Hyperon effects in covariant density functional theory and recent astrophysical observations

Abstract

Motivated by recent observational data, the equations of state with the inclusion of strangeness-bearing $\ensuremath{\Lambda}$ hyperons and the corresponding properties of neutron stars are studied based on the covariant density functional (CDF) theory. To this end, we specifically employ the density-dependent relativistic Hartree-Fock (DDRHF) theory and the relativistic mean field (RMF) theory. The inclusion of $\ensuremath{\Lambda}$ hyperons in neutron stars shows substantial effects in softening the equation of state. Because of the extra suppression effect originating from the Fock channel, large reductions on both the star mass and radius are predicted by the DDRHF calculations. It is also found that the mass-radius relations of neutron stars with $\ensuremath{\Lambda}$ hyperons determined by DDRHF with the PKA1 parameter set are in fairly good agreement with the observational data, where a relatively small neutron-star radius is required. Therefore, it is expected that the exotic degrees of freedom such as the strangeness-bearing structure may appear and play significant roles inside the neutron stars, which is supported further by the systematical investigations on the consistency between the maximum neutron-star mass and $\ensuremath{\Lambda}$-coupling strength.