rayet stars as a function of stellar parameters
Clumping-corrected mass-loss rates of 64 Galactic Wolf-Rayet (WR) starsare used to study the dependence of mass-loss rates, momentum transferefficiencies and terminal velocities on the basic stellar parameters andchemical composition. The luminosities of the WR stars have beendetermined either directly from the masses, using the dependence of L onmass predicted by stellar evolution theory, or they were determined fromthe absolute visual magnitudes and the bolometric corrections. For thispurpose we improved the relation between the bolometric correction andthe spectral subclass. (1) The momentum transfer efficiencies η(i.e. the ratio between the wind momentum loss and radiative momentumloss) of WR stars are found to lie in the range of 1.4 to 17.6, with themean value of 6.2 for the 64 program stars. Such values can probably beexplained by radiative driving due to multiple scattering of photons ina WR wind with an ionization stratification. However, there may be aproblem in explaining the driving at low velocities. (2) We derived thelinear regression relations for the dependence of the terminal velocity,the momentum transfer efficiency and the mass-loss rates on luminosityand chemical composition. We found a tight relation between the terminalvelocity of the wind and the parameters of the hydrostatic core. Thisrelation enables the determination of the mass of the WR stars fromtheir observed terminal velocities and chemical composition with anaccuracy of about 0.1 dex for WN and WC stars. Using evolutionary modelsof WR stars, the luminosity can then be determined with an accuracy of0.25 dex or better. (3) We found that the mass-loss rates(&mathaccent "705Frelax dot;) of WR stars depend strongly onluminosity and also quite strongly on chemical composition. For thecombined sample of WN and WC stars we found that &mathaccent"705Frelax dot; in Mȯyr-1 can be expressed as&mathaccent "705Frelax dot; ≃ 1.0 ×10-11(L/L ȯ)1.29Y1.7Z0.5 (1) with an uncertainty of σ = 0.19dex (4) The new mass-loss rates are significantly smaller than adoptedin evolutionary calculations, by about 0.2 to 0.6 dex, depending on thecomposition and on the evolutionary calculations. For H-rich WN starsthe new mass-loss rates are 0.3 dex smaller than adopted in theevolutionary calculations of Meynet et al. (1994). (5) The lowermass-loss rates, derived in this paper compared to previously adoptedvalues, facilitate the formation of black holes as end points of theevolution of massive stars. However they might create a problem inexplaining the observed WN/WC ratios, unless rotational mixing ormass-loss due to eruptions is important.
| Wind properties of Wolf-Rayet stars at low metallicity: Sk 41 (SMC)
The stellar properties of Sk 41 (AB4, WN5h), the only known singleWolf-Rayet star in the SMC, are derived from ultraviolet (IUE), optical(AAT) and near-IR (NTT) spectroscopy. Contrary to expectations for lowmetal environments, the stellar properties of Sk 41 are typical ofequivalent WN stars in the Galaxy and LMC, with T* ~ 42kK,log(L/Lsun)=5.7, vinfty =1300 km s-1,H/He ~ 2 by number, and dot {M}/sqrt {f}=3x10-5Msunyr-1, where f is the volume filling factor.The stellar luminosity of Sk 41 is 50% below the minimum value predictedby single star evolutionary models at the metallicity of the SMC,suggesting a deficiency in present evolutionary models at lowmetallicity. Emission line luminosities of He II lambda 4686 and C IVlambda lambda 5801-12 in SMC WR stars are not systematically lower thantheir Galactic and LMC counterparts. From 43 late-type and 59 early-typeWN stars, log Llambda HeII= 36.0 ergs-1 and 35.8 erg s-1, respectively, while logLlambdaCIV =36.5 erg s-1. from 25early-type WC stars. This new calibration has application in deriving WRpopulations in young starburst galaxies. Synthetic WN models arecalculated with identical parameters except that metal abundances arevaried. Following the Smith et al. WN classification scheme, CNOequilibrium models reveal that earlier spectral types are predicted atlower metallicity, i.e. WN3-4 at 0.04Zsun versus WN6 at1.0Zsun. This provides an explanation for the trend towardsearlier WN spectral types at low metallicity. Based, in part, onobservations collected at the European Southern Observatory, La Silla,Chile (Proposal No. 61.D--0680 and 63.H--0683)
| The Structure of Wolf-Rayet Winds. II. Observations of Ionization Stratification in the WN Subtype
Motivated by the question of the importance of ionization stratificationin solving the ``momentum problem'' of Wolf-Rayet stellar winds, we havechosen a sample of 14 WN stars for a systematic study. We performedmeasurements of the emission line widths on ultraviolet, optical, andinfrared spectra to obtain data spanning a large range of ionizationpotentials. We provide extensive tables of these measurements as well asline profile classifications. The presence of ionization stratificationin the wind should result in a correlation between ionization potentialand line width. We find most of the winds to be stratified and discussthe level of stratification found in each star. To test the importanceof ionization stratification to efficient radiation-to-wind momentumtransfer, we compare our empirically measured stratification strengthswith two sets of theoretical performance numbers and give thecorrelation statistics in each case.
| 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.
| Quantitative spectroscopy of Wolf-Rayet stars in HD97950 and R136a - the cores of giant HII regions
We present quantitative analyses of Wolf-Rayet stars in the cores of twogiant Hii regions - HD97950 in NGC3603 and R136a in 30 Doradus - basedon archive Hubble Space Telescope (HST) spectroscopy. We confirmprevious WN6h+abs classifications for components A1, B and C in HD97950,while classifications for R136a1-3 are revised from O3If^*/WN6 to WN5h.From detailed non-local thermodynamic equilibrium analyses, we find thatall Wolf-Rayet stars exhibit products of CNO-processed material at theirsurface since they are rich in both helium (H/He~3-6, by number) andnitrogen (N/He~0.002-0.006). Their luminosities, log(L/Lsolar)=6.0-6.3,are amongst the highest known for Wolf-Rayet stars. Consequently theyare very massive stars (M_init>=100Msolar) at a relatively low age(~2Myr), reminiscent of the late WN stars in the Carina Nebula. Weobtain a revised distance modulus of 15.03mag (=10.1kpc) to NGC3603based on available photometry, an updated M_v calibration for early Ostars and a reddening of E(B-V)=1.23mag towards its core. From a censusof the massive stellar content of the two central clusters we concludethat their global properties are comparable. We evaluate thecontribution made by Wolf-Rayet stars to the total Lyman continuumionizing flux and kinetic energy released into the ISM. We discuss howsimple calibrations can be used to estimate stellar luminosities,ionizing fluxes and mass-loss rates of luminous OB stars. Wolf-Rayetstars provide ~20 per cent of the total ionizing flux (~1.3x10^51 Lyphotons^-1) within 0.5pc of their cores, and ~60 per cent of the totalkinetic energy injected into the ISM (5-6x10^38ergs^-1), despiterepresenting only 10 per cent of the massive stellar population. For thelarger R136 cluster in 30 Doradus (r<=10pc), 117 massive starsprovide a total ionizing flux of 4x10^51 Ly photons^-1 and release atotal kinetic energy of 1.6x10^39 ergs^-1 into the ISM, the latter beingdominated by nine WR (43 per cent) and six O3If^*/WN (29 per cent)stars.
| 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 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.
| X-ray and γ-ray emission in open clusters.
We have studied a number of galactic open clusters that lie in the errorboxes of COS B sources. These clusters belong to complexes in whichsimilar star formation processes occur. They have similar ages, stellarpopulation and contain peculiar stars with very high-velocity stellarwinds. We propose that a system of shock fronts, set up at theinterfaces between the hypersonic wind of the peculiar stars and theother cluster members, generates acceleration of cosmic ray particles,whose interactions with the inter-cluster gas concentrations would beresponsible for the observed γ-ray emission. We find variousobservational evidences (including CGRO data, strongly supporting theassociation of the γ-ray source J 2021+37=2CG075+00 with Berk 87)for the presence of such shock fronts in some of these clusters. We showthat the diffuse hard X-ray emission from the clusters regions isconsistent with the geometric scenario inferred from the data and withthe assumed mechanism of cosmic ray acceleration.
| Spectral atlas of the Galactic Wolf-Rayet stars (WN sequence).
Spectra of 62 (i.e. almost all) Galactic single Wolf-Rayet stars of theWN sequence are compiled. These data provided the empirical basis forour comprehensive spectral analyses of these stars published recently.The observations cover wide parts of the visual range and have aspectral resolution λ/{DELTA} λ between 2000 and 3600 inmost cases. Infrared observations around 10830A are included for 18stars. The spectra are preliminary ``rectified'' by division through anestimated stellar continuum. The whole material is displayed in conciseplots sorted by spectral subtype in order to provide an overview. Accessto the full digital data is offered via anonymous file transfer.
| 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
| Interstellar Ultraviolet Extinction Towards the Nitrogen Sequence Wolf-Rayet Stars
With the archive IUE spectra of WR stars collected by Niedzielski andRochowicz (1994) we obtain the UV extinction curve for WN stars. Usingthe two-color diagram method we can reach the goal almost independently,assuming only similar (b-v)_0 for all single galactic WN stars, what isdiscussed on the basis of latest results. The resulting extinction curvediffers strongly shortwards of the 2200 bump from that of Seaton (1979)and is virtually identical to that of Krelowski and Papaj (1992).Assuming the power law shape of WN continua we obtain a new value of(b-v)_0=-0.22 for single WN stars and present E_{b-v} for 34 galactic WNstars.
| Spectral analyses of the Galactic Wolf-Rayet stars: hydrogen-helium abundances and improved stellar parameters for the WN class
Almost all known Galactic single WN stars have been analyzed applyingnon-LTE models for spherically expanding atmospheres. While in aprevious study we performed "coarse" spectral analyses based essentiallyon helium-line equivalent widths compared to a grid of models, we nowpresent improved results from 25 detailed "fine" analyses, i.e. fromfitting the line profiles with individual model calculations. Thenon-LTE models applied now account for a helium-hydrogen composition,and corresponding abundances are determined for each star. 53% of thestudied Galactic WN stars (33 out of 62) are hydrogen-free, populating aluminosity range from 10^4.6^ to 10^5.7^Lsun_. The occurrenceof hydrogen is restricted to those WN stars with lowest stellartemperatures (30...35kK, with few exceptions), comprising most stars oflate subtype (WNL) and part of the weak-lined early-type WN stars(WNE-w). But all WN stars are definitely hydrogen-deficient, compared tothe solar value. Most WN stars showing hydrogen exhibit mass fractionsβ_H_ between 10% and 30%. They cover the whole luminosity range ofthe the hydrogen-free WN stars, but extend also to higher values: agroup of ten WNL stars with hydrogen is brighter than10^5.7^Lsun_, i.e. brighter than any hydrogen-free WN star.Only three WN stars are found with hydrogen mass fractions as small as=~5%. A small group of four WN7 stars with high luminosities(>10^5.6^Lsun_) has outstandingly high hydrogen massfractions (40...53%). The gradual switching from the high mass-lossrates of the hydrogen-free WN stars to the much lower mass-loss rates oftypical OB stars is found to be correlated with the hydrogen abundance.
| 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
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Корма |
| Прямое восхождение: | 07h59m46.25s |
| Склонение: | -28°44'03.1" |
| Видимая звёздная величина: | 11.137 |
| Собственное движение RA: | 0.5 |
| Собственное движение Dec: | 3.7 |
| B-T magnitude: | 11.467 |
| V-T magnitude: | 11.165 |
Каталоги и обозначения:
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