1994A&A...290..119P

Evolutionary calculations are presented of massive close binaries in which the evolution of both components is taken into account. The calculations were performed with Eggleton's stellar evolution code for a variety of close (case A) binaries plus a few wider (case B) systems, with initial primary masses between 8 and 16M_☉. For the systems with 16M_☉ primaries the parameter space of initial mass ratio and orbital period is explored for the formation of contact binaries. It is found that systems with initial mass ratios q₀≲0.7 evolve into contact during the first, rapid mass transfer phase, while systems with larger q₀ but initial orbital period P₀≲1.6d reach contact later on, during the slow mass transfer phase. For the wider case A systems that avoid contact altogether, there is a limited range in initial orbital period and mass ratio where it is possible that the original secondary star reaches the end of its evolution before the primary. In particular, in massive systems the secondary may undergo a supernova explosion before the primary, leading to the ejection of this primary (a helium star) from the system. We suggest that this type of evolution may be relevant to the formation of the Crab nebula, which is helium-rich and rather far from the Galactic plane. For case B systems we find that systems with larger orbital period are more likely to come into contact, even if the primary still has a radiative envelope.