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|Musca - the heavenly fly.|
|Radio observations of the planetary nebula around the OH/IR star OH354.88-0.54 (V1018 Sco)|
We present radio observations of the unique, recently formed, planetarynebula (PN) associated with a very long-period OH/IR variable star V1018Sco that is unequivocally still in its asymptotic giant branch phase.Two regions within the optical nebula are clearly detected innon-thermal radio continuum emission, with radio spectral indicescomparable to those seen in colliding-wind Wolf-Rayet binaries. Wesuggest that these represent shocked interactions between the hot, faststellar wind and the cold nebular shell that represents the PN's slowwind moving away from the central star. This same interface producesboth synchrotron radio continuum and the optical PN emission. The fastwind is neither spherical in geometry nor aligned with any obviousoptical or radio axis. We also report the detection of transientH2O maser emission in this nebula.
|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.
|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.
|The nearest star of spectral type O3: a component of the multiple system HD 150136|
From radial velocities determined in high signal-to-noise digitalspectra, we report the discovery that the brightest component of thebinary system HD 150136 is of spectral type O3. We also present thefirst double-lined orbital solution for this binary. Our radialvelocities confirm the previously published spectroscopic orbital periodof 2.6 d. HeII absorptions appear double at quadratures, but singlelines of NV and NIV visible in our spectra define a radial velocityorbit of higher semi-amplitude for the primary component than do theHeII lines. From our orbital analysis, we obtain minimum masses for thebinary components of 27 and 18 Msolar. The neutral Heabsorptions apparently do not follow the orbital motion of any of thebinary components, thus they most probably arise in a third star in thesystem.
|Cosmic-Ray Neon, Wolf-Rayet Stars, and the Superbubble Origin of Galactic Cosmic Rays|
We report the abundances of neon isotopes in the Galactic cosmic rays(GCRs) using data from the Cosmic Ray Isotope Spectrometer (CRIS) aboardthe Advanced Composition Explorer (ACE). These abundances have beenmeasured for seven energy intervals over the energy range of84<=E/M<=273 MeV nucleon-1. We have derived the22Ne/20Ne ratio at the cosmic-ray source using themeasured 21Ne, 19F, and 17O abundancesas ``tracers'' of secondary production of the neon isotopes. Using thisapproach, the 22Ne/20Ne abundance ratio that weobtain for the cosmic-ray source is0.387+/-0.007(statistical)+/-0.022(systematic). This corresponds to anenhancement by a factor of 5.3+/-0.3 over the22Ne/20Ne ratio in the solar wind. This cosmic-raysource 22Ne/20Ne ratio is also significantlylarger than that found in anomalous cosmic rays, solar energeticparticles, most meteoritic samples of matter, and interplanetary dustparticles. We compare our ACE CRIS data for neon and refractory isotoperatios, and data from other experiments, with recent results fromtwo-component Wolf-Rayet (W-R) models. The three largest deviations ofGCR isotope ratios from solar system ratios predicted by these models,12C/16O, 22Ne/20Ne, and58Fe/56Fe, are indeed present in the GCRs. Infact, all of the isotope ratios that we have measured are consistentwith a GCR source consisting of about 80% material with solar systemcomposition and about 20% W-R material. Since W-R stars are evolutionaryproducts of OB stars, and most OB stars exist in OB associations thatform superbubbles, the good agreement of these data with W-R modelssuggests that superbubbles are the likely source of at least asubstantial fraction of GCRs.
|An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite|
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
|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.
|Metallicity and binarity in WC and WO stars|
|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.
|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.
|The total-to-selective extinction ratio determined from near IR photometry of OB stars|
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
|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.
|Gamma-ray emission from Wolf-Rayet binaries|
In the colliding wind region of early-type binaries, electrons can beaccelerated up to relativistic energies displaying power-law spectra, asdemonstrated by the detection of non-thermal radio emission from severalWR+OB systems. The particle acceleration region, located between thestars, is exposed to strong photon fields in such a way that inverseCompton cooling of the electrons could result in a substantialhigh-energy non-thermal flux. In particular cases, the ratio of theenergy densities of magnetic to photon fields in the colliding windregion will determine whether a given source can produce or notsignificant gamma-ray emission. We present here a study of the binariesWR 140, WR 146, and WR 147 in the light of recent radio and gamma-rayobservations. We show that with reasonable assumptions for the magneticfield strength WR 140 can produce the gamma-ray flux from the EGRETsource 3EG J2022+4317. WR 146 and WR 147 are below the detectionthreshold, but new and forthcoming instruments like INTEGRAL and GLASTmight detect non-thermal emission from them.
|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.
|An updated review on microlensing experiments.|
|Neutral Hydrogen around the Oxygen-Sequence Wolf-Rayet Star WR 102 and the Nebula G2.4 + 1.4|
|Microlensing Maps for the Milky Way Galaxy|
At any instant, there are ~1000 microlensing events to sources brighterthan 20 mag in the Milky Way. Large-scale maps of the microlensingoptical depth and the mean timescale are constructed for a number ofmodels of the Galactic bar and disk, incorporating the effects ofstreaming and spiral structure. Freudenreich's model can reproduce thehigh optical depths toward the bulge. It is also in good agreement withthe data toward the spiral arms (except for the field γ Norma).Spiral structure tends to increase the optical depth by <~20% and themean timescale by <~100%. Different bar morphologies givecharacteristically different shaped contours, especially at low Galacticlatitudes (|b|<2deg). These could be traced out with aK-band microlensing survey, consuming ~100 minutes per night on atelescope such as the Visible and Infrared Survey Telescope forAstronomy.
|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.
|Observation of periodic variable stars towards the Galactic spiral arms by EROS II|
We present the results of a massive variability search based on aphotometric survey of a six square degree region along the Galacticplane at (l = 305o, b = -0.8o) and (l =330o, b = -2.5o). This survey was performed in theframework of the EROS II (Expérience de Recherche d'ObjetsSombres) microlensing program. The variable stars were found among 1 913576 stars that were monitored between April and June 1998 in twopassbands, with an average of 60 measurements. A new period-searchtechnique is proposed which makes use of a statistical variable thatcharacterizes the overall regularity of the flux versus phase diagram.This method is well suited when the photometric data are unevenlydistributed in time, as is our case. 1362 objects whose luminosityvaries were selected. Among them we identified 9 Cepheids, 19 RRLyræ, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular stars.Most of them are newly identified objects. The cross-identification withknown catalogues has been performed. The mean distance of the RRLyræ is estimated to be ~ 4.9 +/- 0.3 kpc undergoing an averageabsorption of ~ 3.4 +/- 0.2 mag. This distance is in good agreement withthat of disc stars that contribute to the microlensing source starpopulation. Our catalogue and light curves are available electronicallyfrom the CDS, Strasbourg and from our Web site http://eros.in2p3.fr.Full Tables 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/389/149 This work is basedon observations made with the MARLY telescope of the EROS collaborationat the European Southern Observatory, La Silla, Chile.
|Filamentary Shell Structures from the AAO/UKST Hα Survey|
Here we present the first results of a search for new optical supernovaremnant candidates and other filamentary objects on films produced bythe Anglo-Australian Observatory/UK Schmidt Telescope Hα Survey.Sixty-one fields, or 26% of the Galactic plane survey fields, have beenvisually examined. This has resulted in the detection of four newlargediameter filamentary structures, and the discovery of extensive newoptical emission in two previously known optical supernova remnantcandidates.
|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.
|H I Shells behind the Coalsack|
We report the discovery of two new large H I shells in the direction ofthe Coalsack Nebula. Both shells were observed with the Parkes RadioTelescope as part of the Southern Galactic Plane Survey. The largestshell, GSH 304-00-12, is at a distance of ~1.2 kpc and has derivedphysical dimensions of 280×200 pc. The second shell, GSH305+01-24, is at a distance of ~2.2 kpc and has derived dimensions of280×440 pc. We present a simple numerical model to show that GSH305+01-24 most likely formed from stellar winds in the Centaurus OB1stellar association. There is associated radio, infrared, and Hαcontinuum emission. Both shells are situated in the Sagittarius-Carinaarm, with GSH 305+01-24 more distant. The far edge of GSH 304-00-12 isat the near side of the arm and opens into the interarm region. We findno evidence of closure at the near side of the shell and thereforedescribe the geometry as conical. Emission from the near side of theshell may be lost in absorption by the Coalsack Nebula.
|Non-thermal emission in Wolf-Rayet stars: are massive companions required?|
We examine the radio spectral indices of 23 Wolf-Rayet (WR) stars toidentify the nature of their radio emission. We identify nine systems asnon-thermal emitters. In seven of these systems the non-thermal emissiondominates the radio spectrum, while in the remaining two it is ofcomparable strength to the thermal, stellar wind emission, giving`composite' spectra. Among these nine systems, seven have knownspectroscopic or visual binary companions. The companions are allmassive O or early B-type stars, strongly supporting a connectionbetween the appearance of non-thermal emission in WR stars and thepresence of a massive companion. In three of these binaries, the originof non-thermal emission in a wind-collision region between the stars hasbeen well established in earlier work. The binary systems that exhibitonly thermal emission are all short-period systems where awind-collision zone is deep within the opaque region of the stellar windof the WR star. To detect non-thermal emission in these systems requiresoptically thin lines of sight to the wind-collision region.
|HST FGS1R Results On the Association Between Binary Wolf-Rayet Stars and Non-Thermal Radio Emission|
Two separate models have been proposed to explain the non-thermalemission detected in some Wolf-Rayet (WR) stars. In models based onsingle WR stars, this emission is proposed to arise via synchrotronradiative processes in the outer (intrinsically unstable) WR wind (e.g.White & Chen 1995). In models based on WR + O systems, thisnon-thermal radio emission is suggested to arise from the WR windcolliding with the wind of a companion (e.g. Williams et al. 1990). Inorder to be observed, the colliding winds region is believed to occur inwide binaries where the interaction zone is outside the WR radiophotosphere (≈30 AU based on spherically symmetric uniform windmodels). HST FGS1R observations of 9 non-thermal and 9, as a controlgroup, purely thermal radio emitting stars attempted to verify thetheory that this non-thermal emission is always a result of binaryinteractions. If the binary model is correct, then most or all of ournon-thermal targets should have companions with projected separations of0.01″
|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.
|Observation of microlensing towards the galactic spiral arms. EROS II. 2 year survey|
We present the analysis of the light curves of 8.5 million starsobserved during two seasons by EROS (Expérience de Recherched'Objets Sombres), in the Galactic plane away from the bulge. Threestars have been found that exhibit luminosity variations compatible withgravitational microlensing effects due to unseen objects. Thecorresponding optical depth, averaged over four directions, is bar tau =0.38+0.53_-0.15 x 10-6. All three candidates havelong Einstein radius crossing times ( ~ 70 to 100 days). For one ofthem, the lack of evidence for a parallax or a source size effectenabled us to constrain the lens-source configuration. Another candidatedisplays a modulation of the magnification, which is compatible with thelensing of a binary source. The interpretation of the optical depthsinferred from these observations is hindered by the imperfect knowledgeof the distance to the target stars. Our measurements are compatiblewith expectations from simple galactic models under reasonableassumptions on the target distances. This work is based on observationsmade at the European Southern Observatory, La Silla, Chile.
|Radio Continuum Measurements of Southern Early-Type Stars. III. Nonthermal Emission from Wolf-Rayet Stars|
The Australia Telescope Compact Array (ATCA) has been used to search forradio continuum emission at 2.4 and 1.4 GHz from a sample of 36 southernWolf-Rayet stars. Seven Wolf-Rayet stars were detected at 2.4 GHz, ofwhich two were also detected at 1.4 GHz. We have identified sixWolf-Rayet stars, WR 14, 39, 48, 90, 105, and 112, that have nonthermalemission. The ATCA data confirm that at least 40% of Wolf-Rayet starswith measured spectral indices have nonthermal emission at centimeterwavelengths. Properties of each of the six sources are discussed. Themeasured spectral indices are between 0 and -1.0, and the radioluminosities are of order 10^29 ergs s^-1. So far 10 confirmed sourcesof nonthermal emission are known, including the six ATCA detections andfour previously known cases, WR 125, 140, 146, and 147. In all cases,the nonthermal radio emission almost certainly originates from aninteraction between the Wolf-Rayet stellar wind and the wind from amassive companion star. The radio observations agree well withtheoretical predictions for colliding winds. Synchrotron emission occursfrom relativistic electrons accelerated in strong shocks. The nonthermalspectral indices are likely to be close to -0.5. For WR 39, the detectedradio emission is offset by ~3" from the optical position of WR 39 andby ~2" from the optical position of WR 38B. We suggest that the radioemission may originate from a wind-wind interaction between WR 39 and WR38B, although this is not confirmed. For WR 11, the radio spectral indexincreases from +0.3 between 3 and 6 cm to +1.2 between 13 and 20 cm.This is interpreted as evidence for a highly attenuated nonthermalcomponent that originates well within the ionized wind of the W-R starfrom an interaction with the wind of the O9 companion star.
|Ultraviolet Interstellar Linear Polarization. V. Analysis of the Final Data Set|
Using recent measurements of ultraviolet interstellar polarization, wehave examined its relationship to ultraviolet extinction and topolarization and extinction measurements in the visible and infrared.The relationship between the relative amount of ultraviolet polarizationand the parameter lambda_max, determined using only visible data, isconfirmed and strengthened, for example, by a tight correlation betweenp(6 mum^-1)/p_max and lambda^-1_max. A good fit to the wavelengthdependence of the polarization from the infrared to the ultraviolet canbe achieved with a five-parameter function combining a power law in theinfrared and a Serkowski-like function in the ultraviolet. Thepolarization efficiency (ratio of polarization to extinction) is less inthe ultraviolet than in the visual, and the ratio of these efficienciesincreases systematically with lambda^-1_max. We relate these effects tosystematic changes in the underlying aligned grain size distribution.The polarization efficiency of the grains causing the 2175 Åextinction bump along most sight lines is so (unusually) small that nostatistically significant polarization feature is detectable in thatwavelength region. Only two of 28 sight lines show a definitepolarization feature. The environments of the two bump sight lines aresimilar but not unique, and the mechanism for producing the polarizationfeature along only these two sight lines is still not known.
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