Dense baryonic matter: Constraints from recent neutron star observations

Updated constraints from neutron star masses and radii impose stronger restrictions on the equation of state for baryonic matter at high densities and low temperatures. The existence of 2M(circle dot) neutron stars rules out many soft equations of state with prominent exotic compositions. The present work reviews the conditions required for the pressure as a function of baryon density to satisfy these constraints. Several scenarios for sufficiently stiff equations of state are evaluated. The common starting point is a realistic description of both nuclear and neutron matter based on a chiral effective field theory approach to the nuclear many-body problem. Possible forms of hybrid matter featuring a quark core in the center of the star are discussed using a three-flavor Polyakov-Nambu-Jona-Lasinio model. It is found that a conventional equation of state based on nuclear chiral dynamics meets the astrophysical constraints. Hybrid matter generally turns out to be too soft unless additional strongly repulsive correlations, e.g., through vector current interactions between quarks, are introduced. The extent to which strangeness can accumulate in the equation of state is also discussed.