We explore the influence of rotation on the evolution of massive stars (40 M⊙). We find the evolution in the HRD, the initial-final mass relation, and the duration of the various WR phases to depend sensitively on the initial rotation rate. We predict that, independent of the initial rotation rate, the majority of WR stars should be slow rotators.

Wolf-Rayet stars may have evolved from massive close binary systems, as a result of the mass transfer processes in such systems. A substantial fraction of all WR stars known is, indeed, found in close binaries, with massive early-type companions. After the outer layers of the initial primary have been removed during Roche Lobe Over-Flow, the remaining stellar core may be a helium-burning helium star, classified observation ally as a WR star. The fraction of such evolved binaries among main-sequence stars can be computed, following recipes from stellar- and galactic evolution and assuming steady, standard-IMF star formation over the last ∼ 109 yr in our Galaxy. Combining these results with the stellar PMF yields the total galactic number of such objects, or alternatively the number expected within, e.g., 3 kpc from the Sun. Comparison with observed numbers shows that the close binary produced WR stars should constitute a significant fraction of all WR stars, the exact fraction being dependent on the lower mass limit for WR star production (and also the duration of WR characteristics in the helium star's lifetime). The mass transfer processes in close binary systems probably allow this limit to be lower than in the case of a single WR star. Another favourable comparison regards the computed and observed numbers of persistent strong, massive X-ray binaries. Finally, second-phase WR stars are likely to comprise 5 — 10 % of the number of normal, first-stage WR binaries.

A detailed study of the Ofpe/WN9 candidate MCA1b in M33 (Willis et al. 1992) is presented, based on new high-resolution observations. We re-classify MCA1b as WN9-10 and determine its physical parameters (T* = 29 kK, logL/L⊙ = 5.9, log· = −4.0, v∞ = 420 km s−1) and composition (H/He = 2.6, N/He = 0.003, C/N = 0.2) using the Wolf-Rayet standard model. Overall, MCA1b is very similar to the LMC stars R84 (WN9, Crowther et al. 1994) and R71 (LBV, Lennon et al. 1994) indicating a similar evolutionary status and metallicity.

Luminous Blue Variables are among the brightest objects and lie at the Humphreys-Davidson empirical limit of maximum luminosity (Lamers 1986b). A distinct feature of these stars is the higher rate of mass loss and the relatively lower terminal velocity than for other stars of the same spectral type. LBV stars show moderate photometric variations with ΔV ⋍ 1 — 2m. They have higher Ṁ and lower v∞ when they are brighter (i.e., have a low Teff). These stars probable represent a short phase in the evolution along the chain O → Of → LBV → WR. After investigating the dynamics of the wind of P Cygni, Lamers (1986a) concluded that the wind was best explained by radiation pressure arising from a large number of optically thin lines in the Balmer continuum. Recently Chen & Marlborough (1993) applied this driving mechanism to Be stars. Application of this mechanism to P Cygni has been criticized by Pauldrach & Puls (1990) who found that P Cygni's wind is driven by radiation pressure due to both optically thick (60 %) and optically thin lines and that it gives rise to the typical β velocity law. In our calculations we used a method developed by Vilkoviskij and Tambovtseva (1992). We assume that Teff, M*, R*, Ṁ and initial velocity v0 at the stellar surface are given. The input parameters have been taken from Lamers (1986b). All four LBV's (except P Cygni) are considered in their maximum phase, i.e., when their winds are expected to be driven by radiation pressure in optically thin lines. A constant value of vo = 20 km/s has been adopted. Force-multiplier parameters k and δ have been taken from Abbott (1982). If we assume that the acceleration of the wind is due to optically thin lines of metals in the Balmer continuum, than we must have α ≪ 1 (Abbott 1982). We stress that there are no calculations available to determine that quantity in the optically thin limit. We will estimate α later for P Cygni but for other stars we will use a as a free parameter. Note that α depends on the ratio of optically thick to optically thin lines in the wind. Basically k depends on the number of lines that produce radiation pressure and in our case a value of k can be even larger than given by Abbott (1982). The results of our calculations are presented in Table I. We give the values of v∞ calculated for the indicated values of input parameters. The terminal velocities are in good agreement with the observational data (Lamers 1986b).

The presence of heated circumstellar dust around WC type Wolf-Rayet stars is reviewed in terms of the episodic or persistent condensation of carbon grains in their stellar winds. At present, 19 WC stars are known to form dust persistently and 7 episodically. Infrared spectroscopy suggests that the grains are amorphous rather than graphitic carbon. The properties of the stars which make dust episodically are reviewed in terms of processes in colliding winds in WR binary systems and the possible applicability of these processes to the stars which make dust persistently in examined.

WC-type Wolf-Rayet stars show episodic or persistent infrared emission attributed to amorphous carbon grains forming in their winds. The process of dust formation in the hydrogen-poor environments characteristic of WC winds is reviewed and compared to the chemical pathway to soot particle production in AGB stars. Emphasis is put on the formation of the dust precursors, and the physical conditions necessary to nucleate them in WC winds. In particular, it is concluded that dust formation around WC stars occurs in a dense, largely neutral, circumstellar disk.

The physical properties of dust in WR shells are investigated in detail. It has been found that carbon grains may grow in the severe conditions of WR shells due to collisions with impinging positive carbon ions. Detailed physical models of carbon dust and dust production rates for selected WC stars have been calculated. It is concluded that graphite rather than amorphous carbon is the most suitable grain material.

I have examined all InfraRed Astronomy Satellite (IRAS) data relevant to the 173 galactic Wolf-Rayet (WR) stars in an updated catalogue provided by van der Hucht (priv. comm.), including the 13 stars newly discovered by Shara et al. (1991). Using the exact coordinates given in these lists, I have examined the IRAS Point Source Catalog (PSC), the Faint Source Catalog, Faint Source Reject Catalog, and generated 1-dimensional spatial profiles (“ADDSCAN”s), and 2-dimensional full-resolution images (“FRESCO”s). The goal was to assemble the best set of observed IRAS colour indices for different WR types, in particular for known dusty WCL objects. These colour indices define zones in the IRAS colour-colour plane. By searching the PSC for otherwise unassociated sources that satisfy these colours, I have identified potential new WR candidates, too faint to have been recognized in previous optical searches. I have extracted these candidates' IRAS Low Resolution Spectrometer (LRS) data and compared the spectra with the highly characteristic LRS shape for known dusty WCL stars. Any surviving candidates must now be examined by optical spectroscopy. This work represents a much more rigorous and exhaustive version of the LRS study that identified IRAS 17380-3031 (WR98a) as the first new WR (WC9) star discovered by IRAS (Cohen et al. 1991).

At first sight, it is surprising that dust nucleation and growth can occur in the harsh environment of a Wolf-Rayet star wind. However, we know that episodic formation occurs in several different types of astrophysical source. Of particular relevance to our efforts to gain a fuller understanding of the process in WC stars is the extensive work that has been performed on classical novae. Thus, although episodic dust formation in other types of object is briefly described, this review concentrates on a comparison with this process as observed in novae in particular. It is argued that for efficient grain growth, clumping of ejecta (perhaps following shock-interactions) is essential.

Many late-type WC stars exhibit strong IR excesses which have usually been explained by hot dust radiation. Alternatively, we found that free-free and free-bound radiation of a small number of high density clumps beyond the effective radio-emission zone may give rise to “dust-like” IR excesses.

In close binary systems of O-type stars, the individual stellar winds will collide between the stars to form shock fronts. The existence of the shock region can be established through observations of excess X-ray emission and orbital phase-related variations in UV wind features and optical emission lines. Here I report on work in progress on the orbital variations of the UV wind lines in a large sample of O-binaries that have been observed with the International Ultraviolet Explorer satellite. In most of these binaries, the blue absorption trough of the P-Cygni lines weakens when the strong wind component is behind its companion, in accordance with expectations for colliding winds. I describe how the radial velocity variations of the wind features differ from the orbital motions. Finally I discuss observations of Hα emission in these systems, with emphasis on HD 47129 (Plaskett's star). Such optical emission lines act as a probe of high density regions in the winds.

We have investigated a recently obtained ROSAT X-ray observation of the massive binary 29 UW Canis Majoris (HR 2781; HD 57060). This short-period binary (4.3934 d) consists of two eclipsing O-type stars and has been observed for about 9 days with the ROSAT PSPC. We discuss the origin of the X-ray emission in the close binary system 29 CMa in the context of the traditional colliding wind scenario as well as the scenario of X-ray emission from shock heated gas in the stellar winds.

In this paper, a review of spectroscopic evidence for colliding wind effects in WR+O binaries is presented with a particular emphasis on variability at ultraviolet wavelengths. In the optical, evidence for colliding winds comes from extra emission formed in the region of shocked gas and superimposed on the strong emission lines formed in the WR wind. At ultraviolet wavelengths, we observe the consequences of the fact that between the two stars the winds are prevented from reaching their terminal velocity because of the wind-wind interaction.

A detailed description of the wind-wind collision (WWC) zones in the Wolf-Rayet binaries V444 Cyg (WN5+O6III-V, P = 4.21 d) and CX Cep (WN5+O5V, P = 2.13 d) is presented. In V444 Cyg, parameters of the WWC zone can be derived from variations of HeI line profiles. There is some evidence for a highly unstable character of the WWC zone. In CX Cep, the complicated dependence of equivalent widths for emission lines of different ionization potential on orbital phase can be explained by a combination of several factors, e.g., (a) additional ionization from the bow shock region; (b) reheating of the WR wind by the O-star companion; and (c) an emission component arising in the WWC region. Some preliminary indications of WWC are revealed in the WR+OB system CQ Cep (WN7+O9I-II?) with the shortest known orbital period: P = 1.64 d.

The Magellanic Clouds provide an ideal laboratory to study the properties of Wolf-Rayet stars. In particular, the 25 WC/WO stars that they contain are nearly all of the same sub-class (21 WC4, one WC5, one WC6 and one WO in the Large Magellanic Cloud; one WO in the Small Magellanic Cloud). This, along with their formation in the same isolated environment, suggests that the wind structure is similar in each star. Thus, a study of other characteristics, such as the effects of binary separation and period on binary wind collisions, can be undertaken with little concern for differing winds. Some 30 high-quality CCD spectra for each of the 25 stars were obtained during three observing missions in the past decade. Although the analysis is not yet complete, the sample clearly contains four large-amplitude WR+O binaries with periods ranging from two to seventeen days and possibly other long-period systems. This paper deals with the wind interaction of the star B22.

Recent studies of variable Wolf-Rayet emission lines reveal a hierarchy of structures, characterized by power laws analogous to what is expected from supersonic compressible turbulence. The collision of inhomogeneous winds can be very different from the case of smooth winds. The difference will mainly depend on two factors: (i) the relative importance of the inhomogeneous compared to the homogeneous component; and (ii) the characteristic filling factor of the inhomogeneous component. Using relations derived from observations of variable line structures (“blobs”), it can be deduced that the flux emitted by the inhomogeneous part of the wind of a WR star is dominated by its smallest structures. This implies that a significant fraction of the underlying emission line profile could be produced by small, undetectable inhomogeneities. It can also be deduced that the volume spanned by the inhomogeneities is dominated by the largest structures. This in turn implies that the filling factor should be low, or that we are dealing with a fractal-like hierarchy. It is suggested that the wind is composed of dense structures separated by large “voids” which may actually be filled by a homogeneous wind component. The interacting zone of two inhomogeneous colliding winds should thus be much more extended in space than for a smooth-wind model, because the dense, inhomogeneous structures are able to penetrate through the large “voids”.

The variable peaks on the flat–topped CIII-569.6nm line of the WC-star in the WR 79–system are interpreted as additional emissions, produced in the colliding wind region of the two stars. The minimum and maximum distances of the appertaining radial velocity curves yield the important parameter i and so the system-masses. Synthetic profiles strongly support the present model.

The significance of 3D effects in simulating colliding winds in WR binaries is demonstrated for the examples V444 Cyg, HD 193793 and γ Vel. The effects decrease with increasing separation of the stars. A comparison with symbiotic systems is drawn and a simulation of the symbiotic system EG And is presented.

The wind interaction processes in supernova precursors can be responsible for the formation of regular structures of the circumstellar matter not only at the distances of 1018 cm (SN1987A), but also at those of less than 1016 cm.

Ever since the earliest work, X-ray observations have been central in the chequered development of the study of interacting high-velocity winds in binaries. This review assesses how confidently the observations of Wolf-Rayet stars may be interpreted as the result of colliding winds, drawing attention to the wide range of X-ray luminosities even among the relatively bright binaries. Although the data do not generally conform to the simplest binary models, some suggestions are made to reconcile theory and observation.