Constraining the Overcontact Phase in Massive Binary Evolution I. Mixing in V382 Cyg, VFTS 352, and OGLE SMC-SC10 108086

TL;DR

This study uses spectroscopic analysis of three massive overcontact binary systems at different metallicities to constrain internal mixing processes, revealing high luminosities without typical surface abundance changes, suggesting additional physical mechanisms influence massive binary evolution.

Contribution

It provides the first detailed spectroscopic constraints on internal mixing in massive overcontact binaries across different metallicities using both 1D and 3D analysis techniques.

Findings

Both components are highly overluminous, indicating efficient mixing or past mass transfer.

No strong evidence of helium or CNO surface abundance changes.

Less massive components are close to more massive ones on the Hertzsprung-Russell diagram.

Abstract

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.