Massive overcontact binaries as progenitors of magnetic massive stars

Recent MHD simulations have demonstrated that magnetic massive stars can be formed via mergers of massive binary systems. This coalescence is preceded by a contact phase, which, while expected to be common, is poorly understood due to a lack of observational constraints: less than ten O-type overcontact binaries are currently known. The nature and degree of internal mixing during the contact phase is extremely important to the final evolutionary outcome of these objects. If the mixing is efficient enough, the stars will enter the Chemically Homogeneous Evolution regime and, instead of expanding as they evolve, the stars may shrink. This pathway has been proposed as a way to form gravitational wave progenitors. If the mixing is less efficient, then these systems may merge, forming objects such as magnetic massive stars, Be stars, LBVs etc. By studying the temperature and chemical abundances, we can determine the degree of internal mixing during this phase, and thus constrain the future evolution. Here, we present a study of several massive overcontact systems in different metallicity regimes. Our findings indicate that, while these systems are rapidly rotating, there is no strong evidence of chemical adjustments on the surface. In fact, the abundances all appear to be consistent with expectations for non-rotating stars. Interestingly, however, all systems show elevated temperatures, and the components of each system lay very close to each other on the HR diagram. This is true even for the unequal mass systems, indicating that the more massive component drives the observed temperature for both components in the system. We show that these results are robust by demonstrating that they are reproducible by a more representative three-dimensional spectroscopic analysis approach as well. Finally, we discuss the implications of our findings for the formation of magnetic massive stars and for massive star evolution in general.