Constraining the overcontact phase in massive binary evolution. I. Mixing in V382 Cyg, VFTS 352, and OGLE SMC-SC10 108086

As potential progenitors of several exotic phenomena including gravitational wave sources, magnetic stars, and Be stars, close massive binary systems probe a crucial area of the parameter space in massive star evolution. Despite the importance of these systems, large uncertainties regarding the nature and efficiency of the internal mixing mechanisms still exist. In this work, we aim to provide robust observational constraints on the internal mixing processes by spectroscopically analyzing a sample of three massive overcontact binaries at different metallicities. Using optical phase-resolved spectroscopic data, we perform an atmosphere analysis using more traditional 1D techniques and using state-of-the-art 3D techniques. We compare and contrast the assumptions and results of each technique and investigate how the assumptions affect the final derived atmospheric parameters. We find that in all three cases, both components of system are highly overluminous indicating either efficient internal mixing of helium or previous non-conservative mass transfer. However, we do not find strong evidence of helium or CNO surface abundance changes usually associated with mixing. Additionally, we find that in unequal mass systems, the measured effective temperature and luminosity of the less massive component places it very close to the more massive component on the Hertzsprung-Russell diagram. These results were obtained independently using both of the techniques mentioned above, which suggests that these measurements are robust. The observed discrepancies between the temperature and the surface abundance measurements when compared to theoretical expectations indicate that unaccounted for additional physical mechanisms may be at play.