Bulk viscosity in superfluid neutron star cores. II. Modified Urca processes in $npe\mu$ matter

摘要

We study the bulk viscosity in neutron star cores due to modified Urcaprocesses involving nucleons, electrons and muons and analize its reduction bysinglet-state or triplet-state superfluidity of nucleons. In combination withthe results of our previous paper on the bulk viscosity due to direct Urcaprocesses, a realistic description of the bulk viscosity in superfluid neutronstar cores is obtained. Switching off direct Urca processes with decreasingdensity in a nonsuperfluid matter lowers the bulk viscosity by 3-5 orders ofmagnitude. The presense of muons opens additional source of bulk viscosity dueto muon Urca processes and lowers the threshold density of the electron directUrca process. The superfluidity may strongly reduce the bulk viscosity andaffect thus damping of neutron star vibrations.