Two possible solutions to the Wolf-Rayet wind momentum problem are discussed: purely radiation driven wind theory, with multi-line effects, and Luminous Magnetic Rotator theory. Several recently developed radiative processes for enhancing M or v∞ are described, and it is concluded that only the winds of rather hot luminous Wolf-Rayet stars could possibly be driven by radiation. These stars should show evidence of acceleration at large radial distances. For the rapid rotators, it is possible to drive a dense equatorial outflow. Limits are discussed regarding the needed surface magnetic fields. With this model, the wind momentum problem is solved in a piece-wise fashion by having the large radio flux of Wolf-Rayet stars come from the equatorial zone and the broad P Cygni lines, arising in the polar wind. The Luminous Magnetic Rotator model can also be tested through observation, primarily through spectropolarimetry.

Specific features both of continua and of line spectra of WR and P Cygni type stars can probably be explained if an extensive deceleration zone is proposed to exist in their stellar winds. The outflowing matter is first accelerated near the stellar surface, then follows the deceleration of the flow and after that the final acceleration of the outflowing matter takes place (i.e. the wind has an ADA-structure: acceleration-deceleration-acceleration). Such a structure of the wind probably arises due to the multiscattering of photons in the envelope having two detached shells, which are optically thick in resonance lines (these shells can form if ionization stratification is present in the envelope).

The mass-loss rates of O stars and B supergiants are of interest because of their influence on the evolution of these massive stars (among other matters). In principle, the ‘safest’ (i.e. most model-independent) method of determining M is to measure the free-free emission from stellar winds at radio wavelengths. This method is complicated, however, by the existence of poorly understood non-thermal emission in some stars, and by the possibility of hydrogen recombination in the winds of B supergiants.

We are in the process of carrying out a VLA survey of OB stars, initially at 3.5cm, to a flux limit of ~0.1mJy. Because all our targets should have thermal emission at detectable levels (based on mass-loss rates from Howarth & Prinja 1989 and terminal velocities from Prinja, Barlow & Howarth 1990), the survey is yielding an unbiassed estimate of the frequency of non-thermal emission. The improved sensitivity of our survey over earlier work defines the log M – log L relationship much more precisely than was previously possible, over a large range in luminosities; and allows us to make definitive statements on recombination in B supergiant winds. Our sample includes the first radio detections of an OC star, of a massive X-ray binary, and of thermal emission from a main-sequence star.

Most Wolf-Rayet binaries show phase-dependent light variations with a broad dip occurring at phase zero, when the WR star passes closest to the observer. When the orbital period is long, or the inclination is low, this dip is buried in the noise. When eclipses of the stars occur, another dip is seen when the companion passes closest to the observer; in this case, both dips are relatively deep and sharp (e.g. V444 Cygni, HD5980).

In the cases when only one dip at phase zero is seen, a simple model is derived. This involves electron scattering of companion starlight as the orbit makes it systematically traverse different amounts of the WR wind, assumed to be spherically symmetric and to follow a monotonic velocity-radius law. The shape and amplitude of the dip yield estimates of M and i (hence M when combined with M sin3i from spectroscopic orbits).

We argue that easily measured, reliable estimates of terminal velocities for early-type stars are provided (1) by the central velocity asymptotically approached by narrow absorption features in unsaturated UV P Cygni profiles, and (2) by the violet limit of zero residual intensity in saturated P Cygni profiles. We use these estimators and high resolution IUE data to determine terminal velocities, v∞, for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OB stars our values are typically 15-20% smaller than the extreme violet edge velocities, vedge, while for WR stars v∞ = 0.76vedge on average. We give new mass-loss rates for WR stars which are thermal radio emitters, taking into account our new terminal velocities and recent revisions to estimates of distances and to the mean nuclear mass per electron. We examine the relationships between v∞, the surface escape velocities, and effective temperatures.

We report on a theoretical investigation of the X-rays observed from early type stars on the global wind dynamics.

We investigate the dependence of the wind parameters on the properties of the parent star by means of an approximate analytical approach, largely based upon dimensional arguments (Bandiera and Turolla, 1990). The main assumption we use are.

I update previous estimates of the separate contributions for radiative energy, integrated total stellar wind mass and dust mass from Wolf-Rayet stars and other massive (OBA) stars. In the context of the intriguing dusty WC9 stars, I: (1) discuss the observability (or otherwise) between 0.4 and 23 μm of the condensation route from hot gas to carbon-rich grains; (2) urge caution in the use of 10 μm infrared spectra of these luminous stars to deduce the importance of silicates as a component of the interstellar medium, and (3) speculate on a possible new method for discovering new members of this relatively rare subtype based on IRAS Low Resolution Spectra. I review the observational evidence for dust condensation around SN 1987A.

The conditions of dust formation in the winds of WC-stars are discussed. Simple analytical estimations for the existence of neutral carbon zones where carbon nucleation might occur are given. It is shown that efficient dust formation can take place in the CI region if the effective temperature of the star is below ~ 25000 K and the density at the base of the wind is larger than ~ 1011 cm–3.

We have identified 21 mass-losing red supergiants (20 M-type, 1 G-type, L > 105 L⊙) within 2.5 kpc of the Sun. These supergiants are highly evolved descendants of main sequence stars with initial masses larger than about 20 M⊙. The surface density projected onto the plane of the Milky Way is between about 1 and 2 kpc–2. Although with considerable uncertainty, we estimate that the mass return by the M supergiants is somewhere between 1 and 3 10-5 M⊙ kpc–2 yr–1. In the hemisphere facing the galactic center there is much less mass loss from M supergiants than from W-R stars, but in the anticenter direction, the M supergiants return more mass than do the W-R stars. The duration of the M supergiant phase appears to be between 2 and 4 105 years. During this phase a star of initially at least 20 M⊙ returns perhaps 3 to 10 M⊙ into the interstellar medium.

Ring nebulae have been found around WR stars, OB and Of stars, and luminous blue variables. Ring nebulae are formed by the interaction between the central stars and their ambient medium via different combinations of stellar winds, ejecta, and radiation. The spectral properties of the nebulae can be used to diagnose the stellar properties, such as luminosity and effective temperature. Correlations between ring nebulae and their central stars may be used to check scenarios of stellar evolution.

To understand the origin of WR-rings it is of interest to consider nebulae associated with the precursors of WR stars and with stars at later stages of evolution. The former are represented by numerous Of stars and the latter by only four WO stars which, according to Barlow and Hummer (1982), are the most extreme WR stars near the end of their lives.

The nebula, G2.4+1.4, is shown to be a highly reddened, photoionised, mass-loss bubble of very high excitation powered by WR 102, the most extreme oxygen sequence Wolf-Rayet star known. It lies at a distance of 3±1kpc, and is about 11 pc in diameter. The exciting star, contains neither hydrogen nor helium in its atmosphere, is losing mass at a velocity of 5530 km.s-1, and has the following properties: log (Tion) = 5.20±0.05; log (R/R⊙) = 0.05±0.20; log (L/L⊙) = 5.85±0.20. We conclude that the star is the ~20M⊙ core of a supermassive star (M ≤ 60M⊙) seen near the end of its life.

Within an ongoing program of long slit spectroscopy of the known WR nebulae, we show here some results for eight objects of the sample selected from the master list of Chu et al. (1983). We present an analysis of their ionisation structure and determine abundances of oxygen, nitrogen and helium in as many positions as possible. The implications of the abundance results for the chemical enrichment of the Interstellar Medium and stellar nucleosynthesis are briefly reviewed.

High spatial (2”) and spectral (7 km s–1) resolution observations of the ring nebula surrounding the LBV star AG Carinae are presented. The data, covering Hα and [NII], were obtained with the UCL echelle spectrograph on the AAT and cover 4 radial slit positions centred on AG Car. The nebular motions are clearly resolved and show that it is a single shell expanding at 70 km s–1 which was ejected from the central star some 7 000 years ago. The data also reveal the presence of a high velocity (~ 83 km s–1) bipolar mass outflow which has distorted the northeastern back edge of the shell.

Long-slit spectrophotometric observations of HII regions around Wolf-Rayet stars in the Galaxy, the Magellanic Clouds and in IC1613 are employed to measure the ionizing radiation from these stars. The sample includes all known WR stars in the SMC and 14 regions in the LMC comprising most of the known ring nebulæ. Apart from previously known HeII λ4686 emitting nebulæ around a few WO stars, HeIII regions were discovered within the nebulæ N76 and N79W which are being photoionized by the strong He+ Lyman continuum emission from the MagellanicWNE stars AB7 and Br2, respectively. The Zanstra blackbody temperatures and luminosities (80 000, 95 000 K and log L/LO = 5.4, 6.0, respectively) agree remarkably well with more elaborate WN model calculations and suggest a much higher bolometric correction (BC = −5.8 mag) than generally admitted for WN stars. The nebular λ4686 fluxes are several times higher than the corresponding broad stellar features suggesting that the highly ionized gas would remain detectable at much larger distances (i.e. in HII galaxies) than the underlying hot WNE population.

Giant HII regions contain highly energetic objects: luminous, massive stars (including Wolf-Rayet stars) generating powerful winds, as well as, often, supernova remnants. These objects interact with the surrounding gas by creating shock waves. Part of the energy input is radiated away in the form of X-rays; also, protons and electrons may be accelerated in situ and generate γ-rays by collisions with the ionized gas. In addition, the stars themselves (including the accompanying low-mass PMS stars) are sources of X-rays, and W-R stars may emit continuum y-rays and are associated with nuclear γ-ray lines seen in the interstellar medium. Therefore, both through the stars they contain and through interactions within the gas, giant HII regions are, in addition to their more traditional properties and over nearly 7 decades in energy, important sources of high-energy radiation.

We present direct observational evidence showing that early type WR stars, and WO stars in particular, are exceedingly hot, and that their ionizing radiation is not fully trapped by their extended atmospheres. We show that extreme WR stars, or Warmers, are found in metal poor as well as in metal rich regions in accordance with theory. Images of HeIII regions around Galactic and Magellanic Cloud Warmers are shown.

A number of Wolf-Rayet stars show variations of up to a factor of ten in their infrared emission on timescales of months to years while their photospheric luminosities remain unchanged. This can be interpreted in terms of variation in the rates at which dust grains form in their stellar winds. Our data show variable circumstellar dust emission from WR 70 (HD 137603), with an episode of enhanced dust formation in early 1989, and fading of emission by dust formed around WR 48a (in 1979) and WR 19 some time before our first observation in 1988. We consider their relation to WR 140 (HD 193793), which forms dust in its wind for a few months at intervals of 7.94 years.

The spectacular ring nebula RCW58 around the Wolf-Rayet star WR40 (HD 96548, WN8) has been observed by the IRAS-Survey instrument and by the IRAS Chopped Photometric Channel (CPC) to study its IR flux and morphology. The survey data were examined with the Groningen Exportable Infra-red High-resolution Analysis system (GEISHA) software, and analysed in terms of line and dust contributions.