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Core collapse of dense massive star clusters is unavoidable and this leads to the formation of massive objects, with a mass up to 1000 M ⊙ and even larger. When these objects become stars, stellar wind mass loss determines their evolution and final fate, and decides upon whether they form black holes (with normal mass or with intermediate mass) or explode as a pair instability supernova. In the present paper, we discuss the evolution of very massive stars and we present a convenient evolution recipe that can be implemented in a gravitational N-body code to study the dynamics of dense massive clusters.

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We discuss differences between massive single star and massive close binary population number synthesis predictions of WR stars. We show that the WC/WN number ratio as function of metallicity depends significantly on whether or not binaries are included. Furthermore, the observed WC(+OB)/WN(+OB) number ratio in the Solar neighborhood seems to indicate that the WR mass loss rates are lower by another factor two compared to recently proposed clumping corrected formalisms. We then demonstrate that the observed lower luminosity distribution of single WN stars can be explained in a satisfactory way by massive single star evolutionary computations where the red supergiant phase is calculated using a stellar wind mass loss rate formalism that is based on recent observations.

Abstract. In the present paper we investigate in detail the effects of binaries with initial period between 1 day and 10 years on theoretical simulations of Wolf-Rayet (WR) type spectral features in starbursts. We focus on the evolution of the nebular H β line in starburst in general, on the intensity ratios I(nebular He II λ4686)/I(H β ), I(blue bump)/I(H β ), and I(red bump)/I(H β ) as a function of the equivalent width of H β of WR galaxies in particular. The binary evolutionary processes that dominate the evolution of the considered spectral features are the Roche lobe overflow in case Br systems, the mass transfer efficiency, and the merger rate. We show that the predictions on nebular He II depend critically on the uncertainties in the theory of WR atmospheres and particularly on uncertainties in the treatment of the subsonic velocity region of the WR wind. The observations of low metallicity starbursts are best reproduced by a theoretical model with a significant number of binaries and with a metallicity-dependent WR wind.