Gravitational signals emitted by a point mass orbiting a neutron star: Effects of stellar structure

The effects that the structure of a neutron star would have on the gravitational emission of a binary system are studied in a perturbative regime, and in the frequency domain. Assuming that a neutron star is perturbed by a point mass moving on a close, circular orbit, we solve the equations of stellar perturbations in general relativity to evaluate the energy lost by the system in gravitational waves. We compare the energy output obtained for different stellar models with that found by assuming that the perturbed object is a black hole with the same mass, and we discuss the role played by the excitation of the stellar modes. Our results indicate that the stellar structure begins to affect the emitted power when the orbital velocity is $v\ensuremath{\gtrsim}0.2 c$ $({\ensuremath{\nu}}_{\mathrm{GW}}\ensuremath{\gtrsim}185 \mathrm{Hz}$ for a binary system composed of two ${1.4M}_{\ensuremath{\bigodot}}$ neutron stars). We show that the differences between different stellar models and a black hole are due mainly to the excitation of the quasinormal modes of the star. Finally, we discuss to what extent and up to which distance the perturbative approach can be used to describe the interaction of a star and a pointlike massive body.