On orbital circulation in late-type close binaries

In a coeval sample of late-type binary stars, all close binaries with periods shorter than a cutoff period have circular orbits, whereas close binaries with larger periods display eccentric orbits. The observed cutoff periods are found to increase monotonically with the evolutionary age of the sample. The three theories based on the tidal-torque mechanism are quite ineffective during the main-sequence lifetime of a late-type binary, being operative during the pre-main-sequence contraction phase only. On the contrary, the observed distribution of cutoff periods with age is consistent with the hydrodynamical mechanism, since it is the only one that meets the test of absolute calibration for late-type binaries evolving on the main sequence - i.e., given plausible values for the Reynolds number in the surface layers, the theoretical circularization time does not exceed the sample age at cutoff period. However, because this mechanism may not be equally efficient during a contraction phase, it is pointed out that the tidal-torque mechanism can be responsible for orbital circularization during the pre-main-sequence phase - the hydrodynamical mechanism being fully responsible for orbital circularization during the main-sequence phase. Such a solution, which has been hitherto ignored, is quite a plausible one since the two competing mechanisms are not mutually exclusive - each one being operative for different values of the parameters.