Magnetic braking at work in binaries

Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
e-mail: wvanrens@vub.be

Received: 26 March 2020
Accepted: 13 July 2020

Abstract

Context. In earlier papers, we aimed to reconstruct the progenitor systems of Algol-type semi-detached binaries. To this end, we developed a binary evolutionary code for the purpose of reproducing the orbital parameters, masses, and location in the HRD of well-observed Algol systems. In this code, the effects of mass and angular momentum losses and tidal coupling were included, but not magnetic braking at that point. In the present paper, we study the effects of magnetic braking on the rotation of the mass gainers in these systems.

Aims. Equatorial velocities have been measured for a number of mass-gaining stars in interacting binaries. Tides tend to synchronize the rotation of the gainer, but many observed low equatorial velocities cannot be explained by tidal interactions alone.

Methods. We added magnetic braking to our code to better reproduce the observed equatorial velocities.

Results. Large equatorial velocities of mass-gaining stars are lowered by tidal interaction and magnetic braking. Tides are mainly at work at short orbital periods, leaving magnetic braking alone at work during longer orbital periods.

Conclusions. Slow rotation of mass gainers in Algol-type binaries is mostly well reproduced by our code. However, (not observed) critical rotation of the gainer in some systems cannot be avoided by our calculations.