Neutron Star Structure and the Equation of State

The structure of neutron stars is considered from theoretical andobservational perspectives. We demonstrate an important aspect of neutron starstructure: the neutron star radius is primarily determined by the behavior ofthe pressure of matter in the vicinity of nuclear matter equilibrium density.In the event that extreme softening does not occur at these densities, theradius is virtually independent of the mass and is determined by the magnitudeof the pressure. For equations of state with extreme softening, or those thatare self-bound, the radius is more sensitive to the mass. Our results show thatin the absence of extreme softening, a measurement of the radius of a neutronstar more accurate than about 1 km will usefully constrain the equation ofstate. We also show that the pressure near nuclear matter density is primarilya function of the density dependence of the nuclear symmetry energy, while thenuclear incompressibility and skewness parameters play secondary roles. In addition, we show that the moment of inertia and the binding energy ofneutron stars, for a large class of equations of state, are nearly universalfunctions of the star's compactness. These features can be understood byconsidering two analytic, yet realistic, solutions of Einstein's equations,due, respectively, to Buchdahl and Tolman. We deduce useful approximations forthe fraction of the moment of inertia residing in the crust, which is afunction of the stellar compactness and, in addition, the presssure at thecore-crust interface.Comment: 51 pages, 10 figures, submitted to Ap