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Modelling spectral line profiles of wind-wind shock emissions from massive binary systems
One of the most intriguing spectral features of Wolf-Rayet (WR) binarystars is the presence of time-dependent line profiles. Long-termobservations of several systems revealed the periodicity of thisvariability, synchronized with the orbital movement. Several partiallysuccessful models have been proposed to reproduce the observed data. Themost promising model assumes that the origin of the emission is thewind-wind interaction zone. In this scenario, two high velocity anddense winds produce a strong shock layer, responsible for most of theX-rays observed from these systems. As the gas cools down, flowing alongthe interaction surface, it reaches recombination temperatures andgenerates the emission lines. Luhrs noted that, as the secondary starmoves along its orbital path, the shock region, of conical shape,changes its position with relation to the line of sight. As aconsequence, the stream-measured Doppler shift presents time variationsresulting in position changes of the spectral line. However, his modelrequires a very thick contact layer and also fails to reproduce recentlyobserved line profiles of several other WR binary systems. In our work,we present an alternative model, introducing turbulence in the shocklayer to account for the line broadening and opacity effects for theasymetry in the line profiles. We showed that the gas turbulence avoidsthe need of an unnaturally large contact layer thickness to reproduceline broadening. Also, we demonstrated that if the post-shock gas isoptically thick at the observed line frequency, the emission from theopposing cone surface is absorbed, resulting in a single-peaked profile.This result fully satisfies the recent data obtained from massive binarysystems, and can help in the determination of both the winds and theorbital parameters. We successfully applied this model to the Br22system and determined its orbital parameters.
| Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| An XMM-Newton view of the young open cluster NGC 6231. I. The catalogue
This paper is the first of a series dedicated to the X-ray properties ofthe young open cluster NGC 6231. Our data set relies on an XMM-Newtoncampaign of a nominal duration of 180 ks and reveals that NGC 6231 isvery rich in the X-ray domain too. Indeed, 610 X-ray sources aredetected in the present field of view, centered on the cluster core. Thelimiting sensitivity of our survey is approximately 6 ×10-15 erg cm-2 s-1 but clearly dependson the location in the field of view and on the source spectrum. Usingdifferent existing catalogues, over 85% of the X-ray sources could beassociated with at least one optical and/or infrared counterpart withina limited cross-correlation radius of 3´´ at maximum. Thesurface density distribution of the X-ray sources presents a slight N-Selongation. Once corrected for the spatial sensitivity variation of theEPIC instruments, the radial profile of the source surface density iswell described by a King profile with a central density of about 8sources per arcmin2 and a core radius close to 3.1 arcmin.The distribution of the X-ray sources seems closely related to theoptical source distribution. The expected number of foreground andbackground sources should represent about 9% of the detected sources,thus strongly suggesting that most of the observed X-ray emitters arephysically belonging to NGC 6231. Finally, beside a few bright but softobjects - corresponding to the early-type stars of the cluster - most ofthe sources are relatively faint (~5 × 10-15 ergcm-2 s-1) with an energy distribution peakedaround 1.0-2.0 keV.
| 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.
| First Ever Polarimetric Detection of a Wind-Wind Interaction Region and a Misaligned Flattening of the Wind in the Wolf-Rayet Binary CQ Cephei
In this paper we present unfiltered and multiband (i.e., UBVRI)polarimetric observations of the short-period Wolf-Rayet binary CQ Cep.Using the basic assumptions of an optically thin, corotating envelopeand pointlike sources (i.e., BME78 assumptions), we determined theorbital parameters of the system (i.e.,i=99deg+/-1deg andΩ=76deg+/-2deg at the 2 σ level) withan accuracy many times better than any previous work. Residual non-BME78variability around phase 0.0 was present in our data, which we associatewith the polarimetric eclipse of the dense central parts of theWolf-Rayet (W-R) wind by the orbiting O star. We attribute the observedphase lag of -0.15 between our residuals and those expected for astandard polarimetric eclipse to a wind-wind interaction (WWI) regiondistorted by Coriolis forces using the model presented by Marchenko etal. This model was also able to explain the strong wavelength dependenceof the polarimetric amplitudes in our multiband observations. Ouranalysis also reveals important epoch-dependent departures of the matterdistribution from spherical symmetry that were not related to theorbital plane and therefore cannot be the result of tidal interaction.We conclude that binarity is not playing an important role in drivingthe wind of the W-R star in CQ Cep and contributing to the observednonspherical matter distribution. On the other hand, this asymmetrycould be explained by a rotationally induced disk misaligned with theorbital plane.
| 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.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| Intermediate scale structure of the interstellar medium towards NGC 6231 in Sco OB1 with FUSE
The FUSE far-ultraviolet interstellar spectra toward seven targets inNGC 6231 show that the molecules H2, HD,and CO as well as various atomic species are distributed in severalclouds. The main absorption component found on the sight lines lies inthe Lupus cloud region at a distance of about 150 pc, and there is aweaker second one, presumably in the Sco OB1 shellsurrounding NGC 6231 (d≃1.8 kpc). H2excitation modelling is used to constrain the radiation field and thedensity in the gas; HD is used to estimate the abundance ofH+. The small angular separation of the target stars allowscolumn density variations to be probed over the field of view, on scalesof 0.05 pc in the case of the Lupus cloud distance. They are 40% forH2 and 60% for H I. A rather strong radiation field insidethe molecular clouds suggests a separation of the gas into smallercloudlets also along the line of sight.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.
| Radio continuum observations of massive stars in open cluster NGC 6231 and the Sco OB1 association
We present results of the Australia Telescope Compact Array (ATCA) radiocontinuum observations of massive stars in the Sco OB1 association. Most stars detected in the program show spectral indices lower thanvalue expected from thermal free-free emission.
| Colliding winds in the WR+OB binary θ Muscae (WR 48, WC5+O6-7V)
Spectra, providing full phase coverage, of the 19 d WC6+OB binaryθ Mus (WR 48, HD 113904), have been obtained and show dramaticvariations of the &CIIIλ5696 emission line profile. We havemodeled these line profile variations assuming the winds from the WRstar and its close OB companion are colliding and forming a shock regionfrom which extra emission originates.
| 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.
| 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.
| 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.
| Optical Polarization Observations of NGC 6231: Evidence for a Past Supernova Fingerprint
We present the first linear multicolor polarization observations for asample of 35 stars in the direction of the Galactic cluster NGC 6231. Wehave found a complex pattern in the angles of the polarimetric vectors.Near the core of this cluster the structure shows a semicircular patternthat we have interpreted as a reorientation of the dust particlesshowing the morphology of the magnetic field. We propose that asupernova event occurred some time ago and produced a shock on the localISM. We discuss in this paper independent confirmations of this event,both from the studies on the diffuse interstellar absorptions and theresults of the pre-main-sequence stars. We also show that a supernova issupported by the evolutionary status of the cluster.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.
| The Unusual 2001 Periastron Passage in the ``Clockwork'' Colliding-Wind Binary WR 140
We follow, using both optical spectroscopy and photometry, the``textbook'' colliding-wind WR+O binary WR 140 through and between theperiastron passages of 1993 and 2001. An extensive collection ofhigh-quality spectra allows us to derive precise orbital elements forboth components simultaneously. We confirm the extremely higheccentricity of the system, e=0.881+/-0.005, find an excellent match ofthe newly derived period to the previous estimates, P=2899.0+/-1.3 days,and improve the accuracy of the time of periastron passage,T0=HJD2,446,147.4+/-3.7. Around periastron, at orbital phasesφ~0.995-1.015, additional emission components appear on the tops ofthe broad Wolf-Rayet emission lines of relatively low ionizationpotential. The phase-dependent behavior of these excess line emissionspoints to their origin in the wind-wind collision zone, which allows usto place some limits on the orbital inclination of the system,i=50deg+/-15deg, and half-opening angle of the bowshock cone, θ=40deg+/-15deg. The relativelysudden appearance and disappearance of the extra emission componentsprobably signify a rapid switch from an adiabatically to a radiativelydominated regime and back again. Multiyear UBV photometry provides onemore surprise: in 2001 at φ=0.02-0.06, the system went through aseries of rapid, eclipse-like events. Assuming these events to berelated to an episode of enhanced dust formation at periastron, weestimate the characteristic size of the dust grains to be a~0.07 μm.
| Improved Hipparcos Parallaxes of Coma Berenices and NGC 6231
A method to reprocess the Hipparcos Intermediate Astrometry Data thatreduces the propagation of the along-scan spacecraft attitude errors wasdeveloped and successfully used to obtain a more consistent parallax ofthe Pleiades (Makarov, published in 2002). The same technique is usednow to correct the Hipparcos parallaxes of the Coma Berenices and NGC6231 open clusters, which are also in error. The new mean parallax ofComa is 12.40+/-0.17 mas (against previously 11.43 mas) and of NGC 62311.7+/-0.4 mas (against previously -0.8 mas). The new data for Coma arein excellent agreement with the pre-Hipparcos main-sequence fittingestimates. All six members of NGC 6231 that have negative parallaxes inHipparcos obtain positive parallaxes. These results suggest that themain source of astrometric error has been correctly identified, and thata more accurate Hipparcos catalog can be computed.
| The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars
The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksecXMM-Newton, observation, implying an upper limit to the X-ray luminosityof LX < 2.5x 1030 erg s-1 andto the X-ray to bolometric luminosity ratio ofLX/Lbol < 4*E-9. This confirmsindications from earlier less sensitive measurements that there has beenno convincing X-ray detection of any single WC star. This lack ofdetections is reinforced by XMM-Newton, and CHANDRA observations of WCstars. Thus the conclusion has to be drawn that the stars withradiatively-driven stellar winds of this particular class areinsignificant X-ray sources. We attribute this to photoelectronicabsorption by the stellar wind. The high opacity of the metal-rich anddense winds from WC stars puts the radius of optical depth unity athundreds or thousands of stellar radii for much of the X-ray band. Webelieve that the essential absence of hot plasma so far out in the windexacerbated by the large distances and correspondingly high ISM columndensities makes the WC stars too faint to be detectable with currenttechnology. The result also applies to many WC stars in binary systems,of which only about 20% are identified X-ray sources, presumably due tocolliding winds.
| The Effect of Binarity and Metallicity in the Spectra of WC and WO Stars
A statistical analysis of the main emission lines common to the WC andWO stars is made based on an extensive set of spectral data. To definethe trends in equivalent width ( Wλ), line ratios, andline widths, median values are derived for single-spectrum stars ofdifferent spectral class. We find that in Galactic WO and WC4 stars,Wλ (C IV 581 nm) is smaller compared to inextragalactic objects. In both Galactic and extragalactic stars,Wλ (O V 559 nm) smoothly increases towards early WCand WO stars. It is argued that differences in stellar wind structure,in combination with the ambient metallicity, may be the cause of theanomalies. Variation of the profile of the 465 nm blend indicates asubstantial contribution of He II 468 nm for the WCE and WO stars. Inaddition, we comment on the carbon abundances in relation to theevolutionary status of these objects. We also give an estimate of theOB/WR continuum flux ratio in composite-spectrum systems.
| Modelling the colliding-winds spectra of the 19-d WR + OB binary in the massive triple system θ Muscae
High signal-to-noise ratio, moderate-resolution spectra, providing fullphase coverage of the 19-d WC6 + OB binary θ Mus (WR 48, HD113904), have been obtained and show dramatic variations of the CIIIλ5696 emission-line profile. We have modelled these lineprofile variations using a purely geometrical model which assumes thatthe emission arises from two regions, an optically thin spherical shellaround the WR star and a cone-shaped region that partially wraps aroundthe OB star. The cone-shaped region represents the shock front arisingfrom the collision between the winds of the two stars. This work buildsupon our earlier study of WR 42 and WR 79, and uses a completely newcode for the modelling, which includes the effects of turbulence. We nowfind much better agreement between the orbital inclination angles foundfor these stars with those determined using other methods. The fittingparameters found via modelling the C IIIλ5696 profile variationsof θ Mus are used to infer that the OB companion most likely hasa spectral type of O6V or O7V. The modelling presented here continues toshow the exciting promise of a better understanding of WR starfundamental parameters.
| 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.
| Physical parameters of the high-mass X-ray binary 4U1700-37
We present the results of a detailed non-LTE analysis of the ultravioletand optical spectrum of the O6.5 Iaf+ star HD153919 - the mass donor in the high-mass X-ray binary4U1700-37. We find that the star has a luminositylog(L*/Lsun)=5.82 +/- 0.07, T_eff=35 000 +/- 1000K, radius R*=21.9+1.3-0.5Rsun, mass-loss rate dot {M}=9.5x 10-6Msun yr-1, and a significant overabundance ofnitrogen (and possibly carbon) relative to solar values. Given theeclipsing nature of the system these results allow us to determine themost likely masses of both components of the binary via Monte Carlosimulations. These suggest a mass for HD 153919 ofM* = 58 +/- 11 Msun - implying the initial mass ofthe companion was rather high (ga 60 Msun). The most likelymass for the compact companion is found to be M_x =2.44+/- 0.27Msun, with only 3.5 per cent of the trials resulting in amass less than 2.0 Msun and none less than 1.65Msun. Such a value is significantly in excess of the upperobservational limit to the masses of neutron stars of 1.45Msun found by Thorsett & Chakrabarthy (\cite{thorsett}),although a mass of 1.86 Msun has recently been reported forthe Vela X-1 pulsar (Barziv et al. \cite{barziv}).Our observational data is inconsistent with the canonical neutron starmass and the lowest black hole mass observed (ga 4.4 Msun;Nova Vel). Significantly changing observationalparameters can force the compact object mass into either of theseregimes but, given the strong proportionality between M* andM_x, the O-star mass changes by factors of greater than 2, well beyondthe limits determined from its evolutionary state and surface gravity.The low mass of the compact object implies that it is difficult to formhigh mass black holes through both the Case A & B mass transferchannels and, if the compact object is a neutron star, wouldsignificantly constrain the high density nuclear equation of state.Based on observations collected at the European Southern Observatory, LaSilla, Chile (64.H-0224).
| 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.
| Ultra-high-resolution observations of interstellar Na I and K I towards the Scorpius OB1 association
I present ultra-high-resolution (R~9×105) observationsof interstellar NaI and KI absorption lines towards three members of theScorpius OB1 association (HD 152235, 152236 and 152249). Theseobservations have, for the first time, resolved the intrinsic linewidthsof most of the discrete absorption components present along thesecomplex sightlines. The aims of the project were twofold: (i) toconstrain the physical conditions prevailing in the highly blueshiftedSco OB1 shell components, and especially to search for evidence ofactive shocks within them; and (ii) to further constrain thelow-velocity structure, where the NaI spectra are fully saturated butthe unsaturated KI lines allow a clear resolution of the individualvelocity components. The results of these analyses are discussed.Perhaps the most surprising result is the lack of any obviouscorrelation between the velocity dispersion of a velocity component(b-value) and its velocity. Specifically, the high-velocity shellcomponents are generally found to be no broader than the low-velocitycomponents attributed to foreground (often molecular) clouds, and cannottherefore be any hotter or more turbulent. Thus, with the possibleexception of the most blueshifted component towards HD 152236, there isno evidence for active shocks in the shell components at present.However, consideration of the relative time-scales for post-shockcooling and grain surface adsorption indicates that shock processing inthe past may still account for the low NaI/CaII ratios of thesecomponents found in previous work.
| A first detailed study of the colliding wind WR+O binary WR 30a
We present a detailed, extensive investigation of the photometric andspectroscopic behaviour of WR 30a. This star is definitely a binarysystem with a period around 4.6d. We propose the value P=4.619d(σ=0.002d). The identification of the components as WO4+O5((f))indicates a massive evolved binary system; the O5 component is amain-sequence or, more likely, a giant star. The radial velocities ofthe O star yield a circular orbit with an amplitude KO=29.9(σ=2.1)kms-1 and a mass function of 0.013(σ=0.003)Msolar. The spectrum of WR 30a exhibits strongprofile variations of the broad emission lines that are phase-lockedwith the orbital period. We report the detection of the orbital motionof the WO component with KWO=189kms-1, but thisshould be confirmed by further observations. If correct, it implies amass ratio MWO /MO=0.16. The star exhibitssinusoidal light variations of amplitude 0.024mag peak-to-peak with theminimum of light occurring slightly after the conjunction with the Ostar in front. On the basis of the phase-locked profile variations ofthe Civ λ4658 blend in the spectrum of the WO, we conclude that awind-wind collision phenomenon is present in the system. We discuss somepossibilities for the geometry of the interaction region.
| Magellanic Cloud WC/WO Wolf-Rayet stars - II. Colliding winds in binaries
A search for evidence of colliding winds is undertaken among the fourcertain Magellanic Cloud WC/WO spectroscopic binaries found in thecompanion Paper I, as well as among two Galactic WC/WO binaries of verysimilar subtype. Two methods of analysis, which allow the determinationof orbital inclination and parameters relating to the shock cone fromspectroscopic studies of colliding winds, are attempted. In the firstmethod, Lührs' spectroscopic model is fitted to the moderatelystrong Ciii 5696-Å excess line emission arising in the shock conefor the stars Br22 and WR 9. The four other systems show only very weakCiii 5696-Å emission. Lührs' model follows well the meandisplacement of the line in velocity space, but is unable to reproducedetails in the line profile and fails to give a reliable estimate of theorbital inclination. In the second method, an alternative attempt isalso made to fit the variation of more global quantities, full width athalf-maximum and radial velocity of the excess emission, with phase.This method also gives satisfactory results in a qualitative way, butshows numerical degeneracy with orbital inclination. Colliding windeffects on the very strong Civ 5808-Å Wolf-Rayet emission line,present in all six binaries, are also found to behave qualitatively asexpected. After allowing for line enhancement in colliding windbinaries, it now appears that all Magellanic Cloud WC/WO stars occupy avery narrow range in spectral subclass: WC4/WO3.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| 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.
| 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.
| Multi-frequency variations of the Wolf-Rayet system HD193793 (WC7pd+O4-5) III. IUE observations
The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35short-wavelength high-resolution spectra. An absorption-lineradial-velocity solution is obtained from the photospheric lines of theO component, by comparison with a single O star. The resulting orbitalparameters, e = 0.87 +/- 0.05, omega = 31degr +/- 9degr andKO star = 25 +/- 15 km s-1, confirm the largeeccentricity of the orbit, within the uncertainties of previous opticalstudies. This brings the weighted mean UV-optical eccentricity to e =0.85 +/- 0.04. Occultation of the O-star light by the WC wind and theWC+O colliding-wind region results into orbital modulation of theP-Cygni profiles of the C ii, C iv and Si iv resonance lines. Nearperiastron passage, the absorption troughs of those resonance-lineprofiles increase abruptly in strength and width, followed by a gradualdecrease. In particular, near periastron the blue black-edges of theP-Cygni absorption troughs shift to larger outflow velocities. Wediscuss that the apparently larger wind velocity and velocity dispersionobserved at periastron could be explained by four phenomena: (i)geometrical resonance-line eclipse effects being the main cause of theobserved UV spectral variability, enhanced by sightline crossing of theturbulent wind-wind collision zone; (ii) the possibility of anorbital-plane enhanced WC7 stellar wind; (iii) possible common-envelopeacceleration by the combined WC and O stellar radiation fields; and (iv)possible enhanced radiatively driven mass loss due to tidal stresses,focused along the orbiting line of centers.
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Datos observacionales y astrométricos
| Constelación: | Escorpio |
| Ascensión Recta: | 16h54m19.60s |
| Declinación: | -41°49'12.0" |
| Magnitud Aparente: | 6.59 |
| Distancia: | 10000000 parsecs |
| Movimiento Propio en Ascensión Recta: | 0 |
| Movimiento Propio en Declinación: | -4.4 |
| B-T magnitude: | 6.833 |
| V-T magnitude: | 6.625 |
Catálogos y designaciones:
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INHABITED SKY
