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X-ray emission characteristics of two Wolf-Rayet binaries: V444 Cyg and CD Cru
Using data from observations made with XMM-Newton, we present an X-rayanalysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru. The X-raylight curves show the phase-locked variability in both binaries, wherethe flux increased by a factor of ~2 for V444 Cyg and ~1.5 for CD Cru,from minimum to maximum. The maximum luminosities in the 0.3-7.5 keVenergy band were found to be 5.8 × 1032 and 2.8 ×1032 erg s-1 for V444 Cyg and CD Cru,respectively. The X-ray spectra of these stars confirmed largeextinction and revealed hot plasma with prominent emission-line featuresof highly ionized Ne, Mg, Si, S, Ar, Ca and Fe; these are found to beconsistent with a two-temperature plasma model. At a temperature of ~0.6keV, the cooler plasma was found to be constant at all phases for bothbinaries, which could be the result of a distribution of small-scaleshocks in radiation-driven outflows. The hot components in thesebinaries were found to be phase-dependent. They varied from 1.85 to 9.61keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption ofthe hard component varied with the orbital phase and was found to bemaximum during the primary eclipse of V444 Cyg. The high plasmatemperature and variability with orbital phase suggest that thehard-component emission is caused by a colliding wind shock between thebinary components.
| The massive Wolf-Rayet binary LSS 1964 (=WR 29). II. The V light curve
Context: WR 29 is a known WN7h+O double-lined binary system with arather short period (3.164 days). Aims: We search for lightvariations to determine the inclination of the system and thus theabsolute masses of both components. Methods: We observedphotometrically the field of WR 29 between December, 2002, and February,2006. Results: We find that the V light of WR 29 varies in phasewith the spectroscopic period of 3.16412 days, presenting two minimacorresponding to the conjunctions of the binary components. Numericalmodels fitted to the light curve indicate an orbital inclination ofabout 44°, and masses of 53 M_? and 42 M_? for the O- andWN-type components, respectively.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/1269 Based on datacollected at CASLEO, CTIO, and LCO. Visiting Astronomer, CASLEO,Argentina, and CTIO and LCO, Chile. Visiting Astronomer, CASLEO,Argentina. In memoriam (1936-2006). Visiting Astronomer, LCO and CTIO,Chile.
| Description of the ``Scenario Machine''
We present an updated description of the “Scenario Machine”code, which is used to carry out population-synthesis analyzes of theevolution of close binary stars.
| A Near-Infrared Survey of the Inner Galactic Plane for Wolf-Rayet Stars. I. Methods and First Results: 41 New WR Stars
The discovery of new Wolf-Rayet (WR) stars in our Galaxy via large-scalenarrowband optical surveys has been severely limited by dust extinction.Recent improvements in infrared technology have madenarrowband-broadband imaging surveys viable again. We report a new J, K,and narrowband imaging survey of 300 deg2 of the plane of theGalaxy, spanning 150 degrees in Galactic longitude and reaching 1 degreeabove and below the Galactic plane. The survey has a useful limitingmagnitude of K = 15 over most of the observed Galactic plane, and K = 14within a few degrees of the Galactic center. Thousands of emission linecandidates have been detected. In spectrographic follow-ups of 173 WRstar candidates we have discovered 41 new WR stars, 15 of type WN and 26of type WC. Star subtype assignments have been confirmed with K-bandspectra, and distances approximated using the method of spectroscopicparallax. A few of the new WR stars are among the most distant known inour Galaxy. The distribution of these new WR stars is seen to followthat of previously known WR stars along the spiral arms of the Galaxy.Tentative radial velocities were also measured for most of the new WRstars.
| Rotational mixing in massive binaries. Detached short-period systems
Models of rotating single stars can successfully account for a widevariety of observed stellar phenomena, such as the surface enhancementsof N and He observed in massive main-sequence stars. However, recentobservations have questioned the idea that rotational mixing is the mainprocess responsible for the surface enhancements, emphasizing the needfor a strong and conclusive test for rotational mixing. We investigatethe consequences of rotational mixing for massive main-sequence stars inshort-period binaries. In these systems the tides are thought to spin upthe stars to rapid rotation, synchronous with their orbital revolution.We use a state-of-the-art stellar evolution code including the effect ofrotational mixing, tides, and magnetic fields. We adopt a rotationalmixing efficiency that has been calibrated against observations ofrotating stars under the assumption that rotational mixing is the mainprocess responsible for the observed surface abundances. We find thatthe primaries of massive close binaries (M1 ? 20{M}?, P_orb ? 3 days) are expected to showsignificant enhancements in nitrogen (up to 0.6 dex in the SmallMagellanic Cloud) for a significant fraction of their corehydrogen-burning lifetime. We propose using such systems to test theconcept of rotational mixing. As these short-period binaries often showeclipses, their parameters can be determined with high accuracy. For theprimary stars of more massive and very close systems (M1 ?50 {M}?, P_orb? 2 days) we find that centrallyproduced helium is efficiently mixed throughout the envelope. The starremains blue and compact during the main sequence evolution and stayswithin its Roche lobe. It is the less massive star, in which the effectsof rotational mixing are less pronounced, which fills its Roche lobefirst, contrary to what standard binary evolution theory predicts. Theprimaries will appear as “Wolf-Rayet stars in disguise”:core hydrogen-burning stars with strongly enhanced He and N at thesurface. We propose that this evolution path provides an alternativechannel for the formation of tight Wolf-Rayet binaries with amain-sequence companion and might explain massive black hole binariessuch as the intriguing system M33 X-7.
| 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.
| β Cephei stars in the ASAS-3 data. II. 103 new β Cephei stars and a discussion of low-frequency modes
Context: The β Cephei stars have been studied for over a hundredyears. Despite this, many interesting problems related to this class ofvariable stars remain unsolved. Fortunately, these stars seem to bewell-suited to asteroseismology. Hence, the results of seismic analysisof β Cephei stars should help us to better understand pulsationsand the main sequence evolution of massive stars, particularly theeffect of rotation on mode excitation and internal structure. It istherefore extremely important to increase the sample of known βCephei stars and select targets that are useful for asteroseismology. Aims: We analysed ASAS-3 photometry of bright early-type stars with thegoal of finding new β Cephei stars. We were particularly interestedin β Cephei stars that would be good for seismic analysis, i.e.,stars that (i) have a large number of excited modes; (ii) showrotationally split modes; (iii) are components of eclipsing binarysystems; (iv) have low-frequency modes, that is, are hybrid βCephei/SPB stars. Methods: Our study was made with a homogeneous sampleof over 4100 stars having MK spectral type B5 or earlier. For thesestars, the ASAS-3 photometry was analysed by means of a Fourierperiodogram. Results: We have discovered 103 β Cephei stars,nearly doubling the number of previously known stars of this type. Amongthese stars, four are components of eclipsing binaries, seven have modesequidistant or nearly equidistant in frequency. In addition, we foundfive β Cephei stars that show low-frequency periodic variations,very likely due to pulsations. We therefore regard them as candidatehybrid β Cephei/SPB pulsators. All these stars are potentially veryuseful for seismic modeling. Moreover, we found β Cephei-typepulsations in three late O-type stars and fast period changes in one, HD168050.Table 2 and Figs. 2-14 are only available in electronic form athttp://www.aanda.org The V photometry for all 103 stars is available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/477/917
| On the X-ray and optical properties of the Be star HD 110432: a very hard-thermal X-ray emitter
HD 110432 is the first proposed, and best studied,member of a growing group of Be stars with X-ray properties similar toγ Cas. These stars exhibit hard-thermal X-raysthat are variable on all measurable timescales. This emission contrastswith the soft emission of “normal" massive stars and with thenon-thermal emission of all well known Be/X-ray binaries - so far, allBe + neutron star systems. In this work we present X-ray spectral andtiming properties of HD 110432 from three XMM-Newtonobservations in addition to new optical spectroscopic observations. Likeγ Cas, the X-rays of HD110432 appear to have a thermal origin, as supported bystrongly ionized Fe XXV and Fe XXVI lines detected in emission. Afluorescent iron feature at 6.4 keV is present in all observations,while the Fe XXVI Lyβ line is present in two of them. Its X-rayspectrum, complex and time variable, is well described in eachobservation by three thermal plasmas with temperatures ranging between0.2-0.7, 3-6, and 16-37 keV. Thus, HD 110432 has thehottest thermal plasma of any known Be star. A sub-solar iron abundance(~0.3-0.5 ×Z_Fe,ȯ) is derived for the hottest plasma, whilelines of less excited ions at longer wavelengths are consistent withsolar abundances. The star has a moderate 0.2-12 keV luminosity of~5×1032 erg s-1. The intensity of the X-rayemission is strongly variable. Recurrent flare-like events on timescales as short as ~10 s are superimposed over a basal flux which varieson timescales of ~5-10×103 s, followed by similarlyrapid hardness variabilities. There is no evidence for coherentoscillations, and an upper limit of ~2.5% is derived on the pulsedfraction for short pulsations from 0.005 to 2.5 Hz. In the opticalregion the strong and quasi-symmetrical profile of the Hα line (EW~ -60 Å) as well as the detection of several metallic lines inemission strongly suggest a dense and/or large circumstellar disk. Also,the double-peaked profiles of metallic lines confirm the nearly edge-onprojection of that disk noted recently by Smith & Balona.HD 110432 has several properties reminiscent of thecataclysmic variables such as a very hot X-ray temperature and some ofits detailed spectral features. This suggests that it might be a Be starharbouring an accreting white dwarf. On the other hand, recent evidenceof magnetic activity reported in the literature of HD110432 suggests an interaction between the surface of the Bestar and its disk can produce the X-rays.Based on the public data archive of the XMM-Newton, an ESA sciencemission with instruments and contributions directly funded by ESA MemberStates and NASA.
| Physical Properties of Wolf-Rayet Stars
The striking broad emission line spectroscopic appearance of Wolf-Rayet(WR) stars has long defied analysis, owing to the extreme physicalconditions within their line- and continuum-forming regions. Recently,model atmosphere studies have advanced sufficiently to enable thedetermination of stellar temperatures, luminosities, abundances,ionizing fluxes, and wind properties. The observed distributions ofnitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and innearby star-forming galaxies are discussed; these imply lower limits toprogenitor masses of ?25, 40, and 75 Mȯ forhydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars,respectively. WR stars in massive star binaries permit studies ofwind-wind interactions and dust formation in WC systems. They also showthat WR stars have typical masses of 10 25 Mȯ, extendingup to 80 Mȯ for H-rich WN stars. Theoretical andobservational evidence that WR winds depend on metallicity is presented,with implications for evolutionary models, ionizing fluxes, and the roleof WR stars within the context of core-collapse supernovae andlong-duration gamma-ray bursts.
| On rejuvenation in massive binary systems
We introduce a set of stellar models for massive stars whose evolutionhas been affected by mass transfer in a binary system, at a range ofmetallicities. As noted by other authors, the effect of such masstransfer is frequently more than just rejuvenation. We find that, whilststars with convective cores which have accreted only H-rich matterrejuvenate as expected, those stars which have accreted He-rich matter(e.g. at the end stages of conservative mass transfer) evolve in a waythat is qualitatively similar to rejuvenated stars of much highermetallicity. Thus, the effects of non-conservative evolution dependstrongly on whether He-rich matter is amongst the portion accreted orejected. This may lead to a significant divergence in binary evolutionpaths with only a small difference in initial assumptions. We compareour models to observed systems and find approximate formulae for theeffect of mass accretion on the effective age and metallicity of theresulting star.
| Direct constraint on the distance of γ2 Velorum from AMBER/VLTI observations
Context: . Interferometry can provide spatially resolved observations ofmassive star binary systems and their colliding winds, which thus farhave been studied mostly with spatially unresolved observations. Aims: . We present the first AMBER/VLTI observations, taken at orbitalphase 0.32, of the Wolf-Rayet and O (WR+O) star binary systemγ2 Velorum and use the interferometricobservables to constrain its properties. Methods: . TheAMBER/VLTI instrument was used with the telescopes UT2, UT3, and UT4 onbaselines ranging from 46 m to 85 m. It delivered spectrally dispersedvisibilities, as well as differential and closure phases, with aresolution R=1500 in the spectral band 1.95-2.17 μm. We interpretthese data in the context of a binary system with unresolved components,neglecting in a first approximation the wind-wind collision zone fluxcontribution. Results: . Using WR- and O-star synthetic spectra,we show that the AMBER/VLTI observables result primarily from thecontribution of the individual components of the WR+O binary system. Wediscuss several interpretations of the residuals, and speculate on thedetection of an additional continuum component, originating from thefree-free emission associated with the wind-wind collision zone (WWCZ),and contributing at most to the observed K-band flux at the 5% level.Based on the accurate spectroscopic orbit and the Hipparcos distance,the expected absolute separation and position angle at the time ofobservations were 5.1±0.9 mas and 66±15°,respectively. However, using theoretical estimates for the spatialextent of both continuum and line emission from each component, we infera separation of 3.62+0.11-0.30 mas and a positionangle of 73+9-11°, compatible with theexpected one. Our analysis thus implies that the binary system lies at adistance of 368+38-13 pc, in agreement with recentspectrophotometric estimates, but significantly larger than theHipparcos value of 258+41-31 pc. Based on observations collected at the European Southern Observatory,Paranal, Chile, within the guaranteed time programme 074.A-9025(A).
| The luminosity excess of OB stars in quasi-stationary X-ray binaries
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| 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.
| 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.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| 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.
| 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.
| 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.
| A revision of the fundamental parameters of the open cluster Hogg 15 and the projected star WR 47
We revise the fundamental parameters of the faint open cluster Hogg 15,for which two recent colour-magnitude diagram (CMD) studies haveobtained significantly different ages. In the present study, we combinea series of methods trying to constrain age, together with otherfundamental parameters. We employ spatial extractions to construct theCMDs, and the cluster integrated spectrum to compare it with those oftemplates of known age. We derive fundamental parameters, in particular,distance, of the closely projected Wolf-Rayet star HDE 311884 (WR 47) -often proposed to be physically related to Hogg 15. Based on the WR 47spectrum and available photometry, we conclude that the short distanceimplied by the Hipparcos parallax (216 pc) is affected by binary motion.From the WR 47 spectrum we estimate a reddening E(B-V) and a distance of1.10 +/- 0.05 and 5.2 +/- 0.9 kpc, respectively. For Hogg 15 we derivean age of 20 +/- 10 Myr, a reddening of 1.10 +/- 0.05, and a distance of3.1 +/- 0.5 kpc. We conclude that Hogg 15 is not related to WR 47 fromthe point of view of origin, since the cluster and the star do notbelong to the same formation event.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| 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.
| Photometric Survey of Southern Open Clusters: Melotte 105, HOGG 15 PISMIS 21 and Ruprecht 140
Not Available
| A multicolour CCD photometric and mass function study of the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253
We derive cluster parameters and mass functions from new UBVRI CCDphotometric observations of ~3500 stars reaching down to V~20mag for thedistant southern open star clusters NGC 3105, NGC 3603, Melotte 105,Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCDdata are presented for the first time. The reddening is non-uniformacross the face of the young (age <300Myr) clusters NGC 3105, NGC3603, Melotte 105, Hogg 15 and Pismis 20, with average values ofE(B-V)=1.06, 1.44, 0.52, 1.15 and 1.20mag respectively, while it isuniform with average values of E(B-V)=0.72 and 0.20mag for the olderclusters NGC 4815 and 6253 respectively. The values of colour excessratios indicate the presence of normal interstellar reddening across thecluster regions studied here. Well-defined main sequences can be seen inall the clusters. However, main-sequence turn-off points and subgiantbranches are well defined only in the older clusters NGC 4815 and 6253.The distances to the clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15,NGC 4815, Pismis 20 and NGC 6253 are 9.5+/-1.5, 7.2+/-1.2, 2.3+/-0.2,3.0+/-0.3, 2.75+/-0.2, 3.55+/-0.35 and 1.8+/-0.12kpc respectively, whilethe corresponding ages derived using theoretical convective coreovershooting stellar evolutionary isochrones are 25+/-10, 3+/-2,250+/-30, 6+/-2, 400+/-50, 6+/-2 and 2500+/-600Myr respectively. In themass range ~1-75Msolar, the mass functions of all clustersexcept for NGC 6253 have been studied. The slopes of their mass spectraagree within errors with the Salpeter value (1.35). The slope of themass function for stars more massive than 10Msolar is almostthe same as for the lower mass stars. The mass function slopes ofclusters younger than 500Myr seem to have no dependence on Galacticlongitude, Galactocentric distance and cluster age. As the inherentuncertainties in the mass function determinations of young Galactic starclusters can produce internal scatter that is larger than the externalscatter, we conclude that, above 1Msolar, the initial massfunction is universal with a slope of Salpeter value.
| 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.
| On the stellar content of the open clusters Melotte 105, Hogg 15, Pismis 21 and Ruprecht 140
CCD observations in the B, V and I passbands have been used to generatecolour-magnitude diagrams reaching down to V ~ 19 mag for two slightlycharacterized (Melotte 105 and Hogg 15) and two almost unstudied (Pismis21 and Ruprecht 140) open clusters. The sample consists of about 1300stars observed in fields of about 4arcmin x4arcmin . Our analysis showsthat neither Pismis 21 nor Ruprecht 140 are genuine open clusters sinceno clear main sequences or other meaningful features can be seen intheir colour-magnitude diagrams. Melotte 105 and Hogg 15 are openclusters affected by E(B-V) = 0.42 +/- 0.03 and 0.95 +/- 0.05,respectively. Their distances to the Sun have been estimated as 2.2 +/-0.3 and 2.6 +/- 0.08 kpc, respectively, while the corresponding agesestimated from empirical isochrones fitted to the Main Sequence clustermembers are ~ 350 Myr and 300 Myr, respectively. The present data arenot consistent with the membership of the WN6 star HDE 311884 to Hogg15. Tables 2 to 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.793.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/370/931
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