THE ORBITAL EVOLUTION OF HIGHLY ECCENTRIC BINARIES

The tidal evolution of close binaries in the limit of e --> 1 is studi ed in this work. We use Hut equations to obtain the time derivatives a nd timescales for the evolution of the eccentricity, semimajor axis, a nd stellar rotation rate in the high-eccentricity binaries. We find th at in some of the highly eccentric binaries the tidal shear changes ne ar periastron on a timescale shorter than the convective timescale, so that the turbulent viscosity could be reduced. We consider three diff erent recently proposed approaches to viscosity reduction and show tha t for all three theories the tidal evolution for highly eccentric bina ries is quite different from that encountered in low-eccentricity syst ems. In particular, the semimajor axis decreases on a timescale much s horter than the eccentricity, and the periastron distance stays consta nt in time. We suggest to test the different approaches to viscosity r eduction by comparing the age of any known highly eccentric binary wit h its tidal timescales. The proposed test is applied to G1 586A, a nea rby binary recently found to have an extremely high eccentricity. The test indicates that this binary may indeed be used to reject the appro ach which assumes a nonreduced viscosity.