Loss of angular momentum of cool close binaries and formation of contact systems

Abstract

A parameter-free formula for angular momentum (AM) loss via a magnetized wind, derived and calibrated from observations of spin-down of single stars, is applied to synchronized, detached binaries, and the resulting spin-up rate is determined. Due to a saturation effect, the spin-up rate depends only weakly on the period for short orbital periods. As a result it takes typically several Gyr for a short-period binary to decrease its period down to a value of 0.33 of a day - a characteristic period for forming a contact system. The period distribution of G-type binaries recently determined by Duquennoy & Mayor with a cut-off period of 2d is adopted as an initial period distribution of newly formed binaries. The time evolution of such a distribution is followed for binaries with different masses, and the results are compared with observations of stellar clusters. The shortest observed period of a main-sequence binary in the Hyades is in agreement with the adopted value of the initial cut-off period, but there exists an excess of the observed short-period binaries in this cluster compared to the evolved Duquennoy-Mayor distribution. The calculations predict a low number of contact systems in young and intermediate age clusters and a substantial increase of this number for clusters older than about 4Gyr. This agrees with the observations. A comparison of the observed and predicted frequencies of contact systems among field stars requires that the contact phase of evolution lasts a few Gyr. A similar time-scale is obtained if we extend the application of the derived formula for AM loss to a contact system. The agreement suggests that a uniform mechanism of AM loss may be operating in single stars, detached binaries and contact systems. A comparison of time-scales for period evolution of a close binary and nuclear evolution of components with different masses indicates that massive contact systems are more likely to form after coalescence has evolved single stars similar to FK Com, whereas low-mass systems will predominantly form main-sequence single stars similar to blue stragglers.