Reconciliation of neutron-star masses and binding of the Λ in hypernuclei

Abstract

By relating scalar and vector couplings of the hyperons to the inferred empirical binding of the \ensuremath{\Lambda} hyperon in saturated nuclear matter, we obtain compatibility of this binding energy with neutron-star masses. Use of the observational constraint on the lower bound on the maximum neutron-star mass and the upper bound of the couplings that are compatible with hypernuclear levels, places bounds on the reduction in neutron-star mass that hyperons produce. For the best current estimate of nuclear-matter properties, the reduction in mass due to conversion of nucleons to hyperons is (0.71\ifmmode\pm\else\textpm\fi{}0.15)${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$. Neutrons comprise a slight majority population \ensuremath{\sim}${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$ neutron stars with mass \ensuremath{\sim}1.5${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$.