Compact Objects

The physics of compact stars is in itself extremely interesting. The features of these objects (forming during and following a supernova explosion) strongly dependent on the equation-of-state (EOS) adopted to describe the core. Its exact determination remains a formidable theoretical problem. There is still no general agreement on the exact composition of dense matter, and on its EOS, especially for densities in excess several times nuclear matter density. Although EOSs could be provided by high energy Nucleon-Nucleon scattering experiments, one has to extrapolate the results to extreme conditions of high density and high neutron-proton asymmetry, i.e. in a regime where the EOS is poorly constrained by nuclear data and experiments.

Neutrinos play an important role in the collapse of massive stars and in the evolution of neutron stars, which cool to temperatures in the MeV range in a time of order minutes. Thus a detailed knowledge of neutrino generation and scattering processes in dense matter with temperatures up to those at which neutron stars are formed, 20-50 MeV, is crucial for understanding the supernova phenomenon and how neutron stars cool. The problems are challenging ones since they require insights from nuclear, particle, and condensed matter physics, as well as from astrophysics. Chris Pethick at Nordita has made important contributions to this subject, most recently by demonstrating that creation of neutrino pairs by bremsstrahlung from electrons scattered by nuclei in the crust of a neutron star is strongly affected by the band structure of electrons due to the lattice of nuclei. Analysing observations of cooling neutron stars in the light of knowledge of neutrino processes holds the promise of putting constraints on the composition of matter in the interiors of neutron stars.