Impact of the nuclear equation of state on the last orbits of binary neutron stars

¹ N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland e-mail: {bejger;haensel;zdunik}@camk.edu.pl
² Laboratoire de l'Univers et de ses Théories, UMR 8102 du CNRS, Observatoire de Paris, Université Paris 7, 92195 Meudon Cedex, France e-mail: {dorota.gondek;eric.gourgoulhon}@obspm.fr
³ Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan e-mail: keisuke@astro.physics.uiuc.edu

Received: 9 June 2004
Accepted: 13 October 2004

Abstract

We present calculations of quasi-equilibrium sequences of irrotational binary neutron stars based on realistic equations of state (EOS) for the whole neutron star interior. Three realistic nuclear EOSs of various softness and based on different microscopic models have been joined with a recent realistic EOS of the crust, giving in this way three different EOSs of a neutron-star interior. Computations of quasi-equilibrium sequences are performed within the Isenberg-Wilson-Mathews approximation to general relativity. For all evolutionary sequences, the innermost stable circular orbit (ISCO) is found to be given by mass-shedding limit (Roche lobe overflow). The EOS dependence on the last orbits is found to be quite important: for two neutron stars, the gravitational wave frequency at the ISCO (which marks the end of the inspiral phase) ranges from 800 Hz to 1200 Hz, depending upon the EOS. Detailed comparisons with 3rd order post-Newtonian results for point-mass binaries reveals a very good agreement until hydrodynamical effects (dominated by high-order functions of frequency) become important, which occurs at a frequency ranging from 500 Hz to 1050 Hz, depending upon the EOS.