Detailed evolutionary models of massive contact binaries: I. Model grids and synthetic populations for the Magellanic Clouds

The majority of massive stars are born in close binary systems, where often both stars overfill their Roche lobes simultaneously and the systems experience contact. Here, we aim at a comprehensive prediction of the observable properties of such massive contact binaries and their fates. We present the first set of detailed binary evolution models for massive contact binaries, assuming conservative mass transfer. We compute two grids of 2790 models each, for Large and Small Magellanic Clouds metallicity, with initial parameters spanning total masses of 20 M$_{\odot}$ to 80 M$_{\odot}$, orbital periods (P) of 0.6 days to 2 days and mass ratios (q) of 0.6 to 1.0. Our results indicate that only a small fraction of massive binaries which undergo nuclear-timescale contact evolution is expected to originate from outside this parameter range. Our nuclear-timescale contact binary models evolve towards equal masses and ultimately merge on the main sequence. From our synthetic populations, we expect about 48 and 12 O-type contact binaries in the Large and Small Magellanic Clouds respectively; these numbers increase if we also include near-contact binaries. The P-q distribution of seven observed contact binaries in the Magellanic Clouds approximately follows our predicted distribution, although their mass ratios are smaller than expected. We also compare our results to massive contact binaries in the Milk Way and find that some systems with total masses more than 40 M$_{\odot}$, have larger periods than expected from our predictions for lower metallicity populations.