Dense Matter and Neutron Stars: Some Basic Notions

TL;DR

This paper reviews fundamental properties of dense matter in neutron stars, including phase transitions, nuclear structures, and superfluidity, providing a semi-quantitative understanding of their physical behavior.

Contribution

It offers a comprehensive overview of dense matter properties in neutron stars, emphasizing simple physical arguments and recent insights into superfluidity and phase transitions.

Findings

Identification of key densities for phase transitions in neutron star matter

Estimation of pairing gaps and superfluid densities in dense nuclear matter

Discussion of the structure and properties of pasta phases in neutron star crusts

Abstract

A number of properties of dense matter can be understood semiquantitatively in terms of simple physical arguments. We begin with the outer parts of neutron stars, and consider the density at which pressure ionization occurs, the density at which electrons become relativistic, the density at which neutrons drip out of nuclei, and the size of the equilibrium nucleus in dense matter. Subsequently, we treat the so-called "pasta" phases expected to occur at densities just below the density at which the transition from the crust to the liquid core of a neutron star occurs. We then consider aspects of superfluidity in dense matter. Estimates of pairing gaps in homogeneous nuclear matter are given, and the effect of the dense medium on the interaction between nucleons is described. Finally, we turn to superfluidity in the crust of neutron stars and especially the neutron superfluid density, an important quantity in the theory of sudden speedups of the rotation rate of some pulsars.