Exotic neutron star equations of state

Abstract

Neutron stars are dense stellar remnants formed out of the core collapse of giant stars in a supernovae. Due to the inferred extreme densities the neutron star interior is assumed to consist of dense nuclear matter, thus provide a laboratory to study matter at high densities. In this work relativistic mean field (RMF) theory is applied to a relativistic description of nuclear matter known as Quantum Hadrodynamics (QHD), or the σ−ω model, where the strong nuclear force is described by the exchange of mesons. Parameters sets in QHD are defined by the meson-baryon coupling constants: QHD-I is an introductory parameter set whilst NL3 is a more advanced version including more mesons and additional interactions. These QHD parameter sets were then expanded to include the neutral lambda hyperon, as a first approximation of hyperonic matter. RMF is applied as an approximation to calculate the bulk properties of the nuclear matter and to study the neutron star equation of state (EoS). The EoS for pure neutron matter model as well as beta-equilibrated, charge neutral matter model were compared to the EoS of beta-equilibrated, charge neutral matter with lambda hyperons. These were then used to compare and contrast the influence on the neutron star mass-radius relationship. Comparing the calculated results to observational data, the presence of the lambda hyperon in the neutron star cannot be ruled out. However, a more complete model would be to include the full baryon octet in the model for dense nuclear matter.