HYDRODYNAMIC MOTIONS AND NEUTRINO EMISSIVITY OF NEUTRON-STARS

The equilibrium number densities of species in particles in the neutro n star core depend on the local density. Hydrodynamic motions, if they exist in the core, can deviate the chemical composition of matter fro m equilibrium, since the relaxation of a moving fluid element towards the new equilibrium requires a finite time. Thus, the moving core plas ma is not entirely in chemical equilibrium, and the departure from equ ilibrium can substantially increase the rate of neutrino energy loss. In the frame of the simplest model, we examine the influence of a hydr odynamic flow on the neutrino emissivity and show that the latter may be enhanced by many orders of magnitude even for a relatively slow mot ion. The neutrino energy loss rate is calculated for a wide range of t he velocity of flow. The effect of an enhanced neutrino emission on co oling is considered for different neutron star models. Cooling turns o ut to be extremely sensitive to the velocity of flow. Owing to this, a study of the thermal evolution of neutron stars may be a powerful dia gnostic of their internal hydrodynamics. Thus, available observational data on the surface temperature are consistent with the theoretical m odels only if there are no motions in the core with velocity V greater than or equal to 10(-9) cm s(-1).