The central density of a neutron star is unaffected by a binary companion at linear order in $\mu/R$

Abstract

Recent numerical work by Wilson, Mathews, and Marronetti [J. R. Wilson, G. J. Mathews and P. Marronetti, Phys. Rev. D 54, 1317 (1996)] on the coalescence of massive binary neutron stars shows a striking instability as the stars come close together: Each star's central density increases by an amount proportional to 1/(orbital radius). This overwhelms any stabilizing effects of tidal coupling [which are proportional to 1/(orbital radius)^6] and causes the stars to collapse before they merge. Since the claimed increase of density scales with the stars' mass, it should also show up in a perturbation limit where a point particle of mass $\mu$ orbits a neutron star. We prove analytically that this does not happen; the neutron star's central density is unaffected by the companion's presence to linear order in $\mu/R$. We show, further, that the density increase observed by Wilson et. al. could arise as a consequence of not faithfully maintaining boundary conditions.