BULK VISCOSITY OF NEUTRON-STARS

Viscosity of neutron stars has been a continuing area of research for many years now. Recently interest in this field has revived because of the possibility of URCA processes in neutron stars. In this paper we report calculation of the bulk viscosity of neutron stars from these p rocesses. For this purpose we have used the beta-decay rates which wer e calculated without making the usual approximations of neglecting the neutrino momentum and using the nuclear mean field theory for the des cription of interacting nuclear matter. Also we have not restricted ou r calculation to the linear regime which corresponds to the assumption that fluctuations in the chemical potential away from beta-equilibriu m remain small: Delta mu/kT much less than 1. We find that for large a mplitude fluctuations, where the linear approximation is not valid, bu lk viscosity increases by many orders of magnitude. Also, as against s trange matter stars, where the viscosity first increases with increasi ng temperature and then starts decreasing beyond 0.1 MeV, we find that the viscosity increases uniformly with temperature at least up to 2 M eV. We discuss the implications of these results for the stability of neutron stars.