Rotational mixing in massive binaries: detached short-period systems

TL;DR

This paper explores how rotational mixing affects massive stars in short-period binary systems, proposing these systems as tests for rotational mixing theories and explaining certain binary evolution pathways.

Contribution

It introduces detailed models of rotational mixing in massive close binaries, highlighting their potential to test stellar evolution theories and explaining the formation of specific binary systems.

Findings

Helium is efficiently mixed in massive close binaries.

Stars remain compact and within Roche lobe during main-sequence.

An alternative formation channel for tight Wolf-Rayet binaries.

Abstract

Models of rotating single stars can successfully account for a wide variety of observed stellar phenomena, such as the surface enhancements of N and He. However, recent observations have questioned the idea that rotational mixing is the main process responsible for the surface enhancements, emphasizing the need for a strong and conclusive test. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In these systems the tides spin up the stars to rapid rotation. We use a state-of-the-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We discuss the surface abundances expected in massive close binaries (M1~20 solar masses) and we propose using such systems to test the concept of rotational mixing. As these short-period binaries often show eclipses, their parameters can be determined with high accuracy, allowing for a direct comparison with binary evolution models. In more massive close systems (M1~50 solar masses, Porb<~2 days) we find that helium is efficiently mixed throughout the envelope. The star remains blue and compact during the main-sequence phase. It stays within its Roche lobe while it gradually becomes a helium star. It is the less massive star, in which the effects of rotational mixing are less pronounced, which fills its Roche lobe first. We propose that this evolution path provides an alternative channel for the formation of tight Wolf-Rayet binaries with a main-sequence companion and might explain massive black hole binaries such as the intriguing system M33 X-7.