Progenitor models of Wolf-Rayet+O binary systems

Since close WR+O binaries are the result of a strong interaction of both stars in massive close binary systems, they can be used to constrain the highly uncertain mass and angular momentum budget during the major mass- transfer phase. We explore the progenitor evolution of the three best suited WR+O binaries HD186943, HD90657 and HD211853, which are characterized by a WR/O mass ratio of ~0.5 and periods of 6..10 days. We are doing this through simple mass loss and angular momentum loss estimates and through binary-evolution models including rotation of both components and a physical model which allows one to compute mass and angular momentum loss from the binary system as a function of time during the mass-transfer process. Our results show that the mass transfer in all three systems must have been highly non-conservative, with on average only ~10% of the transferred mass being retained by the mass-receiving star. This result gives support to our system mass and angular-momentum loss model, which predicts that, in the considered systems, about 90% of the overflowing matter is expelled by the rapid rotation of the mass receiver close to the Ω-limit, which is reached through the accretion of the remaining 10%.