Tidal evolution of eccentric orbits in massive binary systems. A study of resonance locking

We study the tidal evolution of a binary system consisting of a 1.4 M_sun compact object in elliptic orbit about a 10 M_sun uniformly rotating main sequence (MS) star for various values of the initial orbital parameters. We apply our previously published results of 2D non-adiabatic calculations of the non-radial g- and r-mode oscillations of the uniformly rotating MS star, and include the effects of resonant excitation of these modes in the tidal evolution calculations. A high orbital eccentricity enhances the effectiveness of the tidal interaction because of the large number of harmonic components of the tidal potential and the reduced orbital separation near periastron. By including the evolution of the MS star, especially of its rotation rate, many resonance crossings occur with enhanced tidal interaction. We analyse the phenomenon of resonance locking whereby a particular tidal harmonic is kept resonant with a stellar oscillation mode by the combined action of stellar evolution and other tidal harmonics. Resonance locking of prograde g-modes appears an effective mechanism for orbital circularization of eccentric orbits. We consider the orbital evolution of the binary pulsar PSR J0045-7319 and conclude that resonance locking could explain the observed short orbital decay time of this system if the B-star spins in the direction counter to the orbital motion.