On the Orbital Decay of the PSR J0045−7319 Binary

Recent observations of PSR J0045-7319, a radio pulsar in a close eccentricorbit with a massive main sequence B-star companion, indicate that the system'sorbital period is decreasing on a timescale $\sim 5 x 10^{5}$ years (Kaspi etal. 1996). Timing observations of PSR J0045-7319 also indicate that the B-staris rotating rapidly, perhaps close to its breakup rotation rate. For rapid(super-synchronous) prograde rotation of the B-star, tidal dissipation leads toan increasing orbital period for the binary system, while for retrograderotation of any magnitude, the orbital period decreases with time. We show thatif tidal effects are to account for the observed orbital decay of the PSRJ0045-7319 binary, the B-star must have retrograde rotation. This implies thatthe supernova that produced the pulsar in this binary system likely had adipole anisotropy. For a reasonably wide range of retrograde rotation rates, the energy in thedynamical tide of the B-star needs to be dissipated in about one orbital periodin order to account for the observed orbital evolution time for the PSRJ0045-7319 binary. We show, however, that the radiative dissipation of thedynamical tide in a rigidly rotating B-star is too inefficient by a factor of$\approx$ 10$^3$, regardless of the magnitude of the rotation rate. We describehow, when the surface of the B-star is rotating nearly synchronously (which isexpected from the work of Goldreich and Nicholson, 1989), the energy in thedynamical tide is dissipated in less than an orbital period, thus reconcilingthe theoretical and observed rates of orbital evolution.