We present optical spectra of 14 emission-line stars in M33s giant HII regions NGC 592, NGC 595 and NGC 604: five of them are known WR stars, for which we present a better quality spectrogram, eight were WR candidates based on narrow-band imagery and one is a serendipitous discovery. Spectroscopy confirms the power of interference filter imagery to detect emission-line stars down to an equivalent width of about 5 A in crowded fields. We have also used archival HST/WFPC2 images to correctly identify emission-line stars in NGC 592 and NGC 588. emission-line stars in NGC 592 and NGC 588.
This paper gathers, from the literature and private communication, 72 new Galactic Population I Wolf-Rayet stars and 17 candidate WCLd stars, recognized and/or discovered after the publication of The VIIth Catalogue of Galactic Wolf-Rayet Stars. This brings the total number of known Galactic Wolf-Rayet stars to 298, of which 24 (8%) are in open cluster Westerlund 1, and 60 (20%) are in open clusters near the Galactic Center.
The collapsar model requires rapidly rotating Wolf-Rayet stars as progenitors of long gamma-ray bursts. However, Galactic Wolf-Rayet stars rapidly lose angular momentum due to their intense stellar winds. We investigate whether the tidal interaction of a Wolf-Rayet star with a compact object in a binary system can spin up the Wolf-Rayet star enough to produce a collapsar. We compute the evolution of close Wolf-Rayet binaries, including tidal angular momentum exchange, differential rotation of the Wolf-Rayet star, internal magnetic fields, stellar wind mass loss, and mass transfer. The Wolf-Ray…
Observational and theoretical evidence in support of metallicity dependent winds for Wolf-Rayet stars is considered. Well known differences in Wolf-Rayet subtype distributions in the Milky Way, LMC and SMC may be attributed to the sensitivity of subtypes to wind density. Implications for Wolf-Rayet stars at low metallicity include a hardening of ionizing flux distributions, an increased WR population due to reduced optical line fluxes, plus support for the role of single WR stars as Gamma Ray Burst progenitors.
We present new $^{26}$Al stellar yields from rotating Wolf--Rayet stellar models which, at solar metallicity, well reproduce the observed properties of the Wolf-Rayet populations. These new yields are enhanced with respect to non--rotating models, even with respect to non--rotating models computed with enhanced mass loss rates. We briefly discuss some implications of the use of these new yields for estimating the global contribution of Wolf-Rayet stars to the quantity of $^{26}$Al now present in the Milky Way.
Preliminary results from spectrophotometric observations of galaxies with very young starbursts are presented. Starburst galaxies with an age of the burst in the range between 3 and 6 Myr have been observed and new detections of Wolf-Rayet galaxies are reported. We discuss the origin of high excitation nebular lines observed in these galaxies and their possible link with the population of Wolf-Rayet stars.
We report the discovery of emission from Wolf-Rayet stars in a giant HII region 4.5 arcsec South of the nucleus of the IRAS barred spiral galaxy Mrk 712. The ratio of WNL to OV stars, estimated from the luminosity of the HeII 4686 line, is 0.2. By comparison with starburst and stellar evolution models, we find that this high value is only compatible with a very young starburst episode (3 - 4 Myr) and a flat initial mass function (Gamma = -1). The presence of the [ArV] line reveals that the HII region is strongly ionized by the hot Wolf-Rayet stars. The comparison with other barred Wolf-Rayet g…
Wolf-Rayet stars have been detected in a large number of galaxies experiencing intense bursts of star formation. All stars initially more massive than a certain, metallicity-dependent, value are believed to experience the Wolf-Rayet phase at the end of their evolution, just before collapsing in supernova explosion. The detection of Wolf-Rayet stars puts therefore important constraints on the evolutionary status of starbursts, the properties of their Initial Mass Functions and their star formation regime. In this contribution we review the properties of galaxies hosting Wolf-Rayet stars, with s…
If Wolf-Rayet stars are the progenitors of Gamma-ray bursts (GRBs), they must rotate rapidly to produce the GRB. This rotation may effect their stellar-wind bubbles and possibly explain why so many GRB afterglows occur in a constant density medium.
Aims: Recent theoretical predictions for the winds of Wolf-Rayet stars indicate that their mass-loss rates scale with the initial stellar metallicity in the local Universe.We aim to investigate how this predicted dependence affects the models of Wolf-Rayet stars and their progeny in different chemical environments. Methods: We compute models of stellar structure and evolution for Wolf-Rayet stars for different initial metallicities, and investigate how the scaling of the Wolf-Rayet mass-loss rates affects the final masses, the lifetimes of the WN and WC subtypes, and how the ratio of the two p…
We have observed H-alpha and [OIII] emission from eight of the most well defined Wolf-Rayet ring nebulae in the Galaxy. We find that in many cases the outermost edge of the [OIII] emission leads the H-alpha emission. We suggest that these offsets, when present, are due to the shock from the Wolf-Rayet bubble expanding into the circumstellar envelope. Thus, the details of the WR bubble morphology at H-alpha and [OIII] can then be used to better understand the physical condition and evolutionary stage of the nebulae around Wolf-Rayet stars, as well as place constraints on the nature of the stell…
Van Marle et al. (2005) showed that circumstellar absorption lines in early Type Ib/c supernova and gamma-ray burst afterglow spectra may reveal the progenitor evolution of the exploding Wolf-Rayet star. While the quoted paper deals with Wolf-Rayet stars which evolved through a red supergiant stage, we investigate here the initially more massive Wolf-Rayet stars which are thought to evolve through a Luminous Blue Variable (LBV) stage. We perform hydrodynamic simulations of the evolution of the circumstellar medium around a 60 Msol star, from the main sequence through the LBV and Wolf-Rayet sta…
Significant amounts of hydrogen were found in very hot early-type single WN stars in the SMC and the LMC. Recently, we found similar evidence in the Wolf-Rayet star of a short-period LMC binary. We discuss here the relevance of hydrogen for WR star classification, models, the relation to metallicity, and the GRB progenitors.
We present mid-infrared spectrophotometric observations of SS433 with ISOPHOT. The HeI+HeII lines in both spectra of SS433 and of the Wolf-Rayet star WR147, a wind-colliding WN8+BO5 binary system, closely match. The 2.5-12 micron continuum radiation is due to an expanding wind free-free emission in an intermediate case between optically thick and optically thin regimes. The inferred mass loss rate evaluation gives ~10^{-4} Msun/yr. Our results are consistent with a Wolf-Rayet-like companion to the compact object in SS433. A similar study for Cygnus X-3 confirms the Wolf-Rayet-like nature of it…
We present follow-up spectroscopy of emission line candidates detected on near-infrared narrow band images in the inner Galaxy (Homeier et al. 2003). The filters are optimized for the detection of Wolf-Rayet stars and other objects which exhibit emission--lines in the 2 $μ$m region. Approximately three square degrees along the Galactic plane have been analyzed in seven narrow--filters (four emission--lines and three continuum). We have discovered 4 new Wolf-Rayet stars and present coordinates, finding charts, and K-band spectra.
We present the discovery of OB type absorption lines superimposed to the emission line spectrum, and the first double-lined orbital elements for the massive Wolf-Rayet binary HDE 318016 (=WR 98), a spectroscopic binary in a circular orbit with a period of 47.825 days. The semiamplitudes of the orbital motion of the emission lines differ from line to line, indicating mass ratios between 1 and 1.7 for MWR/MOB.
Initial results, techniques, and rationale for a near-infrared survey of evolved emission-line stars toward the Galactic Center are presented. We use images taken through narrow-band emission-line and continuum filters to select candidates for spectroscopic follow-up. The filters are optimized for the detection of Wolf-Rayet stars and other objects which exhibit emission-lines in the 2 micron region. Approximately three square degrees along the Galactic plane have been analyzed in seven narrow-filters (four emission-lines and three continuum). Four new Wolf-Rayet stars have been found which ar…
We report the detection of 3 additional Wolf-Rayet stars in the young cluster Westerlund 1. They were selected as emission-line star candidates based on 1 micron narrow-band imaging of the cluster carried out at OPD/LNA (Brazil), and then confirmed as Wolf-Rayet stars by K-band spectroscopy performed at the 4.1 m SOAR telescope (Chile). Together with previous works, this increases the population of Wolf-Rayet stars detected in the cluster to 22 members. Moreover, it is presented for the first time a K-band spectrum of the luminous blue variable W243, which apparently implies in a higher temper…
High-energy gamma-ray emission is theoretically expected to arise in tight binary star systems (with high mass loss and high velocity winds), although the evidence of this relationship has proven to be elusive so far. Here we present the first bounds on this putative emission from isolated Wolf-Rayet (WR) star binaries, WR 147 and WR 146, obtained from observations with the MAGIC telescope.
Near infrared spectroscopy and photometry of the Wolf-Rayet Star WR 143 (HD 195177) were obtained in the $JHK$ photometric bands. High resolution spectra observed in the J and H bands exhibit narrow 1.083-micron He I line and the H I Pa Beta and the Brackett series lines in emission superposed on the broad emission line spectrum of the Wolf-Rayet star, giving strong indications of the presence of a companion. From the narrow emission lines observed, the companion is identified to be an early-type Be star. The photometric magnitudes exhibit variations in the JHK bands which are probably due to …
We construct a speculative scenario for the evolution of Wolf-Rayet central stars of planetary nebula. It is clear from the latest infra-red observations that a new perspective has to be adopted: the simultaneous presence of carbon- and oxygen-rich dust (double dust chemistry), while being a rare phenomenon for H-rich central stars, is found around the vast majority of cool Wolf-Rayet central stars. This correlation between Wolf-Rayet characteristics and double dust chemistry points to a common mechanism. Within the binary evolution framework established by Soker, two scenarios are proposed, r…
We present the discovery of OB type absorption lines superimposed to the emission line spectrum and the first double-lined orbital elements for the massive Wolf-Rayet binary HDE 318016 (=WR 98), a spectroscopic binary in a circular orbit with a period of 47.825 days. The semiamplitudes of the orbital motion of the emission lines differ from line to line, indicating mass ratios between 1 and 1.7 for M_WR/M_OB.
BAT99-129 in the LMC is one among a handful of extra-galactic eclipsing Wolf-Rayet binaries known. We present blue, medium-resolution, phase-dependent NTT-EMMI spectra of this system that allow us to separate the spectra of the two components of the binary and to obtain a reliable orbital solution for both stars. We assign an O5V spectral type to the companion, and WN3(h)a to the Wolf-Rayet component. We discuss the spectroscopic characteristics of the system: luminosity ratio, radii, rotation velocities. We find a possible oversynchronous rotation velocity for the O star. Surprisingly, the ex…
The collapsar model for long gamma-ray bursts requires a rapidly rotating Wolf-Rayet star as progenitor. We test the idea of producing rapidly rotating Wolf-Rayet stars in massive close binaries through mass accretion and consecutive quasi-chemically homogeneous evolution; the latter had previously been shown to provide collapsars below a certain metallicity threshold for single stars. The binary channel presented here may provide a means for massive stars to obtain the high rotation rates required to evolve quasi-chemically homogeneous and fulfill the collapsar scenario. Moreover, it suggests…
In this paper we present Complete Stellar models (CoStar) for massive stars, which treat the stellar interior and atmosphere, including its wind. Particular emphasis is given to Wolf-Rayet stars. We address the question of the effective temperatures of WNE and WC stars. Our first results show a satisfactory agreement between the CoStar models and the simple temperature correction method applied by Schaller et al. (1992). An analyses of the subphotospheric structure of the WR star models shows the importance of metal opacity. This may be essential for understanding the driving mechanism of Wolf…
BAT99-129 is a rare, short-period eclipsing Wolf-Rayet binary in the Large Magellanic Cloud. We present here medium-resolution NTT/EMMI spectra that allow us to disentangle the spectra of the two components and find the orbital parameters of the binary. We also present VLT/FORS1 spectra of this binary taken during the secondary eclipse, i.e. when the companion star passes in front of the Wolf-Rayet star. With these data we are able to extract, for the first time in absolute units for a WR+O binary, the sizes of the line emitting regions.
We present numerical simulations of the runaway fractions expected amongst O and Wolf-Rayet star populations resulting from stars ejected from binaries by the supernova of the companion. Observationally the runaway fraction for both types of star is similar, prompting the explanation that close dynamical interactions are the main cause of these high-velocity stars. We show that, provided that the initial binary fraction is high, a scenario in which two-thirds of massive runaways are from supernovae is consistent with these observations. Our models also predict a low frequency of runaways with …
To better understand Wolf-Rayet stars as progenitors of gamma-ray bursts, an understanding of the effect metallicity has on Wolf-Rayet mass loss is needed. Using simple analytic models, we study the Mdot - Z relation of a WN star and compare the results to similar models. We find that Mdot roughly follows a power law in Z with index 0.88 from -2.5 < log Z/Z_sun < -1 and appears to flatten by log Z/Z_sun ~ -0.5.
We report on optical imaging and spectroscopic observations of the Wolf-Rayet candidate star MLA 1159 and the surrounding ionization nebula BA 1-642 in the Andromeda Galaxy. Though both objects have been known for many years, our observations (1) confirm the Wolf-Rayet nature of MLA 1159, (2) allow one to determine the nebula as an ionization nebula, and (3) demonstrate the association of MLA 1159 with the nebula. The supersoft X-ray source RX J0045.5+4206 whose error box encompasses the full nebula, seems to be a chance superposition and not related to MLA 1159 and/or BA 1-642.
Recent results with the Potsdam Wolf-Rayet (PoWR) models have shown that Wolf-Rayet mass loss can be explained by radiative wind driving. An inspection of the galactic WR sample, however, reveals that a significant part of the observed WR stars are in conflict with our models. This group is chiefly formed by intermediate spectral subtypes. Among the population of late-type WN stars we find that the enigmatic WN8 subtypes, which are well-known for their photometric variations, are in disagreement with our models. So are there other driving mechanisms working in these objects?
We report the discovery of fifteen previously unknown Wolf-Rayet (WR) stars found as part of an infrared broad-band study of candidate WR stars in the Galaxy. We have derived an empirically-based selection algorithm which has selected ~5000 WR candidate stars located within the Galactic Plane drawn from the GLIMPSE (mid-infrared) and 2MASS (near-infrared) catalogues. Spectroscopic follow-up of 184 of these reveals eleven WN and four WC-type WR stars. Early WC subtypes are absent from our sample and none show evidence for circumstellar dust emission. Of the candidates which are not WR stars, ~1…
Based on 1) a conjecture about the mean free path for particle scattering in perpendicular shock geometries, and 2) a model for Wolf Rayet star winds, we argue that explosions of Wolf Rayet stars can lead through diffusive particle acceleration to particle energies up to 3 10^9 GeV. As a test we first demonstrate that the magnetic fields implied by this argument are compatible with the dynamo limit applied to the convective interior of the main sequence stars which are predecessors of Wolf Rayet stars; second we show that this implies the same strength of the magnetic fields as is suggested by…
We have carried out a search for Wolf-Rayet galaxies in all galaxies with EW(Hb)>2AA in the SDSS DR6. We identify Wolf-Rayet features using a mixture of automatic and visual classification and find a total of 570 galaxies with significant Wolf-Rayet (WR) features and a further 1115 potential candidates, several times more than even the largest heterogeneously assembled catalogues. We discuss in detail the properties of galaxies showing Wolf-Rayet features with a focus on their empirical properties. We are able to accurately quantify the incidence of Wolf-Rayet galaxies with redshift and show t…
Wolf-Rayet stars are the evolved descendents of massive stars. Their extraordinary properties make them useful tracers of the stellar initial mass function (IMF) in a young stellar population. I discuss how the interpretation of spectral diagnostics are complicated by the interplay of stellar, nebular, and dust properties. There is mounting observational evidence for spatial inhomogeneities in the gas and dust distribution. The interplay of these inhomogeneities can significantly alter frequently used star-formation and IMF indicators. Specific examples presented in this contribution are the s…
Recent progress in the quantitative analysis of Wolf-Rayet stars is reviewed, emphasising the role played by choice of spectral diagnostics, clumping and line blanketing on derived stellar properties. The ionizing properties of WR stars are discussed, based on clumped, line blanketed models for WN and WC stars. The role of metallicity and mass-loss is assessed, and the role of HII regions as probes of predicted Lyman continuum distributions. Suggestions are made for differences in observed properties of WCE and WO subtypes.
We have obtained near-infrared images and mid-infrared spectra of the starburst core of the dwarf Wolf-Rayet galaxy He 2-10. We find that the infrared continuum and emission lines are concentrated in a flattened ellipse 3-4'' or 150 pc across which may show where a recent accretion event has triggered intense star formation. The ionizing radiation from this cluster has an effective temperature of 40,000 K, corresponding to $30M_\odot$ stars, and the starburst is $0.5-1.5 \times 10^7$ years old.
We present the main results of the PhD Thesis carried out by López-Sánchez (2006), in which a detailed morphological, photometrical and spectroscopical analysis of a sample of 20 Wolf-Rayet (WR) galaxies was realized. The main aims are the study of the star formation and O and WR stellar populations in these galaxies and the role that interactions between low surface companion objects have in the triggering of the bursts. We analyze the morphology, stellar populations, physical conditions, chemical abundances and kinematics of the ionized gas, as well as the star-formation activity of each sys…
Recent results for Galactic and Magellanic Cloud Wolf-Rayet stars are summarised based on line blanketed, clumped model atmospheres together with UV, optical and IR spectroscopy. The trend towards earlier WN and WC spectral types with decreasing metallicity is explained via the sensitivity of classification diagnostics to abundance/wind density, such that WR mass-loss rates are metallicity dependent. Pre-supernovae masses for WC stars are determined, in reasonable agreement with CO-cores of recent Type-Ic SN.
A 12th Wolf-Rayet star in the SMC has recently been discovered by Massey et al. (2003). In order to determine its spectral type and a preliminary binary status, we obtained 3 high signal-to-noise spectra separated in time at the ESO-NTT. Compared to other WR stars in the SMC, SMC-WR12 appears to belong to the subgroup of faint, single and hydrogen-rich WN stars. We discuss the evolutionary status of WR12 and show that relatively low mass rotating progenitors can better account for the properties of single hydrogen-rich WN stars in the SMC.
We present new low and intermediate-resolution spectroscopic observations of the Wolf Rayet (WR) star population in the massive starburst cluster Westerlund 1. Finding charts are presented for five new WRs - four WNL and one WCL - raising the current total of known WRs in the cluster to 19. We also present new spectra and correct identifications for the majority of the 14 WR stars previously known, notably confirming the presence of two WNVL stars. Finally we briefly discuss the massive star population of Westerlund 1 in comparison to other massive young galactic clusters.
We present new yields of 26Al from Wolf--Rayet stellar winds based on rotating stellar models which account well for numerous observed properties of massive stars. We study the impacts on the yields of a change of initial mass, metallicity and initial rotation velocity. We also consider the effects of a change of mass loss rates during the Wolf--Rayet phase. We show that for surface rotation velocities during the core H--burning phase matching to the observed ones, the quantity of 26Al ejected by a star of a given initial mass and metallicity is roughly doubled when the effects of rotation are…
The stellar winds of massive stars show large changes in mass-loss rates and terminal velocities during their evolution from O-star through the Luminous Blue Variable phase to the Wolf-Rayet phase. The luminosity remains approximately unchanged during these phases. These large changes in wind properties are explained in the context of the radiation driven wind theory, of which we consider four different models. They are due to the evolutionary changes in radius, gravity and surface composition and to the change from optically thin (in continuum) line driven winds to optically thick radiation d…
We present a new compilation of Wolf-Rayet (WR) galaxies and extra-galactic HII regions showing BROAD HeII emission drawn from the literature. Relevant information on the presence of other broad emission lines (NIII 4640, CIV 5808 and others) from WR stars of WN and WC subtypes, and other existing broad nebular lines is provided. In total we include 139 known WR galaxies. Among these, 57 objects show both broad HeII and CIV features. In addition to the broad (stellar) HeII emission, a NEBULAR HeII component is well established (suspected) in 44 (54) objects. We find 19 extra-galatic HII region…
IC 10 X-1 is a bright (Lx=10^38 ergs/s) variable X-ray source in the local group starburst galaxy IC 10. The most plausible optical counterpart is a luminous Wolf-Rayet star, making IC 10 X-1 a rare example of a Wolf-Rayet X-ray binary. In this paper, we report on the detection of an X-ray orbital period for IC 10 X-1of 34.4 hours. This result, combined with a re-examination of optical spectra, allow us to determine a mass function for the system f(m)=7.8 Msun and a probable mass for the compact object of 24-36 Msun. If this analysis is correct, the compact object is the most massive known ste…
We report the discovery of a hitherto undetected population of Wolf-Rayet stars in the young galactic open cluster Westerlund 1. Optical spectroscopy of the cluster identified 11 such objects; provisional classification suggests that 6 are nitrogen rich (WN) and 5 carbon rich (WC). Including the previously identified Blue, Yellow and Red Super- & Hypergiants, Westerlund 1 clearly has a very rich population of massive post-Main Sequence objects. To date, the post-MS population of Westerlund 1 is significantly larger than that of any other galactic young open cluster - with the possible exceptio…
We find that a preliminary classification of LINERs' energetics may be made in terms of the FIR-radio correlation of Wolf-Rayet galaxies. The AGN- or starburst-supported LINERs can be distinguished by their FIR-to-radio ratio, $Q\equiv L(1.4{\rm GHz})/$ $L(60μ{\rm m})>$ or $<0.01$. It is interesting to note that almost all the LINERs with inner rings might be starburst- supported, indicating reduced AGN activities compared with those of the AGN-supported ones. We also find that a shock-heating phase for the warm dust component might be important for some starbursts at the burst age of $\ge 10^…
Context. Wolf Rayet/black hole binaries are believed to exist as a later evolutionary product of high-mass X-ray binaries. Hundreds of such binaries may exist in galaxies, but only a few of them are close enough to be observed as X-ray binaries. Only a couple of candidates have been reported so far. Aims. Based on XMM-Newton observations, we report the positional coincidence of the brightest X-ray source in NGC 300 (NGC 300 X-1) with a Wolf-Rayet candidate. Temporal and spectral analysis of the X-ray source is performed. Methods. We determine an accurate X-ray position of the object, and deriv…
From stellar evolution models and from observations of Wolf-Rayet stars it is known that massive stars are releasing metal-enriched gas in their Wolf-Rayet phase by means of strong stellar winds. Although HII region spectra serve as diagnostics to determine the present-day chemical composition of the interstellar medium, it is not yet reliably explored to what extent the diagnostic HII gas is already contaminated by chemically processed stellar wind matter. In a recent paper, we therefore analyzed our models of radiation-driven and wind-blown HII bubbles around an isolated 85 Msun star with or…
The physical basis for interpreting observations of nebular morphology around massive stars in terms of the evolution of the central stars is reviewed, and examples are discussed, including NGC 6888, OMC-1, and eta Carinae.
We use the rotating stellar models described in the paper by A. Maeder & G. Meynet in this volume to consider the effects of rotation on the evolution of the most massive stars into and during the Wolf--Rayet phase, and on the post-Main Sequence evolution of intermediate mass stars. The two main results of this discussion are the following. First, we show that rotating models are able to account for the observed properties of the Wolf--Rayet stellar populations at solar metallicity. Second, at low metallicities, the inclusion of stellar rotation in the calculation of chemical yields can lead t…
We report on neon abundances derived from {\it Spitzer} high resolution spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of [\ion{Ne}{3}] 15.56 microns. Our targets include four WN stars of subtypes 4--7, and four WC stars of subtypes 4--7. We derive ion fraction abundances $γ$ of Ne^{2+} for the winds of each star. The ion fraction abundance is a product of the ionization fraction $Q_{\rm i}$ in stage i and the abundance by number ${\cal A}_E$ of element E relative to all nuclei. Values generally consistent with solar are obtained for the WN stars, and values in excess of…
We present Gemini-N GMOS and CFHT MOS spectroscopy of Wolf-Rayet candidates in the Local Group dwarf galaxy IC10 that were previously identified by Massey et al. and Royer et al. From the present spectroscopic survey, the WC/WN ratio for IC10 remains unusually high, given its low metallicity, although none of the WC9 stars suspected from narrow-band imaging are confirmed. Our spectroscopy confirms 9 newly discovered Wolf-Rayet candidates from Royer et al., whilst spectral types of 14 Wolf-Rayet stars previously observed by Massey & Armandroff are refined here. In total, there are 26 spectrosco…
In a search for new Galactic planetary nebulae from our systematic scans of the Anglo-Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST) Halpha survey of the Southern Galactic Plane, we have identified a Pop I Wolf-Rayet star of type WN7h associated with an unusual ring nebula that has a fractured rim. We present imagery in Halpha, the 843-MHz continuum from the Molonglo Observatory Synthesis Telescope (MOST), the mid-infrared from the Midcourse Space Experiment (MSX), and confirmatory optical spectroscopy of the character of the nebula and of its central star. The inner edge o…
Massive stars deeply influence their surroundings by their luminosity and the injection of kinetic energy. So far, they have mostly been studied with spatially unresolved observations, although evidence of geometrical complexity of their wind are numerous. Interferometry can provide spatially resolved observations of massive stars and their immediate vicinity. Specific geometries (disks, jets, latitude-dependent winds) can be probed by this technique. The first observation of a Wolf-Rayet (WR) star (γ^2 Vel) with the AMBER/VLTI instrument yielded to a re-evaluation of its distance and an impro…
We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich Wolf-Rayet stellar models characterized by differing hydrogen abundances. We find that modes with intermediate radial orders are destabilized by a kappa mechanism operating on an opacity bump at an envelope temperature log T ~ 6.25. This `deep opacity bump' is due primarily to L-shell bound-free transitions of iron. Periods of the unstable modes span ~ 11-21 hr in the model containing some hydrogen, and ~ 3-12 hr in the hydrogen-depleted model. Based on the latter finding, we suggest that self-excited g modes may be the…
We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission, including the Fe K-alpha line complex, characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to…
We stress the importance of Wolf-Rayet stars for the understanding of the AGN phenomenon in galaxies. WR stars provide an unique opportunity to explore from the ground whether non-thermal nuclear activity and circumnuclear starbursts are connected. We review the known reported WR signatures observed so far in AGNs and point out some intrincacies related to the analysis of the spectra, linked to reddening correction, the origin of the Hbeta line, etc. Finally, we advocate that integral field spectroscopy is a very promising tool to study this problem and present preliminary results of a long-te…
We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $λ$4650 and $λ$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $λ$4686 and C IV $λ$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predic…
It is clear from stellar evolution and from observations of WR stars that massive stars are releasing metal-enriched gas through their stellar winds in the Wolf-Rayet phase. Although HII region spectra serve as diagnostics to determine the present-day chemical composition of the interstellar medium, it is far from being understood to what extent the HII gas is already contaminated by chemically processed stellar wind. Therefore, we analyzed our models of radiative and wind bubbles of an isolated 85 Msun star with solar metallicity (Kröger et al. 2006) with respect to the chemical enrichment of…
Observed properties of Wolf-Rayet stars at high metallicity are reviewed. Wolf-Rayet stars are more common at higher metallicity, as a result of stronger mass-loss during earlier evolutionary phases with late WC subtypes signatures of solar metallicity or higher. Similar numbers of early (WC4-7) and late (WC8-9) stars are observed in the Solar neighbourhood, whilst late subtypes dominate at higher metallicities, such as Westerlund 1 in the inner Milky Way and in M83. The observed trend to later WC subtype within metal-rich environments is intimately linked to a metallicity dependence of WR sta…
We describe the radio evolution of SN 2001ig in NGC 7424, from 700 days of multi-frequency monitoring with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). We find that deviations of the radio light curves at each frequency from the standard "minishell" model are consistent with density modulations in the circumstellar medium (CSM), which seem to recur with a period near 150 days. One possibility is that these are due to enhanced mass-loss from thermal pulses in an AGB star progenitor. A more likely scenario however is that the progenitor was a Wolf-Rayet star, whos…
