We numerically investigate the impact of heat conduction by thermal electrons on the physics of colliding winds by means of a 1D model problem. Compared to ideal flows, the hot plasma has significantly lower temperatures and has both a high- and low-density component. Consequently, the shocks in WR binaries become generally more radiative and the emitted X-ray spectrum is softer, shows more emission lines, and is a composite of the emission from high- and low density regions.

We present the first results from our analysis of WR model atmospheres in the UV domain. Specifically, we test the model atmospheres derived for WR 124 and WR 18 from the Hillier & Miller (1998) and Schmutz (1997) codes through photo-ionization modelling of the associated nebula or H II region. Both codes assume non-LTE and line-blanketing, while photon-loss is implemented only in Schmutz (1997) code. The comparison of the predicted nebular properties with observations indicates that: (i) line-blanketing is essential in reconciling the stellar and nebular properties of WR stars; and (ii) in the case of WR 18 the inclusion of photon-loss produces a slightly more ionized nebula than observed. However, the effect of different model properties such as mass-loss rate has still to be investigated.

We have started an extensive program of systematic observations of planetary nebulae excited by [WR] nuclei with the aim of understanding the evolutionary status of the central stars. Detailed analysis of the nebular and stellar properties might reveal the presence of abundance variations across the nebulae, and allows to detect possible interactions between the massive stellar wind and the nebula. Such an analysis requires spatially resolved spectra, together with a reliable procedure to derive chemical abundances. This is best achieved by constructing individual photo-ionization models that reproduce the main observed properties. Here we present the spectroscopic analysis made for different knots of PB6, NGC2452, NGC2867, NGC6905 and He2–55. All these nebulae are ionized by [WC 2–3] stars and present very high ionization degrees.

The CGPS is a project undertaken at the Dominion Radio Astrophysical Observatory (DRAO), in collaboration with a consortium of university astronomers, to image the atomic hydrogen and the radio continuum emission from the interstellar medium of the Galaxy. We describe the contributions this project can make to WR star research, in particular, and to the study of stellar wind sources, in general. Specific examples are presented.

We report on recent developments in simulating colliding wind X-ray emission, and present a 3D model of the WR binary V444 Cyg (WN5+O6) which self-consistently includes radiative driving.

We present the X-ray lightcurve of the canonical colliding-wind binary WR 140 from the time of the last periastron through late 1997. Variations in column density, and intrinsic and absorbed X-ray flux agree well with the expectations of colliding wind theory in which the winds from the stars are spherically symmetric.

We report preliminary results of monitoring the flux from the Wolf-Rayet object WR 146 with the Westerbork Synthesis Radio Telescope at 21 cm since 1989. We find the average flux density slowly rising in the period 1989–1997, with evidence of shorter time-scale variability.

Studies of the atomic and molecular components of the interstellar medium in the vicinity of hot, massive stars have the potential of revealing a wealth of information on the way the star affects the distribution and physical state of the surrounding ISM during its various evolutionary phases, as well as potentially providing some information on the star itself and its past evolution.

The Two Micron All Sky Survey (2MASS) has observed a number of Wolf-Rayet stars and galaxies at J (1.25 μm), H (1.65 μm), and Ks (2.17 μm), as part of its systematic coverage of the entire sky with automated 1.3-m telescopes at Mt. Hopkins, AZ, and CTIO, Chile. The cameras observe the sky in the three channels simultaneously, using 256 x 256 HgCdTe-detector arrays. The survey samples the sky in 6° x 8.'3 tiles. The 2MASS Production Processing System transforms the raw binary survey data to final atlas images and source extractions with precise photometric calibration and astrometric positions. The survey's 10σ sensitivity is 15.8 mag at J, 15.1 at H, and 14.3 at Ks. In this poster we will present results from our analysis of several objects. We have observed WR stars both with and without associated ring nebulae. 2MASS imaging of the WR galaxies is far less affected by extinction, which can obscure much of the star formation activity. We will place these starburst dwarf galaxies in context with other galaxies, based on their near-IR colors and properties.

WR147 was first discovered to be a strong radio, X-ray and infra-red emission source by Caillault et al. (1985). It was classified as a WN8 type star with a very large mass loss rate and unusual radio emission (α < 0) for a thermal source. This unusual behaviour was explained by MERLIN (Moran et al. 1989) and VLA (Churchwell et al. 1992) observations in which it was resolved into two components: a ‘thermal’ source associated with the Wolf-Rayet star and a non-thermal component ∼ 600 mas to the north. Churchwell et al. showed that the southern component had the expected spectral index of α ≃ 0.6 for free-free emission from a stellar wind. They also showed that the northern component had a spectral index of α ≃ −0.5 and was therefore a non-thermal source, now thought to be due to a colliding wind region.

We present results on the eclipsing binary V444 Cyg (WN5+O6) with the X-ray satellite ASCA. The observations were performed at orbital phases 0.0, 0.25 and 0.5 (the 06 star is in front at phase 0.5 and vice versa at phase 0.0). Two-temperature plasma model could reproduce X-ray spectra in each phase. The temperature of the soft component is lower (kT ≃ 0.6 keV), which is attributed to the individual O6/WN5 stars. The temperature of the hard component is higher (kT ≃ 2 keV), which exhibited phase-related time-variabilities in absorption column NH and luminosity LX; NH was maximal at phase 0.0 while LX was minimal at phase 0.5. These variabilities are consistent with the scenario that X-rays are emitted from plasma heated-up by wind-wind collision near the surface of the O6 star.

The geometry structure of the magnetic field in colliding stellar winds is studied. It is shown that the magnetic field influence in the interaction region depends mainly on the ratio of the wind ram pressures of the components, the ratio of the stellar linear rotational velocity to the wind velocity of the magnetized star, and the stellar separation. For the radiative colliding winds the magnetic field influence increases with the importance of the radiative losses. An asymmetric magnetic field structure appears for a given set of binary parameters and the interaction region might be an asymmetric source of non-thermal radio emission.

The singularity at the stagnation point in steady-state colliding winds has a big influence on the structure of the radiative interaction region. None of the existing numerical models treats properly this mathematical problem. As a result, all the available models cannot be used for deriving the stellar winds parameters by making a comparison with the observed X-ray properties.

At the heart of the galactic giant H II region NGC 3603, low-mass (1–4 M⊙) stars slowly contract toward the main sequence, while peacefully cohabiting with the three most luminous, hydrogen-rich Wolf-Rayet stars in the Galaxy, a dozen hot O3–4 stars and their more modest but numerous O and B companions. This is the story of a very young and dense starburst cluster …

We argue that the WR phenomenon is simply a consequence of very high mass-loss rates (i.e., it is independent of initial mass, composition, or evolutionary status) and that it occurs in very massive main-sequence stars. We explore this argument using new HST observations of R 136a, the compact cluster containing three high-mass WR stars on the main sequence. We suggest that the underlying cause of the WR phenomemon in these stars is mass-loss enhanced by rapid rotation. Finally, we explore the implications of luminous, main-sequence WR stars for interpreting unresolved WR star clusters and galaxies.

I review the properties of two nearby giant H II regions — 30 Doradus and NGC 604, and of two nearby young star complexes now past the H II region phase — Constellation III and NGC 206. I discuss the stellar populations, mode of star formation, gas content, and kinematics as clues to what conditions may be like in more distant starburst environments.

We consider what we've learned about massive star evolution from observations of the resolved stellar content of Local Group galaxies. Studies of mixed-age (galaxy-wide) and coeval (single associations) populations reveal much about massive star evolution, and how it is controlled by metallicity, demonstrating the ‘Conti scenario’ in action! The number of WC stars to WN stars increases with increasing metallicity, as expected: in regions of higher metallicity stars of somewhat lower luminosity can evolve all the way to the WC stage. The exception is the starburst galaxy IC 10, for which I speculate that the IMF may be weighted towards high mass stars. The highest luminosity red supergiants are lacking in galaxies of higher metallicity, suggesting that the stars that would have become these RSGs are spending more of their time as WRs. The presence of luminous RSGs is highly correlated with the presence of WC and WN stars in OB associations, suggesting that many massive stars evolve through both a RSG and WR stage. The relative number of RSGs and WRs does decrease strongly with increasing metallicity, again consistent with higher metallicity systems leading to increased time in the WR phase. The various WC subclasses appear to be the result of the influence of metallicity on stellar wind structure in these stars, and are not due to to differences in mass or luminosity. Data on the field population in the Magellanic Clouds suggest that stars more massive than 30 become WRs in the LMC, while the limit may be more like 50 in the SMC, again as expected. Studies of the turn-off masses in clusters and associations in the MCs and Milky Way are nearing completion, while investigations in the more distant galaxies of the Local Group are just getting underway. For the LMC we find the following: WNE stars come from a large mass range of progenitor (30–100 ), and have very large (negative) bolometric corrections (−6 to −8 mag). The Ofpe/WN9 stars seem to come from lower mass progenitor (20–30 ), and have more modest BCs (−1 to −3 mag). WC stars come from stars with masses > 60–70 , and have BCs of −3 to −4 mag. Both ‘B2I+WN3’ systems and LBV stars like S Doradus are found only in clusters containing very high turn off masses (>70–90 ).

The winds of massive stars play a major role in setting up and controlling both the phase structure of the interstellar medium in galaxies and the vertical distribution of gas in galactic disks. In turn, these processes provide feedback into control of the star formation rate. Thanks to their proximity, these processes can be studied in detail in the Magellanic Clouds. Here we describe the results of a deep emission-line image survey of the ring nebula population, and a global high-resolution H I and H II survey of the Large Magellanic Cloud. These data provide a new insight into the process of feedback between the disk and the halo gas in disk galaxies in general, and the LMC in particular.

The central parsec of our Galaxy is powered by a cluster of young massive hot stars which formed a few million years ago. Within that cluster the seven most luminous (L >105.75 L⊙) and moderately hot (T < 104.5 K) blue supergiants contribute half of the ionizing luminosity of that region. These stars probably formed when a dense cloud fell into the center < 107 years ago, was highly compressed there, and became gravitationally unstable. Over six years of high spatial resolution, near-infrared imaging and spectroscopy have made it possible to carry out a detailed investigation of the stars in the central cluster and its enclosed mass. As one result of a detailed variability study of the central cluster stars we found that the bright He I star IRS 16SW is a short-period variable with a period of ∼9.72 days. It is most likely an eclipsing binary with a lower mass limit of 100 solar masses. Line of sight velocities and proper motions have been measured for these hot stars (as well as ∼200 other stars) down to separations of less than five light days from the compact radio source Sgr A* at the dynamic center of the Milky Way. These confirmed measurements imply the presence of a central dark mass of 2.6 × 106 solar masses. The dark mass at the center of the Milky Way is currently the most compelling case for a massive black hole. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants, substellar condensations or a degenerate gas of elementary particles but that at least 103 to 105 solar masses must be in the form of a massive black hole associated with Sgr A* itself.

The three massive clusters in the Galactic Center are not only the most massive young clusters in the Galaxy, but they harbor more Wolf-Rayet stars than any other starburst region in the Local Group. An understanding of their stellar content will be valuable for extending models to starburst regions in other galaxies. We present HST-NICMOS images, luminosity functions, and color-magnitude diagrams of two of these: the Quintuplet and Arches clusters. The images allow the detection of stars over 6 magnitudes fainter than ever before and reveal previously undetected multiple star systems. For the first time, we clearly identify the main sequence in the Quintuplet cluster, and we extend earlier detections of the main sequence in the Arches cluster to Minitial < 10 M⊙. We estimate that the Arches cluster has an initial mass function slope which is greater than the Salpeter value. Given their stellar content, the Galactic Center clusters provide both the best nearby examples of super star clusters and the best nearby locale in which to investigate WR phenomena in starburst galaxies and galactic nuclei. We discuss the content of the Galactic Center clusters, with a particular emphasis on how they compare to other massive clusters of the local group. We expect that many of the massive stars in the Galactic Center will soon evolve to become WR stars, and eventually become supernovae at a rate of ∼ 1 per 20 000 years for the next several Myr. We note that our preliminary N-body simulations suggest that such dense clusters are short-lived in the strong tidal field of the Galactic Center, consistent with the fact that no older dense clusters are seen in the central 50 pc. This implies a star formation rate of 5(10−3) M⊙ yr−1 in the Galactic Center.