Rotational mixing in massive binaries. Detached short-period systems

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 observed in massive main-sequence stars. 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 for rotational mixing. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In these systems the tides are thought to spin up the stars to rapid rotation, synchronous with their orbital revolution. We use a state-of-the-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We adopt a rotational mixing efficiency that has been calibrated against observations of rotating stars under the assumption that rotational mixing is the main process responsible for the observed surface abundances. We find that the primaries of massive close binaries (M₁ ≈ 20 {M}_⊙, P_orb ≲ 3 days) are expected to show significant enhancements in nitrogen (up to 0.6 dex in the Small Magellanic Cloud) for a significant fraction of their core hydrogen-burning lifetime. 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. For the primary stars of more massive and very close systems (M₁ ≈ 50 {M}_⊙, P_orb≲ 2 days) we find that centrally produced helium is efficiently mixed throughout the envelope. The star remains blue and compact during the main sequence evolution and stays within its Roche lobe. It is the less massive star, in which the effects of rotational mixing are less pronounced, which fills its Roche lobe first, contrary to what standard binary evolution theory predicts. The primaries will appear as “Wolf-Rayet stars in disguise”: core hydrogen-burning stars with strongly enhanced He and N at the surface. 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.