STRUCTURE AND STABILITY OF ROTATING RELATIVISTIC NEUTRON-STARS

The aim of this work is twofold: (1) We investigate the influence of rotation on the bulk properties of neutron stars. (2) The limiting angular velocities at which instability against gravitational radiation-reaction sets in are calculated for rotating neutron stars of gravitational masses M = 1.5 M. and M = 1.9 M. The calculations are based on four different, realistic neutron matter equations of state derived in the framework of relativistic nuclear field theory. Two of them are calculated in the relativistic Hartree and Hartree-Fock approximation for electrically charge neutral many-baryon matter at zero temperature. The influence of two-particle correlations on the bulk properties of both rotating and nonrotating neutron stars is demonstrated through neutron matter equations of state calculated for the relativistic ladder approximation to the two-particle scattering matrix in matter by using the Holinde-Erkelenz-Alzetta (HEA) and Machleidt-Holinde-Elster (Bonn) meson-exchange potentials. We obtain from these two equations of state maximum rotating star models having Keplerian frequencies of 1.19 x 10(3) s-1 (HEA) and 9.8 x 10(3) s-1 (Bonn) and gravitational masses of 2.51 and 2.25 M., respectively. However, from the study of the onset of instability modes caused by gravitational radiation and moderated by (shear) viscosity, performed for all four models and at two masses of 1.5 and 1.9 M., it follows that an m = 4 and/or m = 5 instability mode is probably responsible for limiting the angular velocity of rotating, hot neutron stars (and not the Keplerian frequency). To be more specific, we find that these instability modes are excited at frequencies of 60%-65% (depending on the equation of state as well as on the mass of the star) of the Keplerian value. The limiting frequency of an old neutron star being spun up and reheated up to temperature T almost-equal-to 10(7) K by mass accretion is set by the m = 3 mode. The related instability frequencies are 77%-92% and 80%-91% of the Keplerian value for the M = 1.5 M. and M = 1.9 M. star models, respectively. For T congruent-to 10(6) K the m = 2 mode is excited first; however, since its value is of comparable size with the Keplerian frequency, mass shedding can be expected to set the limit on stable neutron star rotation in this case. These findings have important implications for the possible future discovery of fast pulsars.