Axion effects on quark matter and quark-matter cores in massive hybrid stars

TL;DR

This study explores how axions influence quark matter properties and the structure of hybrid stars, revealing that axions can enlarge quark-matter cores in massive neutron stars despite softening the overall equation of state.

Contribution

It introduces a model incorporating axion effects into quark matter and hybrid star structure, showing how axions affect phase transitions and core sizes in neutron stars.

Findings

Axions cause periodic variations in quark matter properties.

Axions lower the density threshold for the hadron-quark transition.

Axions increase the size of quark-matter cores in massive hybrid stars.

Abstract

Using a three-flavor Nambu--Jona-Lasinio model to describe the charge-parity violating effects through axion field, we investigate the axion effects on quark matter and quark-matter cores in massive hybrid stars. The properties of quark matter vary with the scaled axion field $a/f_a$ in a periodic manner, with a period of $2\pi$. Within the range from 0 to $\pi $, axion field decrease the baryon chemical potential of the first-order phase transition, leading to an increase in normalized pressure and stiffening of the quark matter equation of state. The effect of axions on hybrid star matter that includes the hadron-quark phase transition is contrary to expectations. The axion field shifts the onset of the hadron-quark mixed phase to lower densities but slightly softens the equation of state of the mixed phase matter, which also results in a slight decrease in the maximum mass and corresponding radius of the hybrid stars. However, we also find that the lowering of the onset of the mixed phase significantly increases the radius and mass of the quark-matter core in the hybrid star. Therefore, our results indicate with axion effects, a sizable quark-matter core can appear in $2M_{\odot}$ massive neutron stars.