Hyperons in neutron-star cores: confronting maximum mass observational constraint of ${\boldsymbol 2{M}_{\bf\odot}} $

Abstract：

We employ a comprehensive set of relativistic mean-field (RMF) models to investigate the role of hyperons (Λ, $ \Sigma^{\pm,0} $, and $ \Xi^{-,0} $) in dense nuclear matter. We consider various RMF models that span a wide range of high-density behaviors of equations of state (EoSs), symmetry energy coefficients, and hyperon-meson coupling schemes. Our aim is to assess how the inclusion of hyperons in nucleonic matter influences the key neutron star properties, including the maximum mass ( M$ _{\rm max} $), stellar radius ( R$ _{\rm max} $), and tidal deformability ($ \Lambda_{\rm max} $). By varying the vector meson-hyperon coupling strength ($ X_{\omega Y} $) over a wide range and considering the SU(6) symmetry, we find that a decrease in $ X_{\omega Y} $ results in an increased hyperon population. This leads to a significant softening of the EoS and a reduction in the maximum mass of a neutron star. The models with strong vector repulsion (larger value of $ X_{\omega Y} $) show a dominance of Λand $ \Xi^- $ hyperons, with $ \Xi^0 $ appearing only at higher densities. The neutron star properties, such as M$ _{\rm max} $, R$ _{\rm max} $, and $ \Lambda_{\rm max} $, are strongly affected by the hyperonization for all RMF models. It is observed that the canonical star properties like R$ _{1.4} $ and $ \Lambda_{1.4} $ remain largely unaffected by the presence of hyperons in nucleonic EoSs under fixed vector coupling strengths, except when couplings are based on SU(6) symmetry. This behavior can be attributed to the fact that, although hyperons appear in the very centre of a 1.4 M$ _{\odot} $ star, their population fraction is extremely small and therefore has a negligible effect on global stellar properties like R$ _{1.4} $ and $ \Lambda_{1.4} $. Furthermore, to support a star with observational constraint of M$ _{\rm max} $ $ \ge $ 2 M$ _{\odot} $, the vector coupling strength X$ _{\omega Y} $ must lie in the range 0.8−0.9. Our results highlight the critical role of vector coupling strength in governing hyperonization and its impact on neutron star observables. It is found that increasing X$ _{\omega Y} $ improves compliance with the 2 M$ _{\odot} $ mass constraint by suppressing early hyperonization. The critical role of the slope of symmetry energy ( L) in regulating the impact of hyperonization on neutron star observables is also studied.