Effects of limited core rejuvenation on the properties of massive contact binaries

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
² Department of Astrophysics and Planetary Science, 800 E Lancaster Ave., Villanova, PA 19085, USA

^⋆ Corresponding author; jelle.vandersnickt@student.kuleuven.be

Received: 7 December 2024
Accepted: 8 February 2025

Abstract

Context. Massive contact binaries are both stellar-merger and gravitational-wave progenitors, but their evolution is still uncertain. An open problem in the population synthesis of massive contact binaries is the predicted mass ratio distribution. Current simulations evolve quickly to mass ratios close to unity, which is not supported by the sample of observed systems. It has been shown that modifying the near core mixing properties of massive stars can alter the evolution of contact binaries, but this has not been tested on a whole population.

Aims. We implement a prescription of the convective core overshooting based on the molecular gradient. The goal of the implementation is to limit the rejuvenation efficiency of the accretor. We aim to investigate the effects of the reduced rejuvenation on the mass ratio distribution of massive contact binaries.

Methods. We calculated a grid of 4896 models using the binary-evolution code MESA with our convective overshoot formulation, and we compared the simulations with the known observations of massive contact binaries.

Results. We find that by limiting core rejuvenation through the convective core overshoot, the predicted mass ratio distribution shifts significantly to values away from unity. This improves the theoretical predictions of the mass ratios of massive contact binaries.

Conclusions. The core rejuvenation of the components in massive contact binaries is a key parameter for their predicted mass ratio distribution. Establishing the rejuvenation efficiency within the contact binary population should therefore be possible. The sample size and uncertainties associated with the characterization of contact binaries, however, prevents us from doing so, and other methods like asteroseismology can place constraints on the rejuvenation process.