2011MNRAS.411.2804K

The numerical computations of the circularization of close binary systems at the pre-main-sequence (pre-MS) stage of evolution (reported by Zahn and Bouchet in their now well-known paper) are repeated for an extended stellar mass interval of M= 0.1-2.7 M_☉, based on modern evolutionary pre-MS stellar models and on the modified Zahn theory. The new results mostly corroborate those obtained by Zahn and Bouchet in the mass interval M= 0.5-1.25 M_☉ covered by their work. To compare the theoretically expected results with observations, a catalogue is compiled of 70 eclipsing binaries with eccentric orbits and bona fide photometric elements and masses. The pre-MS lifetimes of all these systems are computed as fractions of the circularization time-scale t_PMS/t_circ in terms of the Zahn theory and assuming the conservation of angular momentum L. All systems of this catalogue fill almost uniformly the entire domain of the e_obs-t_PMS/t_circ diagram independently of t_PMS/t_circ[i.e. the (t_PMS/t_circ)_cr= 4 threshold does not separate circular and eccentric systems in this diagram]. This result shows that the computations of the orbital circularization at the pre-MS stage based on the concept of the conservation of L have nothing to do with the actually observed distribution of the eccentricities of close binary systems. The results obtained lead us to the following conclusions. (i) Binaries appear to cross the Hayashi phase in a more detached state than we observe them on the MS, and therefore they are unaffected by tidal circularization at this stage. (ii) Binaries must lose an appreciable fraction of their angular momentum while evolving from the Hayashi phase to the zero-age main sequence. This loss may be a result of the dynamic interaction with the parent disc and other components of the multiple system.