Neutrino emission in neutron stars

Neutrino emissivities in a neutron star are computed for the neutrino bremsstrahlung process. In the first part, the electroweak nucleon-nucleon bremsstrahlung is calculated in free space in terms of an on-shell T matrix using a generalized low-energy theorem. In the second part, the emissivities are calculated in terms of the hadronic polarization at the two-loop level. Various medium effects, such as finite particle width, Pauli blocking in the T matrix are considered. Compared to the pioneering work of Friman and Maxwell in terms of (antisymmetrized) one-pion exchange, the resulting emissivity is about a factor 4 smaller at saturation density.