paper

Orbital Decay of the PSR J0045-7319/B Star Binary System: Age of Radio Pulsar and Initial Spin of Neutron Star

arXiv:astro-ph/9605096 · doi:10.1086/310166

Abstract

Recent timing observations of PSR J0045-7319 reveal that the neutron star/B star binary orbit is decaying on a time scale of $|\Porb/\dot\Porb|=0.5$ Myr, shorter than the characteristic age ( Myr) of the pulsar (Kaspi et al.~1996a). We study mechanisms for the orbital decay. The standard weak friction theory based on static tide requires far too short a viscous time to explain the observed $\dot\Porb$. We show that dynamical tidal excitation of g-modes in the B star can be responsible for the orbital decay. However, to explain the observed short decay timescale, the B star must have some significant retrograde rotation with respect to the orbit --- The retrograde rotation brings lower-order g-modes, which couple much more strongly to the tidal potential, into closer ``resonances'' with the orbital motion, thus significantly enhancing the dynamical tide. A much less likely possibility is that the g-mode damping time is much shorter than the ordinary radiative damping time. The observed orbital decay timescale combined with a generic orbital evolution model based on dynamical tide can be used as a ``timer'', giving an upper limit of Myr for the age of the binary system since the neutron star formation. Thus the characteristic age of the pulsar is not a good age indicator. Assuming standard magnetic dipole braking for the pulsar and no significant magnetic field decay on a timescale $\lo 1$ Myr, the upper limit for the age implies that the initial spin of the neutron star at birth was close to its current value.

AASTeX, 9 pages, 3 ps figures. ApJ Letters, in press

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