Modeling contact binaries

Abstract

Context. It is common for massive stars to engage in binary interactions. In close binaries, the components can enter a contact phase, when both stars simultaneously overflow their respective Roche lobes. While observational constraints on the stellar properties of such systems exist, the most detailed stellar evolution models that feature a contact phase are not fully reconcilable with those measurements. Aims. We aim to consistently model the contact phases of binary stars in a 1D stellar evolution code. To this end, we have developed a methodology to account for energy transfer in the common contact layers. Methods. We implemented an approximative model for energy transfer between the components of a contact binary based on the von Zeipel theorem in the stellar evolution code MESA. We compared structure and evolution models both with and without this transfer. We then analyzed the implications for the observable properties of the contact phase. Results. Implementing energy transfer helps in eliminating baroclinicity in the common envelope between the components of a contact binary, which (if present) would drive strong thermal flows. We find that accounting for energy transfer in massive contact binaries significantly alters the mass ratio evolution and can extend the lifetime of an unequal mass ratio contact system.