Axion contribution to the mass-radius relation of neutron stars

TL;DR

This paper investigates how axions, hypothetical particles, could influence the mass-radius relationship of neutron stars by affecting their internal structure, with implications depending on density and temperature conditions.

Contribution

It introduces an effective Lagrangian approach to study axion-neutron interactions and their impact on neutron star structure, considering trapping effects and causality constraints.

Findings

Axions can significantly affect neutron star mass under certain densities and temperatures.

Causality limits the regime where axion trapping influences neutron star structure.

Results are adaptable to specific axion models.

Abstract

An effective Lagrangian for the interaction between a pseudo-scalar (axion-like) field and massive fermions is considered. At high density and non-zero temperature axions can be produced through bremsstrahlung. If the axion-neutron interaction is greater than a certain value we can have a mean free path smaller than the size of a neutron star. The influence of such trapped axions on the mass-radius function of the neutron star is investigated by solving numerically the Tolman-Oppenheimer-Volkoff equations. We show that causality limits the applicability of the trapping regime. We find specific ranges of central densities and temperatures of a neutron star for which axions give a conspicuous contribution to the neutron-star mass. For other densities and temperatures axions do not have a significant effect on the structure of a neutron star. Since we use an effective approach our results can be easily applied to specific axion models.