Rotating neutron stars with exotic cores: masses, radii, stability

TL;DR

This paper reviews theoretical models of rotating neutron stars with exotic cores, analyzing their mass-radius relations, stability limits, and observational constraints, highlighting the complexities of their internal structure and rotational stability.

Contribution

It provides a comprehensive review of mass-radius relations, stability criteria, and the effects of exotic cores in rotating neutron stars across various dense matter theories.

Findings

Mass-radius relations vary with exotic core composition.

Rotation limits are constrained by mass-shedding and instability thresholds.

Core phase transitions can lead to metastability and star quakes.

Abstract

A set of theoretical mass-radius relations for rigidly rotating neutron stars with exotic cores, obtained in various theories of dense matter, is reviewed. Two basic observational constraints are used: the largest measured rotation frequency (716 Hz) and the maximum measured mass ($2\;M_\odot$). Present status of measuring the radii of neutron stars is described. The theory of rigidly rotating stars in general relativity is reviewed and limitations of the slow rotation approximation are pointed out. Mass-radius relations for rotating neutron stars with hyperon and quark cores are illustrated using several models. Problems related to the non-uniqueness of the crust-core matching are mentioned. Limits on rigid rotation resulting from the mass-shedding instability and the instability with respect to the axisymmetric perturbations are summarized. The problem of instabilities and of the back-bending phenomenon are discussed in detail. Metastability and instability of a neutron star core in the case of a first-order phase transition, both between pure phases, and into a mixed-phase state, are reviewed. The case of two disjoint families (branches) of rotating neutron stars is discussed and generic features of neutron-star families and of core-quakes triggered by the instabilities are considered.