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Discovery and Interpretation of an X-Ray Period in the Galactic Center Source CXOGC J174536.1-285638
We present X-ray and infrared observations of the X-ray source CXOGCJ174536.1-285638. Previous observations suggest that this source may bean accreting binary with a high-mass donor (HMXB) or a colliding windbinary (CWB). Based on the Chandra and XMM-Newton light curve, we havefound an apparent 189+/-6 day periodicity with better than 99.997%confidence. We discuss several possible causes of this periodicity,including both orbital and superorbital interpretations. We explore indetail the possibility that the X-ray modulation is related to anorbital period and discuss the implications for two scenarios; one inwhich the variability is caused by obscuration of the X-ray source by astellar wind, and the other in which it is caused by an eclipse of theX-ray source. We find that in the first case, CXOGC J174536.1-285638 isconsistent with both CWB and HMXB interpretations, but in the second,CXOGC J174536.1-285638 is more likely a HMXB.
| The luminosity excess of OB stars in quasi-stationary X-ray binaries
Not Available
| 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.
| 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.
| Constraining the mass transfer in massive binaries through progenitor evolution models of Wolf-Rayet+O binaries
Since close WR+O binaries are the result of a strong interaction of bothstars in massive close binary systems, they can be used to constrain thehighly uncertain mass and angular momentum budget during the major masstransfer phase. We explore the progenitor evolution of the three bestsuited WR+O binaries HD 90657, HD 186943 and HD 211853, which arecharacterized by a WR/O mass ratio of ~0.5 and periods of 6...10 days.We are doing so at three different levels of approximation: predictingthe massive binary evolution through simple mass loss and angularmomentum loss estimates, through full binary evolution models withparametrized mass transfer efficiency, and through binary evolutionmodels including rotation of both components and a physical model whichallows to compute mass and angular momentum loss from the binary systemas function of time during the mass transfer process. All three methodsgive consistently the same answers. Our results show that, if thesesystems formed through stable mass transfer, their initial periods weresmaller than their current ones, which implies that mass transfer hasstarted during the core hydrogen burning phase of the initially moremassive star. Furthermore, the mass transfer in all three cases musthave been highly non-conservative, with on average only ~10% of thetransferred mass being retained by the mass receiving star. This resultgives support to our system mass and angular momentum loss model, whichpredicts that, in the considered systems, about 90% of the overflowingmatter is expelled by the rapid rotation of the mass receiver close tothe Ω-limit, which is reached through the accretion of theremaining 10%.
| 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.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| Rotating Progenitors of the Wolf-Rayet Binaries HD186943 and HD90657
We present rotating progenitor models for the WR+O binary systemsHD186943 and HD90657 (van der Hucht, 2001) calculated with theevolutionary code described by Wellstein (2001). Due to rotation, theeffective accretion onto the secondary star is reduced as it is spun-upclose to its break-up velocity and thereby undergoes strong mass loss.We investigate the progenitor evolution of the two observed WR+O binarysystems through stable mass transfer. We conclude that these systemsevolved through Case A mass transfer as they have short orbital periodsnow (8...10 days), and the progenitor systems must have started with ashorter or equal period. We show that there has to be a significant massloss from the binary system to produce WR+O systems similar to theobserved ones. The accretion efficiency of the secondary star in ourrotating models is 10%. We compare properties of the observed andmodelled systems in Table 1.
| Wolf-Rayet Stars, Black Holes, and Gamma-Ray Bursters in Close Binaries
We consider the evolutionary status of observed close binary systemscontaining black holes and Wolf-Rayet (WR) stars. When the componentmasses and the orbital period of a system are known, the reason for theformation of a WR star in an initial massive system of two main-sequencestars can be established. Such WR stars can form due to the action ofthe stellar wind from a massive OB star (M OB≥50M ȯ),conservative mass transfer between components with close initial masses,or the loss of the common envelope in a system with a large (up to25) initial component mass ratio. The strong impact ofobservational selection effects on the creation of samples of closebinaries with black holes and WR stars is demonstrated. We estimatetheoretical mass-loss rates for WR stars, which are essential for ourunderstanding the observed ratio of the numbers of carbon and nitrogenWR stars in the Galaxy . We also estimate the minimum initial masses ofthe components in close binaries producing black holes and WR stars tobe 25M ȯ. The spatial velocities of systems with black holesindicate that, during the formation of a black hole from a WR star, themass loss reaches at least several solar masses. The rate of formationof rapidly rotating Kerr black holes in close binaries in the Galaxy is3×10-6 yr-1. Their formation may be accompanied by a burst ofgamma radiation, possibly providing clues to the nature of gamma-raybursts. The initial distribution of the component mass ratios for closebinaries is dNdq=dM 2/M 1 in the interval 0.04≲q 0≤1,suggesting a single mechanism for their formation.
| 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.
| Evolution of Wolf-Rayet Stars in Binary Systems: An Analysis of the Mass and Orbital-Eccentricity Distributions
We have undertaken a statistical study of the component mass ratios andthe orbital eccentricities of WR + O close binary, detachedmain-sequence (DMS), contact early-type (CE), and semidetached (SD)systems. A comparison of the characteristics of WR + O systems and ofDMS, CE, and SD systems has enabled us to draw certain conclusions aboutthe evolutionary paths of WR + O binaries and to demonstrate that up to90% of all known WR + O binaries formed as a result of mass transfer inmassive close O + O binary systems. Since there is a clear correlationbetween the component masses in SD systems with subgiants, the absenceof an anticorrelation between the masses of the WR stars and O stars inWR + O binaries cannot be considered evidence against the formation ofWR + O binaries via mass transfer. The spectroscopic transitionalorbital period P tr sp corresponding to the transition from nearlycircular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is14d for WR + O systems and 2d 3d for OB + OB systems. Theperiod range in which all WR + O orbits are circular &$(1mathop dlimits_. 6 ≤slant P ≤slant 14(d) ); is close to the range for SD systems with subgiants, &0mathop dlimits_. 7 ≤slant P ≤slant 15(d); . The large difference between the P tr sp values for WR + O and OB +OB systems suggests that a mechanism of orbit circularization additionalto that for OB + OB systems at the DMS stage (tidal dissipation of theorbital energy due to radiative damping of the dynamical tides) acts inWR + O binaries. It is natural to suggest mass transfer in the parent O+ O binaries as this supplementary orbit-circularization mechanism.Since the transitional period between circular and elliptical orbits forclose binaries with convective envelopes and ages of 5×109 yearsis &P_{tr} = 12mathop dlimits_. 4$; , the orbits of most known SD systems with subgiants had enough timeto circularize during the DMS stage, prior to the mass transfer. Thus,for most SD systems, mass transfer plays a secondary role incircularization of their orbits. In many cases, the initial orbitaleccentricities of the O + O binary progenitors of WR + O systems arepreserved, due to the low viscosity of the O-star envelopes and theshort timescale for their nuclear evolution until the primary O starfills its Roche lobe and the mass transfer begins. The mass transfer inthe parent O + O systems is short-lived, and the number of orbitalcycles during the early mass-transfer stage is relatively low (lowerthan for the progenitors of SD systems by three or four orders ofmagnitude). The continued transfer of mass from the less massive to themore massive star after the component masses have become equal leads tothe formation of a WR + O system, and the orbit's residual eccentricityincreases to the observed value. The increase of the orbitaleccentricity is also facilitated by variable radial mass loss via thewind from the WR star in the WR + O system during its motion in theelliptical orbit. The result is that WR + O binaries can haveconsiderable orbital eccentricities, despite their intense masstransfer. For this reason, the presence of appreciable eccentricitiesamong WR + O binaries with large orbital periods cannot be consideredfirm evidence against mass transfer in the parent O + O binary systems.Only for the WR + O binaries with the longest orbital periods (4 of 35known systems, or 11 %) can the evolution of the parent O + O binariesoccur without filling of the Roche lobe by the primary O star, beinggoverned by radial outflow in the form of the stellar wind and possiblyby the LBV phenomenon, as in the case of HD 5980.
| Progenitor models of Wolf-Rayet binaries: short-period WNE+O binaries with mass ratios q = 0.5
We identify two possible paths for the progenitor evolution of observedWNE+O binaries with WNE/O mass ratios close to 0.5 and periods between 7and 10 d. We show, through detailed binary evolution models, that withthe assumption that the O-type star expels most of the matter flowing atit during mass transfer, one possibility to obtain the observed systemsis through Case A mass transfer. We find a second solution usingstandard common envelope evolution. We conclude that in either case theO-type star in the three investigated systems did not accretesignificant amounts of mass. We discuss the intricate situation that inother cases massive close binaries may evolve conservatively.
| Sums of investigation of the linear polarization behaviour of binary systems with a Wolf-Rayet component
Analysis of the long-term (on a scale of years) behaviour of linearpolarization of four WR binary systems (CQ Cep, CX Cep, V444 Cyg and HD211853) is presented. Common features of the long-term polarizationvariations of CQ Cep, CX Cep and HD 211853 in combination with theresults of the harmonic analysis of their polarization curves allowed usto make assumptions on the causes of the found variability. The basicreason of the long-term polarization variability is likely to be thephysical activity of the WR components which manifests itself in theepisodic swelling of the WR envelopes and subsequent expulsion of theiroutermost layers. The involvement into the study of five wider "WR+O"pairs (HDE 311884, HD 90657, HD 97152, HD 152270 and HD 186943) allowedus to confirm these assumptions. The results of the analysis of thepolarization curves of nine WR binary systems are summed up. Threeconfirmations of high massiveness of the WR comnponent HDE 311884 havebeen derived.
| 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.
| Wolf-Rayet Stars and Cosmic Gamma-ray Bursts
The observational properties of cosmic gamma-ray bursts and ofWolf-Rayet (WR) stars and their CO cores at the end of their evolutionare analyzed. WR stars do not have hydrogen envelopes, facilitating thetransformation of the energy of collapse into observable gamma rays. Ofthe ≈90 well-localized gamma-ray bursts, 21 have opticalidentifications, of which 16 have measured redshifts (z=0.4 4.5). Thedistribution of gamma-ray bursts in energy N(ΔE) has a largescatter, from 3×1051 to 2×1054 erg. There is some evidencethat the distribution N(ΔE) is bimodal if we include the gamma-rayburst GRB 980425, which is associated with the peculiar type Icsupernova SN 1998bw in the nearby elliptical galaxy ESO 184-G82, forwhich ΔE γ≈1048erg. These characteristics of gamma-raybursts are reminiscent of the distribution of final masses for the COcores of WR stars, which uniformly covers a broad range: M CO=(1 2)Mȯ-(20 44)M ȯ. The possible bimodality of the gamma-ray burstenergy distribution (E 1=1048 erg; ΔE2=3×1051-2×1054erg) could be associated with the bimodalmass distribution for stellar relativistic objects (MNS=(1.35±0.15)M ȯ; M BH=4 15M ȯ). The fact that SN1998bw is a “peculiar” type Ic supernova, not typical forthe collapses of WR stars (which usually give rise to type Ib/csupernovae), could be related to the rotation of the collapsing CO core.This “drags out/rd the time for the collapse, leading to theformation of a neutron star, a decrease in the gamma-ray burst energy,and an increase in the fraction of kinetic energy transferred to thesupernova envelope. The expected rate of collapse of the CO cores of WRstars in the Galaxy is ≈10-3/yr. This is at least three orders ofmagnitude higher than the mean frequency of gamma-ray bursts per galaxy(≈10-6 10-7/yr). Two models for gamma-ray bursts with WR stars asprogenitors are considered: the hypernova model of Paczynski (1998) andthe pulsation instability CO-core collapse model proposed by Gershte&$/set{lower0.5emhbox{smashriptscriptstylesmile}}{l} $; n (2000). In both models, the rate of CO-core collapses can be broughtinto agreement with the observed rate of gamma-ray bursts by taking intoaccount the anisotropy of the gamma radiation, associated with either arelativistic jet or the random character of the initial CO-core collapsedue to instabilities. It is concluded that WR stars could be theprogenitors of gamma-ray bursts. This hypothesis predicts the existenceof two types of gamma-ray bursts, corresponding to the bimodal massdistribution for stellar relativistic objects, and of three types ofoptical afterglow, associated with collapses of the CO cores of WR starsthat are single, in WR+O binaries, and in hypothetical WR+(A-M) systems.The paper also briefly examines a model of gamma-ray bursts as transientphenomena in the early stages of the evolution of galaxies (z>1),when very massive stars (M>100M ȯ) weak in heavy elements couldform. Such massive stars should also lose their hydrogen envelopes andbe transformed into massive WR stars, whose collapses could beaccompanied by gamma-ray bursts. It is suggested that WR galaxies arethe most probable candidates for the host galaxies of gamma-ray bursts.
| 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.
| 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.
| 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.
| Refined Orbital Parameters of HD 193928
Not Available
| 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.''
| Apsidal Motion in Double Stars. I. Catalog
A catalog of 128 double stars with measured periods of apsidal motion iscompiled. Besides the apsidal periods, the orbital elements of binariesand physical parameters of components (masses, radii, effectivetemperatures, surface gravities) are given. The agreement of the apsidalperiods found by various authors is discussed.
| 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.
| The WR and O-type star population predicted by massive star evolutionary synthesis
Evolutionary calculations of massive single stars and of massive closebinaries that we use in the population number synthesis (PNS) code arepresented. Special attention is given to the assumptions/uncertaintiesinfluencing these stellar evolutionary computations (and thus the PNSresults). A description is given of the PNS model together with theinitial statistical distributions of stellar parameters needed toperform number synthesis.We focus on the population of O-type stars andWR stars in regions where star formation was continuous in time and instarburst regions. We discuss the observations that have to be explainedby the model. These observations are then compared to the PNSpredictions.We conclude that: . probably the majority of the massivestars are formed as binary components with orbital period between 1 dayand 10 yr; most of them interact. . at most 8% of the O-type stars arerunaways due to a previous supernova explosion in a binary; recentstudies of pulsar space velocities and linking the latter to the effectof asymmetrical supernova explosions, reveal that only a smallpercentage of these runaways will have a neutron star companion. . withpresent day stellar evolutionary computations, it is difficult toexplain the observed WR/O number ratio in the solar neighbourhood and inthe inner Milky Way by assuming a constant star formation rate, with orwithout binaries. The observed ratio for the Magellanic Clouds is betterreproduced. . the majority of the single WR stars may have had a binarypast. . probably merely 2-3% (and certainly less than 8%) of all WRstars have a neutron star companion. . a comparison between theoreticalprediction and observations of young starbursts is meaningful only ifbinaries and the effect of binary evolution are correctly included. Themost stringent feature is the rejuvenation caused by mass transfer.
| 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.
| Clumping-corrected mass-loss rates of Wolf-Rayet stars
Mass-loss rates of Galactic Wolf-Rayet stars have been determined fromtheir radio emission power and spectral index (alpha = dln {f_ν} /dln ν), accounting for the clumped structure and (potential) variableionization in their outer winds. The average spectral index between mm-and cm- wavelengths is ~ 0.77 for WN stars and ~ 0.75 for WC stars, incontrast with ~ 0.58 expected for smooth winds. The observed wavelengthdependence of alpha can be explained using clumped wind models in somecases, with shocks (at 30-100 stellar radii) producing a higherionization zone in the outer wind. We obtain an empirical formularelating mass-loss with observed optical emission line equivalentwidths, with application to stars without measured radio fluxes.Clumping-corrected mass-loss rates are generally lower than thoseobtained by current smooth wind models. Specifically we find log\mdot(clumpy)-log \mdot(smooth)=-0.19 (sigma=0.28) for WN stars, and log\mdot(clumpy)-log \mdot(smooth)=-0.62 (sigma=0.19) for WC stars. Newmass-loss rate estimates agree very well with (clumping independent)determinations of WR components in binary systems.
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| Сазвежђа: | Лисица |
| Ректацензија: | 19h46m15.94s |
| Deклинација: | +28°16'19.1" |
| Apparent магнитуда: | 10.311 |
| Proper motion RA: | -3 |
| Proper motion Dec: | -5.5 |
| B-T magnitude: | 10.574 |
| V-T magnitude: | 10.333 |
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INHABITED SKY
