Sound velocity, equation of state and strangeness in neutron star matter

TL;DR

This paper reviews the speed of sound and equation of state in neutron star matter, emphasizing the constraints from observations and exploring the role of strangeness and hyperons in resolving the hyperon puzzle.

Contribution

It provides a comprehensive review of sound velocity constraints and discusses the impact of hyperon-nuclear forces on neutron star composition and the hyperon puzzle.

Findings

Neutron star observations imply a very stiff equation of state.

Phase transitions in neutron star cores are tightly constrained.

Repulsive hyperon-nuclear three-body forces may resolve the hyperon puzzle.

Abstract

This presentation in two parts starts with a brief review on the speed of sound in neutron star matter as inferred from observational data. It is pointed out that, in view the strong constraints imposed by observed properties of the heaviest neutron stars, the equation of state must be very stiff. Stringent limits can be set on the possible appearance of phase transitions in neutron star cores. The second part discusses issues concerning the occurance of strangeness degrees of freedom, in particular of hyperons, in neutron stars. Here a focus is on the role of repulsive hyperon-nuclear three-body forces, potentially resolving the so-called hyperon puzzle.