Abstract. We present evolutionary calculations for 74 close binaries systems with initial primary masses in the range 12...25 M , and initial secondary masses between 6 and 24 M . The initial periods were chosen such that mass overflow starts during the core hydrogen burning phase of the primary (Case A), or shortly thereafter (Case B). We use a newly developed binary code with up-to-date physics input. Of particular relevance is the use of OPAL opacities, and the time-dependent treatment of semiconvective and thermohaline mixing. We assume conservative evolution for contact-free systems, i.e., no mass or angular momentum loss from those system except due to stellar winds. We investigate the borderline between contact-free evolution and contact, as a function of the initial system parameters. The fraction of the parameter space where binaries may evolve while avoiding contact -which we found already small for the least massive systems considered -becomes even smaller for larger initial primary masses. At the upper end of the considered mass range, no contact-free Case B systems exist. While for primary masses of 16 M and higher the Case A systems dominate the contact-free range, at primary masses of 12 M contact-free systems are more frequent for Case B. We identify the drop of the exponent x in the main sequence mass-luminosity relation of the form L ∝ M x as the main cause for this behaviour. For systems which evolve into contact, we find that this can occur for distinctively different reasons. While Case A systems are prone to contact due to reverse mass transfer during or after the primary's main sequence phase, all systems obtain contact for initial mass ratios below ∼ 0.65, with a merger as the likely outcome. We also investigate the effect of the treatment of convection, and found it relevant for contact and supernova order in Case A systems, particularly for the highest considered masses. For Case B systems we find contact for initial periods above ∼ 10 d. However, in that case (and for not too large periods) contact occurs only after the mass ratio has been reversed, due to the increased fraction of the donor's convective envelope. As most of the mass transfer occurs conservatively before contact is established, this delayed contact is estimated to yield to the ejection of only a fraction of the donor star's envelope. Our models yield the value of β, i.e., the fraction of the primaries envelope which is accreted by the secondary. We derive the observable properties of our systems after the major mass transfer event, where the mass gainer is a main sequence or supergiant O or early B type star, and the mass loser is a helium star. We point out that the assumption of conservative evolution for contact-free systems could be tested by finding helium star companions to O stars. Those are also predicted by non-conservative models, but with different periods and mass ratios. We describe strategies for increasing the probability to find helium star companions in observational search programs.

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We present models for the complete life and death of a 60 M ⊙ star evolving in a close binary system, from the main sequence phase to the formation of a compact remnant and fallback of supernova debris. After core hydrogen exhaustion, the star expands, loses most of its envelope by Roche lobe overflow, and becomes a Wolf-Rayet star. We study its post-mass transfer evolution as a function of the Wolf-Rayet wind mass loss rate (which is currently not well constrained and will probably vary with initial metallicity of the star). Varying this mass loss rate by a factor 6 leads to stellar masses at collapse that range from 3.1 M ⊙ up to 10.7 M ⊙ . Due to different carbon abundances left by core helium burning, and non-monotonic effects of the late shell burning stages as function of the stellar mass, we find that, although the iron core masses at collapse are generally larger for stars with larger final masses, they do not depend monotonically on the final stellar mass or even the C/O-core mass. We then compute the evolution of all models through collapse and bounce. The results range from strong supernova explosions (E kin > 10 51 erg) for the lower final masses to the direct collapse of the star into a black hole for the largest final mass. Correspondingly, the final remnant masses, which were computed by following the supernova evolution and fallback of material for a time scale of about one year, are between 1.2 M ⊙ and 10 M ⊙ . We discuss the remaining uncertainties of this result and outline the consequences of our results for the understanding of the progenitor evolution of X-ray binaries and gamma-ray burst models.

We discuss which fraction of the matter flowing to the companion during a Roche lobe overflow phase can actually be accreted by the secondary star. Employing new evolutionary models for massive close binaries which include the effects of rotation for both components as well as angular momentum accretion and spin-orbit coupling, we propose a physical model to calculate the accretion efficiency in Case A and B systems. We provide examples showing that both cases, high and low accretion efficiency, do occur within these models, as it seems required by observed post-mass transfer systems. Furthermore, we discuss late evolutionary stages of such binaries, with emphasis on the formation of compact objects: what are their spin rates, which systems can produce black holes, which gamma-ray bursts?