EQUATION OF STATE OF NUCLEON MATTER AND NEUTRON-STAR STRUCTURE

Properties of dense nucleon matter and the structure of neutron stars are studied using variational chain summation methods and the new Argo nne vip two-nucleon interaction, which provides an excellent fit to al l of the nucleon-nucleon scattering data in the Nijmegen database. The neutron star gravitational mass limit obtained with this interaction is 1.67M.. Boost corrections to the two-nucleon interaction, which giv e the leading relativistic effect of order (upsilon/c)(2), as well as three-nucleon interactions, are also included in the nuclear Hamiltoni an. Their successive addition increases the mass limit to 1.80 and 2.2 0 M.. Hamiltonians including a three-nucleon interaction predict a tra nsition in neutron star matter to a phase with neutral pion condensati on at a baryon number density of similar to 0.2 fm(-3). Neutron stars predicted by these Hamiltonians have a layer with a thickness on the o rder of tens of meters, over which the density changes rapidly from th at of the normal to the condensed phase. The material in this thin lay er is a mixture of the two phases. We also investigate the possibility of dense nucleon matter having an admixture of quark matter, describe d using the bag model equation of state. Neutron stars of 1.4M. do not appear to have quark matter admixtures in their cores. However, the h eaviest stars are predicted to have cores consisting of a quark and nu cleon matter mixture. These admixtures reduce the maximum mass of neut ron stars from 2.20 to 2.02 (1.91) M. for bag constant B=200 (122) MeV /fm(3). Stars with pure quark matter in their cores are found to be un stable. We also consider the possibility that matter is maximally inco mpressible above an assumed density, and show that realistic models of nuclear forces limit the maximum mass of neutron stars to be below 2. 5M.. The effects of the phase transitions on the composition of neutro n star matter and its adiabatic index Gamma are discussed.