תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| 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.
| The massive eclipsing LMC Wolf-Rayet binary BAT99-129. I. Orbital parameters, hydrogen content and spectroscopic characteristics
BAT99-129 in the LMC is one among a handful ofextra-galactic eclipsing Wolf-Rayet binaries known. We present blue,medium-resolution, phase-dependent NTT-EMMI spectra of this system thatallow us to separate the spectra of the two components of the binary andto obtain a reliable orbital solution for both stars. We assign an O5Vspectral type to the companion, and WN3(h)a to the Wolf-Rayet component.We discuss the spectroscopic characteristics of the system: luminosityratio, radii, rotation velocities. We find a possible oversynchronousrotation velocity for the O star. Surprisingly, the extracted Wolf-Rayetspectrum clearly shows the presence of blueshifted absorption lines,similar to what has been found in all single hot WN stars in the SMC andsome in the LMC. We also discuss the presence of such intrinsic lines inthe context of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binaryevolution and GRB progenitors. Altogether, BAT99 129 is theextragalactic counterpart of the well-known Galactic WR binaryV444 Cygni.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| 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.
| On the Nature of the LBV/WR Eclipsing Binary System HD 5980
HD 5980 is a multiple star system in the Small Magellanic Cloud thatunderwent a luminous blue variable-like eruption in 1993-1994. Theerupting star (star A) forms part of a close eclipsing binary whosecompanion's (star B) Wolf-Rayet nature has recently been questioned. Thedata analyzed in this paper cover the period 1979-2002, and support theWNE classification of star B. The orbital phase-dependent variations ofultraviolet Pabsorption lines imply that the wind speed of star A is1700-2200s-1 and that of star B is 2600-3100s-1. These allow an estimateof the wind-wind collision (wwc) shock cone geometry and line profilesfrom the wwc region. Given the assumed mass-loss rates, only a smallfraction of the observed emission line profiles may originate in the wwcregion. The star A+star B binary pair is likely the result of evolutionwith mass transfer and/or evolution with rapid rotation, and provides amagnificent opportunity for studying the processes that occur in massivemultiple systems at the stages of stellar evolution just prior to thesupernova event.
| An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| Hydrogen in the atmosphere of the evolved WN3 Wolf-Rayet star WR 3: defying an evolutionary paradigm?
WR 3 is the brightest very early-type WN star in the sky. Based onseveral years of time-resolved spectroscopy and precision photometry onvarious time-scales, we deduce that WR 3 is most likely a single,weak-lined star of type WN3ha (contrary to its current catalogue-type ofWN3 + O4), with H lines occurring both in emission and absorption in itswind. This conclusion is confirmed and strengthened via detailedmodelling of the spectrum of WR 3. Given the similarity of WR 3 withnumerous H-rich WNE stars in the Large Magellanic Cloud and especiallythe Small Magellanic Cloud, and its location towards the metal-deficientexterior of the Galaxy, we conclude that rotationally induced meridionalcirculation probably led to the apparently unusual formation of this hotGalactic WN star with enhanced hydrogen. Although we cannot completelyrule out the possibility of a binary with a low orbital inclinationand/or long period, we regard this latter possibility as highlyunlikely.
| Radio observations of interstellar bubbles surrounding massive stars}
We show radio continuum observations of the WR ring nebulae around WR101 and WR 113 obtained using the VLA and HI 21 cm line data of theinterstellar bubble around the O type stars BD +24 deg 3866 and BD+25deg 3952 obtained with the DRAO Synthesis Telescope. We review previousradio continuum and HI line results toward WR and O-type stars.
| The enigmatic WR46: A binary or a pulsator in disguise. II. The spectroscopy
We present spectroscopic monitoring of the Wolf-Rayet (WR) starWR 46 between 1989 and 1998, which has been obtainedsimultaneously with multicolour photometry (Veen et al. \cite{Veen02a},Paper I). The spectroscopic monitoring data show that the radiativefluxes of the optical emission lines (O Vi 3811/34, O Vi 5290, N V 4944,N V 4604/20, He Ii 4686, He Ii 4859, He Ii 5411, He Ii 6560) vary inconcert with the photometric single-wave (sw) frequency f_sw (Paper I),and also the difference of that period between 1989 and 1991. Theline-flux variability does not provide obvious support for a shortsecond period (Paper I). The radial-velocity variations show aremarkable behaviour: usually, they display a coherent single-wave onthe time scale of the double-wave period, while during some nights theradial velocity appears surprisingly to stay constant (see alsoMarchenko et al. \cite{Marchenko00}). These so-called stand-stills maybe related to the observed time-delay effects. A time-delay effectmanifests itself in several phenomena. Firstly, the line flux showssmall, but persistent, time-delays for lines originating from loweroptical depths, the outer-wind lines (N V 4604/20 and He Ii). Secondly,the radial-velocity variations display much larger time-delays than theline fluxes and their behaviour appears less consistent. Assuming thatthe double-wave period controls the radial velocity, the stand-still isobserved to start when the radial motion is in anti-phase with thepresumed orbital motion. Thirdly, the outer-wind lines are observed toenter a stand-still much later than the inner-wind lines. Fourthly, theradial-velocity variations of the peaks of the emission lines precedethe radial-velocity variations of the wings of those lines. In additionto line-flux- and radial-velocity variability, the He Ii 4686 emissionline shows pronounced line-profile changes on a time scale of hours. Ourmonitoring is not sufficient to study this in detail. Furthermore, wediscern a flaring behaviour, i.e., an emission bump appeared on the bluewing of two He Ii-lines (around -1700 km s-1) lasting lessthan 5 min. Finally, the line fluxes follow the observed brightenings,also on a time scale of years. We conclude that the short-term cyclicvariability confirms the WR nature as established from the WR standardmodel analysis by Crowther et al. (\cite{Crowther95}; hereafter referredto as CSH). The various time-delay effects are consistent with theformation of the spectrum in a stratified stellar wind. The outer layerstrail the inner ones. The variability is inconsistent with the formationof the spectrum in a stellar disc as proposed by Niemela et al.(\cite{Niemela95}) and Steiner & Diaz (\cite{Steiner98}). Thelong-term cyclic variability of the brightness and line fluxes isrelated to an increase of the mass-loss-rate, and, possibly, to theperiod changes. The interpretation of the nature of the variability isdeferred to Veen et al. (\cite{Veen02b}, Paper III). Based onobservations collected at the European Southern Observatory (ESO), LaSilla, Chile.
| Kinematical Structure of Wolf-Rayet Winds. I.Terminal Wind Velocity
New terminal wind velocities for 164 Wolf-Rayet stars (from the Galaxyand LMC) based on PCyg profiles of lambda1550 CIV resonance line werederived from the archive high and low resolution IUE spectra availableform the INES database. The high resolution data on 59 WR stars (39 fromthe Galaxy and 20 from LMC) were used to calibrate the empiricalrelation lambda_min^Abs- lambda_peak^Emis vs terminal wind velocity,which was then used for determinations of the terminal wind velocitiesfrom the low resolution IUE data. We almost doubled the previous mostextended sample of such measurements. Our new measurements, based onhigh resolution data, are precise within 5-7%. Measurements, based onthe low resolution spectra have the formal errors of approx 40-60%. Acomparison of the present results with other determinations suggestshigher precision of approx 20%. We found that the terminal windvelocities for the Galactic WC and WN stars correlate with the WRspectral subtype. We also found that the LMC WN stars have winds slowerthan their Galactic counterparts, up to two times in the case of the WNEstars. No influence of binarity on terminal wind velocities was found.Our extended set of measurements allowed us to test application of theradiation driven wind theory to the WR stars. We found that, contrary toOB stars, terminal wind velocities of the WR stars correlate only weaklywith stellar temperature. We also note that the terminal to escapevelocity ratio for the WR stars is relatively low: 2.55 pm 1.14 for theGalactic WN stars and 1.78 pm 0.70 for the Galactic WCs. This ratiodecreases with temperature of WR stars, contrary to what is observed inthe case of OB stars. The presented results show complex influence ofchemical composition on the WR winds driving mechanism efficiency. Ourkinematical data on WR winds suggest evolutionary sequence: WNL -->WNE --> WCE --> WCL.
| Empirical Bounds for the Ionizing Fluxes of Wolf-Rayet Stars
Hα photometry and spectroscopic data have been obtained for 10Wolf-Rayet nebulae, representing a wide variety of WN spectral types. Weuse these data to constrain the ionizing flux of the exciting Wolf-Rayetstar, calculating lower bounds for the Lyman continuum flux(Q0) and for the He0- and He+-ionizingfluxes (Q1 and Q2). We find that Q0appears independent of WN spectral type, and lower bound estimates tendto cluster around 48 dex. Finally, we discuss the effects of potentialshock excitation and density bounding on these nebulae and compare ourresults to recent models. Our results are consistent with thepredictions of line-blanketed ISA-wind models and nonblanketed CMFGENmodels but are consistent with only some of the line-blanketed CMFGENmodels.
| 2 Micron Narrowband Adaptive Optics Imaging in the Arches Cluster
Canada-France-Hawaii-Telescope adaptive optics bonnette images throughnarrowband filters in the K band are presented for the Arches cluster.Continuum fluxes, line fluxes, and equivalent widths are derived fromhigh angular resolution images, some nearly diffraction limited, for thewell-known massive stars in the Arches cluster. Images were obtained inthe lines He I 2.06 μm, H I Brγ (2.17 μm), and He II 2.19μm, as well as continuum positions at 2.03, 2.14, and 2.26 μm. Inaddition, fluxes are presented for H I Pα (1.87 μm) and anearby continuum position (1.90 μm) from Hubble Space Telescopearchival data.2 The 2 μm and Pαdata reveal two new emission-line stars and three fainter candidateemission-line objects. Indications for a spectral change of one objectbetween earlier observations in 1992-1993 and our data from 1999 arefound. The ratio of He II 2.19 μm to Brγ emission exhibits anarrow distribution among the stars, suggesting a narrow evolutionaryspread centered predominantly on spectral types O4 If or Wolf-Rayetstars of the WN7 subtype. From the approximate spectral types of theidentified emission-line stars and comparisons with evolutionary modelswe infer a cluster age between ~2 and 4.5 Myr.
| A Mid-Infrared Spectral Survey of Galactic Wolf-Rayet Stars
We present 8-13 μm spectra at resolution R~600 of 29 northernGalactic Wolf-Rayet stars, including the first ever reportedmid-infrared (MIR) spectrum for many. Among the subtypes of the starsstudied were 14 WC, 13 WN, 1 WN/WC, and an additional reclassified WN.Lines of He I and He II, along with fine-structure lines of Ne II and SIV, are strongly present in 22 of the sources observed, while six of thesources exhibit the powerful emission of heated circumstellar carbondust. We point out similarities between our spectra and Infrared SpaceObservatory (ISO) observations of several of the same sources and notean unresolved discrepancy between the two data sets for the WC6 star WR146. We investigate the diagnostic power of MIR He I and He II lines forsubtype discrimination and find the line ratio Wλ(9.7μm He II)/Wλ(11.3 μm He I+He II) can providemoderate discrimination within the WN and WC types, though the smallnumber of stars with corresponding line pairs detected made suchassessment difficult.
| The VIIth catalogue of galactic Wolf-Rayet stars
The VIIth catalogue of galactic PopulationI Wolf-Rayet stars providesimproved coordinates, spectral types and /bv photometry of known WRstars and adds 71 new WR stars to the previous WR catalogue. This censusof galactic WR stars reaches 227 stars, comprising 127 WN stars, 87 WCstars, 10 WN/WC stars and 3 WO stars. This includes 15 WNL and 11 WCLstars within 30 pc of the Galactic Center. We compile and discuss WRspectral classification, variability, periodicity, binarity, terminalwind velocities, correlation with open clusters and OB associations, andcorrelation with Hi bubbles, Hii regions and ring nebulae. Intrinsiccolours and absolute visual magnitudes per subtype are re-assessed for are-determination of optical photometric distances and galacticdistribution of WR stars. In the solar neighbourhood we find projectedon the galactic plane a surface density of 3.3 WR stars perkpc2, with a WC/WN number ratio of 1.5, and a WR binaryfrequency (including probable binaries) of 39%. The galactocentricdistance (RWR) distribution per subtype shows RWRincreasing with decreasing WR subtype, both for the WN and WC subtypes.This RWR distribution allows for the possibility ofWNE-->WCE and WNL-->WCL subtype evolution.
| Wolf-Rayet Stars and Relativistic Objects: Distinctions between the Mass Distributions in Close Binary Systems
The observed properties of Wolf-Rayet stars and relativistic objects inclose binary systems are analyzed. The final masses M CO f for thecarbon-oxygen cores of WR stars in WR + O binaries are calculated takinginto account the radial loss of matter via stellar wind, which dependson the mass of the star. The analysis includes new data on the clumpystructure of WR winds, which appreciably decreases the requiredmass-loss rates for the WR stars. The masses M CO f lie in the range (12)M ȯ (20 44)M ȯ and have a continuous distribution. Themasses of the relativistic objects M x are 1 20M ȯ and have abimodal distribution: the mean masses for neutron stars and black holesare 1.35 ± 0.15M ȯ and 8 10M ȯ, respectively, with agap from 2 4M ȯ in which no neutron stars or black holes areobserved in close binaries. The mean final CO-core mass is &$/line M _{CO}(f) = 7.4 - 10.3M_ ȯ$; , close to the mean mass for the black holes. This suggests that it isnot only the mass of the progenitor that determines the nature of therelativistic object, but other parameters as well-rotation, magneticfield, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binariesthat may have low-mass companions (main-sequence or subgiant M-A stars)are identified; these could be the progenitors of low-mass X-raybinaries with neutron stars and black holes.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| A Morphological Diagnostic for Dynamical Evolution of Wolf-Rayet Bubbles
We have observed Hα and [O III] emission from eight of the mostwell-defined Wolf-Rayet ring nebulae in the Galaxy. We find that in manycases the outermost edge of the [O III] emission leads the Hαemission. We suggest that these offsets, when present, are due to theshock from the Wolf-Rayet bubble expanding into the circumstellarenvelope. Thus, the details of the WR bubble morphology at Hα and[O III] can then be used to understand better the physical condition andevolutionary stage of the nebulae around Wolf-Rayet stars, as well as toplace constraints on the nature of the stellar progenitor and itsmass-loss history.
| Exospheric models for the X-ray emission from single Wolf-Rayet stars
We review existing ROSAT detections of single Galactic Wolf-Rayet (WR)stars and develop wind models to interpret the X-ray emission. The ROSATdata, consisting of bandpass detections from the ROSAT All-Sky Survey(RASS) and some pointed observations, exhibit no correlations of the WRX-ray luminosity (LX) with any star or wind parameters ofinterest (e.g. bolometric luminosity, mass-loss rate or wind kineticenergy), although the dispersion in the measurements is quite large. Thelack of correlation between X-ray luminosity and wind parameters amongthe WR stars is unlike that of their progenitors, the O stars, whichshow trends with such parameters. In this paper we seek to (i) test byhow much the X-ray properties of the WR stars differ from the O starsand (ii) place limits on the temperature TX and fillingfactor fX of the X-ray-emitting gas in the WR winds. Adoptingempirically derived relationships for TX and fXfrom O-star winds, the predicted X-ray emission from WR stars is muchsmaller than observed with ROSAT. Abandoning the TX relationfrom O stars, we maximize the cooling from a single-temperature hot gasto derive lower limits for the filling factors in WR winds. Althoughthese filling factors are consistently found to be an order of magnitudegreater than those for O stars, we find that the data are consistent(albeit the data are noisy) with a trend of fx ∝(Mν&infy;)-1 in WR stars, as is also the casefor O stars.
| X-ray Nova Binary Systems
The physical properties of X-ray novae as close binary systems areanalysed. Observational data in X-ray, UV, optical, IR and radio rangesare summarized. Modern theoretical considerations of the problem ofX-ray novae, taking into account some new ideas and results, aredescribed. Properties of optical stars in X-ray novae are analysed. Dataabout the masses of black holes in X-ray binary systems are summarized.Possible evolutionary links between WR stars in close binary systems andX-ray novae are analysed.
| 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.
| Spectra of Wolf-Rayet stars and planetary nebulae
Some spectrograms of Wolf-Rayet (WR) stars and planetary nebulae (PNe)are captured with a home-made spectroscope attached to aSchmidt-Cassegrain telescope, and the spectra are briefly analysed. Theconstruction and potential of a low resolution stellar spectroscope isdiscussed together with techniques to capture images.
| The Interstellar Medium around Galactic WN Stars: WR 2, WR 128, and WR 151
The neutral hydrogen distribution has been studied in the direction ofthree Galactic Wolf-Rayet (W-R) stars, using the 100 m Effelsberg radiotelescope. Neutral hydrogen voids, regions of low H I emissivity, areobserved over a 10-16 km s^-1 velocity range for the stars WR 2 (=HD6327), WR 128 (=HD 187282), and WR 151 (=CX Cep). These minima areinterpreted as the observable 21 cm counterpart of the interstellarbubble created by the powerful winds of W-R stars and their progenitors.The H I cavities are elongated structures having an axial ratio rangingfrom 1.3 (WR 151) to 2.0 (WR 128). As pointed out in previous studies,the W-R stars are always off-center with respect to either the geometriccenter of the H I void or the absolute minimum inside the cavity. Thisoffset ranges from 40% to 70% of the H I hole's minor axis. The majoraxes of these cavities are ~30 pc for both WR 2 and WR 128, and ~50 pcfor WR 151. The missing H I mass amounts to 96 M_solar (WR 128), 200M_solar (WR 2), and 480 M_solar (WR 151). Some of the physicalparameters of the ring nebula associated with WR 128, such as itsexpansion velocity, 80+/-22 km s^-1, radius, 6.3+/-0.2 pc, andcharacteristic age, (7.4+/-2.4)x10^4 yr, are derived from a simplegeometric model that accounts for the discrepancy between the radialvelocity of the H I hole and the ring nebula related to WR 128.
| 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.''
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| 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.
| A spectropolarimetric survey of northern hemisphere Wolf-Rayet stars
We present a homogeneous, high signal-to-noise spectropolarimetricsurvey of 16 northern hemisphere Wolf-Rayet (WR) stars. A reduction inpolarization at emission-line wavelengths - the `line effect' - isidentified in four stars: WRs 134, 137, 139, and 141. The magnitude ofthe effect in WR 139 (V444 Cyg) is variable, while WR 136, previouslyreported to show the line effect, does not show it in our data. Assumingthe line effect generally to arise from axisymmetric distortions ofstellar winds, we show that a model in which all WRs have the sameintrinsic (equator-on) polarization, with the observed variations solelya result of inclination effects, is inconsistent with the observations.A model in which the intrinsic polarizations are uniformly distributedis more plausible, but best-fitting results are obtained if thedistribution of polarizations is biased towards small values, with only~20 per cent of stars having intrinsic polarizations greater than ~0.3per cent. Radiative transfer calculations indicate that the observedcontinuum polarizations can be matched by models with equator:poledensity ratios of 2-3. The model spectra have electron-scattering wingsthat are significantly stronger than observed (in both intensity andpolarized flux), confirming that the winds of stars showing intrinsicpolarization must be clumped on small scales as well as being distortedon large scales. We combine the results of our survey with observationsfrom the literature to give a sample of 29 stars which have bothaccurate spectropolarimetric observations and physical parametersderived from standard-model analyses. We find that the line-effect starsare clustered at high M, L in the luminosity-mass-loss rate plane(although they are unexceptional in the terminal velocity-subtype andthe surface-mass-flux-temperature planes). The mass-loss rates derivedfrom radio-continuum observations for these stars are in good accordwith the results of optical emission-line analyses, suggesting that (i)the wind structure of line-effect stars has a density contrast which iseffectively constant with radius, and (ii) the high M values may beartefacts of large-scale wind structure. Assuming that observedspectroscopic and photometric variability of the line-effect stars isrelated to the WR rotation period, we compute equatorial rotationvelocities. These velocities correspond to ~10 per cent of the corebreakup rates, and may be large enough to produce significantwind-compression effects according to the models of Ignace, Cassinelli& Bjorkman.
| The relationship between the WR classification and stellar models. II. The WN stars without hydrogen
We consider the relationships between the classification parameters ofWN stars in the new 3-dimensional classification of Smith et al. (1996)and the corresponding and related parameters that define stellaratmosphere models. Specifically, we consider: FWHM of HeII 4686 vs.v_infty ; hydrogen content by direct inspection vs. hydrogen content bymodelling and vs. colour (b-v)_0; ionisation subclass and M_v; vs.effective temperature. From these data we argue that the WN b and onlythe WN b stars (i.e. stars with EW 5411 > 40 Angstroms \ or FWHM 4686> 30 Angstroms) are entirely free of hydrogen. For the WN b stars, weconsider the relationships of EW 5411 and FWHM 4686 to the derivedtemperature T_*; the mass loss rate; and the surface mass flux. Itappears that, to first approximation, the stars are a one-parameterfamily and the spectral classification criteria are sufficient to givean indication of the intrinsic colour, absolute magnitude (not veryaccurately), effective temperature T_* and terminal velocity.Theoretical models suggest that the critical parameter defining most ofthe properties of a WN b star is its present mass. However, thebehaviour of FWHM 4686 suggests the presence of a second parameter thataffects the mass loss rate and terminal velocity of the wind. We suggestthat the second parameter may be either (or a combination of) theinternal mean molecular weight or the rotation rate of the star. Wefurther compare the relationships predicted by evolutionary models withthose found for observed stars (using atmosphere models), highlightingthe present difficulties in these comparisons.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | חץ |
| התרוממות ימנית: | 19h48m32.20s |
| סירוב: | +18°12'03.7" |
| גודל גלוי: | 10.457 |
| תנועה נכונה: | 0.3 |
| תנועה נכונה: | -7.4 |
| B-T magnitude: | 10.49 |
| V-T magnitude: | 10.46 |
קטלוגים וכינוים:
|
