The orbital evolution of highly eccentric binaries

The tidal evolution of close binaries in the limit of e approaching 1 is studied in this work. We use Hut equations to obtain the time derivatives and timescales for the evolution of the eccentricity, semimajor axis, and stellar rotation rate in the high-eccentricity binaries. We find that in some of the highly eccentric binaries the tidal shear changes near periastron on a timescale shorter than the convective timescale, so that the turbulent viscosity could be reduced. We consider three different recently proposed approaches to viscosity reduction and show that for all three theories the tidal evolution for highly eccentric binaries is quite different from that encountered in low-eccentricity systems. In particular, the semimajor axis decreases on a timescale much shorter than the eccentricity, and the periastron distance stays constant in time. We suggest to test the different approaches to viscosity reduction by comparing the age of any known highly eccentric binary with its tidal timescales. The proposed test is applied to Gl 586A, a nearby binary recently found to have an extremely high eccentricity. The test indicates that this binary may indeed be used to reject the approach which assumes a non-reduced viscosity.