Close binary evolution

II. Impact of tides, wind magnetic braking, and internal angular momentum transport

¹ College of Physics, Guizhou University, Guiyang City, 550025 Guizhou Province, PR China
² Geneva Observatory, Geneva University, 1290 Sauverny, Switzerland
e-mail: georges.meynet@unige.ch
³ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, PR China
⁴ Guangxi Key Laboratory for Relativistic Astrophysics, Department of Physics, Guangxi University, 530004 Nanning, PR China
⁵ Department of Physics, Royal Military College of Canada, Ontario, K7K 7B4, Canada

Received: 30 April 2017
Accepted: 5 September 2017

Abstract

Context. Massive stars with solar metallicity lose important amounts of rotational angular momentum through their winds. When a magnetic field is present at the surface of a star, efficient angular momentum losses can still be achieved even when the mass-loss rate is very modest, at lower metallicities, or for lower-initial-mass stars. In a close binary system, the effect of wind magnetic braking also interacts with the influence of tides, resulting in a complex evolution of rotation.

Aims. We study the interactions between the process of wind magnetic braking and tides in close binary systems.

Methods. We discuss the evolution of a 10 M_⊙ star in a close binary system with a 7 M_⊙ companion using the Geneva stellar evolution code. The initial orbital period is 1.2 days. The 10 M_⊙ star has a surface magnetic field of 1 kG. Various initial rotations are considered. We use two different approaches for the internal angular momentum transport. In one of them, angular momentum is transported by shear and meridional currents. In the other, a strong internal magnetic field imposes nearly perfect solid-body rotation. The evolution of the primary is computed until the first mass-transfer episode occurs. The cases of different values for the magnetic fields and for various orbital periods and mass ratios are briefly discussed.

Results. We show that, independently of the initial rotation rate of the primary and the efficiency of the internal angular momentum transport, the surface rotation of the primary will converge, in a time that is short with respect to the main-sequence lifetime, towards a slowly evolving velocity that is different from the synchronization velocity. This “equilibrium angular velocity” is always inferior to the angular orbital velocity. In a given close binary system at this equilibrium stage, the difference between the spin and the orbital angular velocities becomes larger when the mass losses and/or the surface magnetic field increase. The treatment of the internal angular momentum transport has a strong impact on the evolutionary tracks in the Hertzsprung-Russell Diagram as well as on the changes of the surface abundances resulting from rotational mixing. Our modelling suggests that the presence of an undetected close companion might explain rapidly rotating stars with strong surface magnetic fields, having ages well above the magnetic braking timescale. Our models predict that the rotation of most stars of this type increases as a function of time, except for a first initial phase in spin-down systems. The measure of their surface abundances, together, when possible, with their mass-luminosity ratio, provide interesting constraints on the transport efficiencies of angular momentum and chemical species.

Conclusions. Close binaries, when studied at phases predating any mass transfer, are key objects to probe the physics of rotation and magnetic fields in stars.