General-relativistic coupling between orbital motion and internal degrees of freedom for inspiraling binary neutron stars

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Abstract
We analyze the coupling between the internal degrees of freedom of neutron stars in a close binary and the stars’ orbital motion. Our analysis is based on the method of matched asymptotic expansions and is valid to all orders in the strength of internal gravity in each star, but is perturbative in the “tidal expansion parameter” (stellar radius)/(orbital separation). At first order in the tidal expansion parameter, we show that the internal structure of each star is unaffected by its companion, in agreement with post-1-Newtonian results of A. Wiseman [Phys. Rev. Lett. 79, 1189 (1997)]. We also show that relativistic interactions that scale as higher powers of the tidal expansion parameter produce qualitatively similar effects to their Newtonian counterparts: there are corrections to the Newtonian tidal distortion of each star, both of which occur at third order in the tidal expansion parameter, and there are corrections to the Newtonian decrease in the central density of each star (Newtonian “tidal stabilization”), both of which are sixth order in the tidal expansion parameter. There are additional interactions with no Newtonian analogues, but these do not change the central density of each star up to sixth order in the tidal expansion parameter. These results, in combination with previous analyses of Newtonian tidal interactions, indicate that (i) there are no large general-relativistic crushing forces that could cause the stars to collapse to black holes prior to the dynamical orbital instability and (ii) the conventional wisdom with respect to coalescing binary neutron stars as sources of gravitational-wave bursts is correct: namely, finite-stellar-size corrections to the gravitational waveform will be unimportant for the purpose of detecting coalescences.