Hyperons in neutron-star cores and two-solar-mass pulsar

TL;DR

This paper develops a non-linear relativistic mean field model that incorporates hyperons and meson interactions, successfully explaining the existence of two-solar-mass neutron stars despite hyperon presence.

Contribution

It introduces a new RMF model with quartic meson terms and hidden-strangeness mesons, compatible with hypernuclear data and supporting massive neutron stars with hyperon cores.

Findings

The EOS with hyperons can support neutron stars over 2 solar masses.

Quartic meson terms stiffen the EOS at high densities.

Model aligns with semi-empirical nuclear and hypernuclear data.

Abstract

Recent measurement of mass of PSR J1614-2230 rules out most of existing models of equation of state (EOS) of dense matter with high-density softening due to hyperonization or a phase transition to quark matter or a boson condensate. We look for a solution of an apparent contradiction between the consequences stemming from up-to-date hypernuclear data, indicating appearance of hyperons at 3 nuclear densities and existence of a two-solar-mass neutron star. We consider a non-linear relativistic mean field (RMF) model involving baryon octet coupled to meson fields. An effective lagrangian includes quartic terms in the meson fields. The values of the parameters of the model are obtained by fitting semi-empirical parameters of nuclear matter at the saturation point, as well as potential wells for hyperons in nuclear matter and the strength of the Lambda-Lambda attraction in double-Lambda hypernuclei. We propose a non-linear RMF model which is consistent with up-to-date semiempirical nuclear and hypernuclear data and allows for neutron stars with hyperon cores and M larger than 2 solar masses. The model involves hidden-strangenes scalar and vector mesons, coupled to hyperons only, and quartic terms involving vector meson fields. Our EOS involving hyperons is stiffer than the corresponding nucleonic EOS (with hyperons artificially suppressed) above five nuclear densities. Required stiffening is generated by the quartic terms involving hidden-strangeness vector meson.