The Equation of State of Neutron-Rich Matter at Fourth Order of Chiral Effective Field Theory and the Radius of a Medium-Mass Neutron Star

TL;DR

This paper presents neutron star predictions based on chiral effective field theory equations of state, including up to fourth-order three-nucleon forces, to estimate the radius of a typical 1.4 solar mass neutron star.

Contribution

It extends ab initio nuclear force calculations to fourth order in chiral EFT and applies them to neutron star structure predictions, especially the star radius.

Findings

Predicted neutron star radius around 12 km for 1.4 solar masses.

Chiral EFT equations of state are consistent with recent observational constraints.

Higher-order three-nucleon forces significantly influence the equation of state.

Abstract

We report neutron star predictions based on our most recent equations of state. These are derived from chiral effective field theory, which allows for a systematic development of nuclear forces, order by order. We utilize high-quality two-nucleon interactions and include all three-nucleon forces up to fourth order in the chiral expansion. Our ab initio predictions are restricted to the domain of applicability of chiral effective field theory. However, stellar matter in the interior of neutron stars can be up to several times denser than normal nuclear matter at saturation, and its composition is essentially unknown. Following established practices, we extend our microscopic predictions to higher densities matching piecewise polytropes. The radius of the average-size neutron star, about 1.4 solar masses, is sensitive to the pressure at normal densities, and thus it is suitable to constrain ab initio theories of the equation of state. For this reason, we focus on the radius of medium-mass stars. We compare our results with other theoretical predictions and recent constraints.