Neutron star structure: theory, observation, and speculation.

The broad physical aspects of the neutron-neutron interaction in dense matter are reviewed, and an examination is made of the extent to which the equation of state of neutron star matter is influenced by the various phase transitions which have been proposed for the high-density regime. The dependence of the maximum neutron star mass and the stellar structure on the neutronneutron interaction is studied through calculations of the equation of state of neutron matter based on four different models for this interaction: the Reid (R) and Bethe-Johnson (BJ) models which are based on the phenomenological potentials suggested by these authors, a tensor-interaction (TI) model which assumes that the attraction between nucleons comes from the higher order contribution of the pion-exchange tensor interaction, and a mean field (MF) model which assumes that all the attraction between nucleons is due to the exchange of an effective scalar meson. It is shown that the harder equations of state which result from the BJ, TI, and MF models give rise to significant modifications in the structure of neutron stars; heavy neutron stars (> 1 M0) have both larger radii and thicker crusts than were predicted using the R model. These stars are then used as a basis for comparing theory with observation for the mass and structure of neutron stars such as the Crab and Vela pulsars, and the compact X-ray sources Her X-1 and Vela X-1. We find that both theory and observation tend to favor an equation of state that is stiff in the region of g and that a neutron star such as Her X-1 (M 1.3 M0) has a radius of the order of 15 km with a crust thickness of order 5 km. On the basis of starquake theory, it is concluded that the Crab pulsar could have a mass as large as 1.3 M0, with a critical strain angle 10- , comparable to that suggested for Her X- 1. The possibility of solid-core neutron stars is discussed together with some of their observational consequences. Subject headings: dense matter - equation of state - nuclear reactions - pulsars - stars: interiors - stars: neutron