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X-ray Emission from Nitrogen-Type Wolf-Rayet Stars
We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR)stars obtained in a limited survey aimed at establishing the X-rayproperties of WN stars across their full range of spectral subtypes.None of the detected stars is so far known to be a close binary. Wereport Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6),and an XMM-Newton detection of WR79a (WN9ha). These observations clearlydemonstrate that both WNE and WNL stars are X-ray sources. We alsodiscuss Chandra archive detections of the WN6h stars WR 20b, WR 24, andWR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). TheX-ray spectra of all WN detections show prominent emission lines and anadmixture of cool (kT < 1 keV) and hot (kT > 2 keV) plasma. Thehotter plasma is not predicted by radiative wind shock models and otheras yet unidentified mechanisms are at work. Most stars show X-rayabsorption in excess of that expected from visual extinction (AV), likely due to their strong winds or cold circumstellargas. Existing data suggest a falloff in X-ray luminosity toward laterWN7-9 subtypes, which have higher L bol but slower, denserwinds than WN2-6 stars. This provides a clue that wind properties may bea more crucial factor in determining emergent X-ray emission levels thanbolometric luminosity.
| C IV Emission-Line Detection of the Supernova Remnant RCW 114
We report the detection of the C IV ??1548,1551 emissionline in the region of the RCW 114 nebula using the FIMS/SPEAR data. Theobserved C IV line intensity indicates that RCW 114 is much closer to usthan WR 90, a Wolf-Rayet star that was thought to be associated with RCW114 in some of the previous studies. We also found the existence of asmall H I bubble centered on WR 90, with a different local standard ofrest velocity range from that of the large H I bubble which wasidentified previously as related to RCW 114. These findings imply thatthe RCW 114 nebula is likely an old supernova remnant that is notassociated with WR 90. Additionally, the global morphologies of the CIV, H?, and H I emissions show that RCW 114 has evolved in anon-uniform interstellar medium.
| X-ray spectroscopy of stars
Non-degenerate stars of essentially all spectral classes are soft X-raysources. Their X-ray spectra have been important in constrainingphysical processes that heat plasma in stellar environments totemperatures exceeding one million degrees. Low-mass stars on the coolerpart of the main sequence and their pre-main sequence predecessorsdefine the dominant stellar population in the galaxy by number. TheirX-ray spectra are reminiscent, in the broadest sense, of X-ray spectrafrom the solar corona. The Sun itself as a typical example of amain-sequence cool star has been a pivotal testbed for physical modelsto be applied to cool stars. X-ray emission from cool stars is indeedascribed to magnetically trapped hot gas analogous to the solar coronalplasma, although plasma parameters such as temperature, density, andelement abundances vary widely. Coronal structure, its thermalstratification and geometric extent can also be interpreted based onvarious spectral diagnostics. New features have been identified inpre-main sequence stars; some of these may be related to accretionshocks on the stellar surface, fluorescence on circumstellar disks dueto X-ray irradiation, or shock heating in stellar outflows. Massive, hotstars clearly dominate the interaction with the galactic interstellarmedium: they are the main sources of ionizing radiation, mechanicalenergy and chemical enrichment in galaxies. High-energy emission permitsto probe some of the most important processes at work in these stars,and put constraints on their most peculiar feature: the stellar wind.Medium and high- resolution spectroscopy have shed new light on theseobjects as well. Here, we review recent advances in our understanding ofcool and hot stars through the study of X-ray spectra, in particularhigh-resolution spectra now available from XMM -Newton and Chandra. Weaddress issues related to coronal structure, flares, the composition ofcoronal plasma, X-ray production in accretion streams and outflows,X-rays from single OB-type stars, massive binaries, magnetic hot objectsand evolved WR stars.
| Recombination lines and free-free continua formed in asymptotic ionized winds: Analytic solution for the radiative transfer
In dense hot star winds, the infrared and radio continua are dominatedby free-free opacity and recombination emission line spectra. In thecase of a spherically symmetric outflow that is isothermal and expandingat constant radial speed, the radiative transfer for the continuumemission from a dense wind is analytic. Even the emission profile shapefor a recombination line can be derived. Key to these derivations isthat the opacity scales with only the square of the density. Theseresults are well-known. Here an extension of the derivation is developedthat also allows for line blends and the inclusion of an additionalpower-law dependence beyond just the density dependence. The additionalpower-law is promoted as a representation of a radius dependent clumpingfactor. It is shown that differences in the line widths and equivalentwidths of the emission lines depend on the steepness of the clumpingpower-law. Assuming relative level populations in LTE in the upperlevels of He II, an illustrative application of the model to{Spitzer}/IRS spectral data of the carbon-rich star WR 90 is given.
| Observational test of the CH cation oscillator strengths
We revise measurements of the positions and oscillator strengths usingspectral features in the CH+ A-X system, and by usinghigh-resolution, echelle spectra of 36 stars and assuming that itswavelength and oscillator strength as given in the literature for the(0, 0) transition, i.e. 4232.548 Å and 0.00545 respectively, arecorrect. The recommended oscillator strengths of the lines at 3957.689,3745.308, 3579.024, and 3447.077 Å are found to be (in units of10-5) 342, 172, 75, and 40, respectively. The estimatedcolumn densities of the CH cation toward the observed targets are alsopresented.
| Near Infrared Spectra of Galactic Wolf-Rayet Stars
Spectra of 37 Galactic WR stars were observed and reduced in thespectral range 790--895 nm. The main spectral features are identifiedand the equivalent widths and FWHMs of the strongest emission lines aremeasured. The equivalent width of the diffuse interstellar band at 862nm is also measured and the new estimates of color excessesE(B-V) are derived by using an empirical relationship betweenthe equivalent width and the color excess. The equivalent width ratiosfor the lines C III 850 nm, C IV 886 nm and C II 880 nm were found tocorrelate well with the WC subtype.
| Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars
We report on neon abundances derived from Spitzer high resolutionspectral data of eight Wolf-Rayet (WR) stars using the forbidden line of[Ne III] 15.56 μm. Our targets include four WN stars of subtypes 4-7,and four WC stars of subtypes 4-7. We derive ion fraction abundancesγ of Ne2+ for the winds of each star. The ion fractionabundance is a product of the ionization fraction Qi in stagei and the abundance by number AE of element E relativeto all nuclei. Values generally consistent with solar are obtained forthe WN stars, and values in excess of solar are obtained for the WCstars.
| Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysicalprocesses likely to play a role in the production of non-thermalemission in massive stars, with emphasis on massive binaries. Eventhough the discussion will start in the radio domain where thenon-thermal emission was first detected, the census of physicalprocesses involved in the non-thermal emission from massive stars showsthat many spectral domains are concerned, from the radio to the veryhigh energies. First, the theoretical aspects of the non-thermalemission from early-type stars will be addressed. The main topics thatwill be discussed are respectively the physics of individual stellarwinds and their interaction in binary systems, the acceleration ofrelativistic electrons, the magnetic field of massive stars, and finallythe non-thermal emission processes relevant to the case of massivestars. Second, this general qualitative discussion will be followed by amore quantitative one, devoted to the most probable scenario wherenon-thermal radio emitters are massive binaries. I will show how severalstellar, wind and orbital parameters can be combined in order to makesome semi-quantitative predictions on the high-energy counterpart to thenon-thermal emission detected in the radio domain. These theoreticalconsiderations will be followed by a census of results obtained so far,and related to this topic. These results concern the radio, the visible,the X-ray and the γ-ray domains. Prospects for the very highenergy γ-ray emission from massive stars will also be addressed.Two particularly interesting examples—one O-type and oneWolf-Rayet binary—will be considered in details. Finally,strategies for future developments in this field will be discussed.
| Spectral atlas of massive stars around He I 10 830 Å
We present a digital atlas of peculiar, high-luminosity massive stars inthe near-infrared region (10 470-11 000 Å) at medium resolution(R≃7000). The spectra are centered around He I 10 830 Å,which is formed in the wind of those stars, and is a crucial line toobtain their physical parameters. The instrumental configuration alsosampled a rich variety of emission lines of Fe II, Mg II, C I, N I, andPa γ. Secure identifications for most spectral lines are given,based on synthetic atmosphere models calculated by our group. We alsopropose that two unidentified absorption features have interstellarand/or circumstellar origin. For the strongest one (10 780 Å) anempirical calibration between E(B-V) and equivalent width is provided.The atlas displays the spectra of massive stars organized in fourcategories, namely Be stars, OBA Iape (or luminous blue variables, LBVcandidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars.For comparison, the photospheric spectra of non emission-line stars arepresented. Selected LBVs were observed in different epochs from 2001 to2004, and their spectral variability reveals that some stars, such asη Car, AG Car and HR Car, suffered dramatic spectroscopic changesduring this time interval.Based on observations made at Observatório do Pico dos Dias/LNA(Brazil). Figures 5 to 18 are only available in electronic form athttp://www.aanda.org Electronic version of the spectra (fichiers FITS)is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/993
| Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR140
We use hydrodynamical models of the wind-collision region in thearchetype colliding-wind system WR140 to determine the spatial andspectral distributions of the radio, X-ray, and γ-ray emissionfrom shock-accelerated electrons. Our calculations are for orbital phase0.837 when the observed radio emission is close to maximum. Using theobserved thermal X-ray emission at this phase in conjunction with theradio emission to constrain the mass-loss rates, we find that the O starmass-loss rate is consistent with the reduced estimates for O4-5supergiants by Fullerton, Massa & Prinja, and the wind-momentumratio, η = 0.02. This is independent of the opening angle deducedfrom radio very long baseline interferometry observations of the WCRthat we demonstrate fail to constrain the opening angle.We show that the turnover at ~3 GHz in the radio emission is due tofree-free absorption, since models based on the Razin effect have anunacceptably large fraction of energy in non-thermal electrons. We findthat the spectral index of the non-thermal electron energy distributionis flatter than the canonical value for diffusive shock acceleration,namely p < 2. Several mechanisms are discussed that could lead tosuch an index. Our inability to obtain fits to the radio data with p> 2 does not exclude the possibility of shock modification, butstronger evidence than that which currently exists is necessary for itssupport.Tighter constraints on p and the nature of the shocks in WR140 will beobtained from future observations at MeV and GeV energies, for which wegenerally predict lower fluxes than those in previous works. Since thehigh stellar photon fluxes prevent the acceleration of electrons beyondγ >~ 105-106, TeV emission fromcolliding-wind binary systems will provide unambiguous evidence ofpion-decay emission from accelerated ions. We finish by commenting onthe emission and physics of the multiple wind collisions in densestellar clusters, paying particular attention to the Galactic Centre.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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 Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])
We present new Spitzer IRS observations of HD 164270 (WC9, WR103). Aquantitative analysis of the UV, optical, near-, and mid-IR spectra ofHD 164270 is presented, allowing for line blanketing and wind clumping,revealing T*~48 kK, logL/Lsolar~4.9, andM˙~10-5 Msolar yr-1 for a volumefilling factor of f~0.1. Our models predict that He is partiallyrecombined in the outer stellar wind, such that recent radio-derivedmass-loss rates of WC9 stars have been underestimated. We obtainC/He~0.2 and O/He~0.01 by number from optical diagnostics. Mid-IRfine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm areobserved, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. Fromthese we obtain Ne/He~Ne+/He=2.2×10-3 bynumber, 7 times higher than the solar value (as recently derived byAsplund et al.), and S/He~S2+/He=5.1×10-5 bynumber. From a comparison with similar results for other WC subtypes weconclude that WC9 stars are as chemically advanced as earlier subtypes.We consider why late WC stars are exclusively observed inhigh-metallicity environments. In addition, we compare theUV/optical/mid-IR spectroscopic morphology of HD 164270 with theplanetary nebula central star BD +30 3639 ([WC9]). Their UV and opticalsignatures are remarkably similar, such that our quantitativecomparisons confirm similarities in stellar temperature, wind densities,and chemistry first proposed by Smith & Aller, in spite ofcompletely different evolutionary histories, with HD 164270 presently afactor of 10 more massive than BD +30 3639. At mid-IR wavelengths, thedust from the dense young nebula of BD +30 3639 completely dominates itsappearance, in contrast with HD 164270.
| Five WC9 stars discovered in the AAO/UKST Hα survey
We report the discovery of five massive Wolf-Rayet (WR) stars resultingfrom a programme of follow-up spectroscopy of candidate emission-linestars in the Anglo-Australian Observatory United Kingdom SchmidtTelescope (AAO/UKST) Southern Galactic Plane Hα survey. The6195-6775 Åspectra of the stars are presented and discussed. A WC9class is assigned to all five stars through comparison of their spectrawith those of known late-type WC stars, bringing the known total numberof Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibitnear-infrared (NIR) excesses characteristic of hot dust emission (asseen in the great majority of known WC9 stars), we find that two of thestars show no discernible evidence of such excesses. This increases thenumber of known WC9 stars without NIR excesses to seven. Reddenings anddistances for all five stars are estimated.
| 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.
| A spectroscopic search for the non-nuclear Wolf-Rayet population of the metal-rich spiral galaxy M 83
We present a catalogue of non-nuclear regions containing Wolf-Rayetstars in the metal-rich spiral galaxy M 83 (NGC 5236). From a total of283 candidate regions identified using He ii λ4686 imaging withVLT-FORS2, Multi Object Spectroscopy of 198 regions was carried out,confirming 132 WR sources. From this sub-sample, an exceptional contentof ~1035 ± 300 WR stars is inferred, with N(WC)/N(WN) ~ 1.2,continuing the trend to larger values at higher metallicity amongstLocal Group galaxies, and greatly exceeding current evolutionarypredictions at high metallicity. Late-type stars dominate the WCpopulation of M 83, with N(WC8-9)/N(WC4-7) = 9 and WO subtypes absent,consistent with metallicity dependent WC winds. Equal numbers of late toearly WN stars are observed, again in contrast to current evolutionarypredictions. Several sources contain large numbers of WR stars. Inparticular, #74 (alias region 35 from de Vaucouleurs et al.) contains~230 WR stars, and is identified as a Super Star Cluster from inspectionof archival HST/ACS images. Omitting this starburst cluster would resultin revised statistics of N(WC)/N(WN) ~ 1 and N(WC8-9)/N(WC4-7) ~ 6 forthe "quiescent" disk population. Including recent results for thenucleus and accounting for incompleteness in our spectroscopic sample,we suspect the total WR population of M 83 may exceed 3000 stars.
| Observations of the Diffuse Far-Ultraviolet Background with the Far Ultraviolet Spectroscopic Explorer
We have used observations taken under the Far Ultraviolet SpectroscopicExplorer (FUSE) S405/505 channel realignment program to explore thediffuse far-ultraviolet (FUV; 1000-1200 Å) radiation field. Of the71 independent locations in that program, we have observed a diffusesignal in 32, ranging in brightness from 1600 to a maximum of2.9×105 photons cm-2 sr-1s-1 Å-1 in Orion. The FUSE data confirm thatthe diffuse FUV sky is patchy with regions of intense emission, usuallynear bright stars, but also with dark regions, even at low Galacticlatitudes. We find a weak correlation between the FUV flux and the 100μm ratio but with wide variations, perhaps due to differences in thelocal radiation field.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.
| 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.
| The influence of binaries on galactic chemical evolution
Understanding the galaxy in which we live is one of the greatintellectual challenges facing modern science. With the advent of highquality observational data, the chemical evolution modeling of ourgalaxy has been the subject of numerous studies in the last years.However, all these studies have one missing element which is theevolution of close binaries. Reason: their evolution is very complex andsingle stars only perhaps can do the job. (Un)Fortunately at present weknow that the majority of the observed stars are members of a binary ormultiple system and that certain objects can only be formed throughbinary evolution. Therefore galactic studies that do not account forclose binary evolution may be far from realistic.Because of the large expertise developed through the years in stellarevolution in general and binary evolution in particular at the BrusselsAstrophysical Institute, we found ourselves in a privileged position tobe the first to do chemical evolutionary simulations with the inclusionof detailed binary evolution. The complexity of close binary evolutionhas kept many astronomers from including binary stars into theirstudies. However, it is not always the easiest way of living that givesyou the most excitement and satisfaction.
| The chemical evolution of the Galaxy: the importance of stars with an initial mass larger than 40 Msolar
In the present paper we investigate in how far stars with an initialmass larger than 40 Msolar affect the chemical enrichment ofthe Galaxy. We illustrate the importance for chemical yields of a mostup-to-date treatment of the various stellar wind mass loss episodes instellar evolutionary codes and we discuss the effects of a possiblesupernova-like outburst prior to massive black hole 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.
| The RCW 114 nebula: An old supernova remnant or a WR wind-blown bubble?
We present medium resolution spectra (R ~ 7 km s-1) of theinterstellar NaI D1 & D2 absorption lines observed towards 7early-type stars with distances ranging from 221 to ~ 1.5 kpc in theline-of-sight to the RCW 114 nebula. Our observed pattern of absorptionis in conflict with the findings of Bedford et al. (1984) who placed anupper limit to the distance to this nebula of <200 pc. Instead, ourspectral data are consistent with a marked increase in NaI absorptioncomplexity occuring only for sight-lines with distances >1 kpc. Asimilar pattern of absorption behavior is seen for the SiII lambda 1304Å interstellar line observed in the ultraviolet towards four ofthe aforementioned stars. We propose a scenario in which the absorptioncomponents with velocities <-10 km s-1 seen towards RCW114 are caused by an expanding stellar wind-blown bubble associated withthe Wolf-Rayet (WR) star, HD 156385 (d ~ 1.5 kpc), and that theHα and [SII] nebular emission probably arises in shockedfilamentary gas associated with a pre-existing evolved supernova remnantcavity produced by the high-mass progenitor of the present WR star.
| 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 effects of binaries on the evolution of Wolf-Rayet type spectral features in starbursts
In the present paper we investigate in detail the effects of binarieswith initial period between 1 day and 10 years on theoreticalsimulations of Wolf-Rayet (WR) type spectral features in starbursts. Wefocus on the evolution of the nebular Hbeta line instarburst in general, on the intensity ratios I(nebular He II;lambda4686)/I(Hbeta ), I(blue bump)/I(Hbeta ), and I(redbump)/I(Hbeta ) as a function of the equivalent width ofHbeta of WR galaxies in particular. The binary evolutionaryprocesses that dominate the evolution of the considered spectralfeatures are the Roche lobe overflow in case Br systems, the masstransfer efficiency, and the merger rate. We show that the predictionson nebular He II depend critically on the uncertainties in the theory ofWR atmospheres and particularly on uncertainties in the treatment of thesubsonic velocity region of the WR wind. The observations of lowmetallicity starbursts are best reproduced by a theoretical model with asignificant number of binaries and with a metallicity-dependent WR wind.
| 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.
| Synthetic High-Resolution Line Spectra of Star-forming Galaxies below 1200 Å
We have generated a set of far-ultraviolet stellar libraries usingspectra of OB and Wolf-Rayet stars in the Galaxy and the Large and SmallMagellanic Cloud. The spectra were collected with the Far UltravioletSpectroscopic Explorer and cover a wavelength range from 1003.1 to1182.7 Å at a resolution of 0.127 Å. The libraries extendfrom the earliest O to late-O and early-B stars for the Magellanic Cloudand Galactic libraries, respectively. Attention is paid to the complexblending of stellar and interstellar lines, which can be significant,especially in models using Galactic stars. The most severe contaminationis due to molecular hydrogen. Using a simple model for the H2line strength, we were able to remove the molecular hydrogen lines in asubset of Magellanic Cloud stars. Variations of the photospheric andwind features of C III λ1176, O VI λλ1032, 1038, PV λλ1118, 1128, and S IV λλ1063, 1073, 1074are discussed as a function of temperature and luminosity class. Thespectral libraries were implemented into the LavalSB and Starburst99packages and used to compute a standard set of synthetic spectra ofstar-forming galaxies. Representative spectra are presented for variousinitial mass functions and star formation histories. The valid parameterspace is confined to the youngest ages of less than ~=10 Myr for aninstantaneous burst, prior to the age when incompleteness of spectraltypes in the libraries sets in. For a continuous burst at solarmetallicity, the parameter space is not limited. The suite of models isuseful for interpreting the rest-frame far-ultraviolet in local andhigh-redshift galaxies. Based on observations made with theNASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operatedfor NASA by the Johns Hopkins University under NASA contract NAS5-32985.
| Stellar and wind properties of LMC WC4 stars. A metallicity dependence for Wolf-Rayet mass-loss rates
We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based(2.3 m MSSSO, NTT) spectroscopy to determine the physical parameters ofsix WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellarparameters are revised significantly relative to Gräfener et al.(\cite{Grafener1998}) based on improved observations and moresophisticated model atmosphere codes, which account for line blanketingand clumping. We find that stellar luminosities are revised upwards byup to 0.4 dex, with surface abundances spanning a lower range of 0.1 leC/He le 0.35 (20-45% carbon by mass) and O/He le 0.06 (<=10% oxygenby mass). Relative to Galactic WC5-8 stars at known distance, andanalysed in a similar manner, LMC WC4 stars possess systematicallyhigher stellar luminosities, ~ 0.2 dex lower wind densities, yet asimilar range of surface chemistries. We illustrate how theclassification C III lambda 5696 line is extremely sensitive to winddensity, such that this is the principal difference between the subtypedistribution of LMC and Galactic early-type WC stars. Temperaturedifferences do play a role, but carbon abundance does not affect WCspectral types. We illustrate the effect of varying temperature andmass-loss rate on the WC spectral type for HD 32257 (WC4, LMC) and HD156385 (WC7, Galaxy) which possess similar abundances and luminosities.Using the latest evolutionary models, pre-supernova stellar masses inthe range 11-19 Msun are anticipated for LMC WC4 stars, with7-14 Msun for Galactic WC stars with known distances. Thesevalues are consistent with pre-cursors of bright type-Ic supernovae suchas SN 1998bw (alias GRB 980425) for which a minimum total mass of C andO of 14 Msun has been independently derived. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer, and NASA-ESA Hubble Space Telescope. Also based onobservations collected at the European Southern Observatory in program63.H-0683, and at the Australian National University Siding SpringObservatory.
| 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.
| IUE Absorption-Line Observations of the Moderately and Highly Ionized Interstellar Medium toward 164 Early-Type Stars
We present measurements of Galactic interstellar Al III, Si IV, and C IVabsorption recorded in high-resolution archival ultraviolet spectra of164 hot early-type stars observed by the International UltravioletExplorer (IUE) satellite. The objects studied were drawn from the listof hot stars scheduled to be observed with the Far UltravioletSpectroscopic Explorer (FUSE) satellite as part of observing programsdesigned to investigate absorption by O VI in the Galactic disk andhalo. Multiple IUE echelle-mode integrations have been combined toproduce a single ultraviolet (1150-1900 Å) spectrum of each starwith a spectral resolution of ~25 km s-1 (FWHM). Selectedabsorption-line profiles are presented for each star along with plots ofthe apparent column density per unit velocity for each line of the AlIII, Si IV, and C IV doublets. We report absorption-line equivalentwidths, absorption velocities, and integrated column densities based onthe apparent optical depth method of examining interstellar absorptionlines. We also determine column densities and Doppler parameters fromsingle-component curve-of-growth analyses. The scientific analysis ofthese observations will be undertaken after the FUSE satellite producessimilar measurements for absorption by interstellar O IV, Fe III, S III,and other ions. Based on archival data from observations obtained withthe International Ultraviolet Explorer (IUE) satellite sponsored byNASA, SERC, and ESA.
| 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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