Tidal evolution of close binary stars. II. Orbital circularization oflate-type binaries.

The premain-sequence evolution of close binary systems with masses ranging from 0.5 to 1.25 solar masses is described. It is demonstrated that most of the orbital circularization occurs during the Hayashi phase, and that the subsequent decrease in eccentricity on the main-sequence is negligible. The theoretical cutoff period, which separates the circular from the eccentric systems, lies between 7.2 and 8.5 days, depending on the masses and on the assumptions made about the initial conditions; it agrees well with the observed period of about 8 days. This cutoff period is a function of the initial radius of the stars on the Hayashi track. In these calculations, the values predicted by the star formation theory of Stahler et al. (1980) are used; hence the good agreement mentioned above provides a new validation of that theory. It is also shown that systems around that cutoff period reach the zero-age main-sequence with their components rotating faster than the orbital rate.