Roles of Hyperons in Neutron Stars

Abstract

We examine the roles the presence of hyperons in the cores of neutron stars may play in determining global properties of these stars. The study is based on estimates that hyperons appear in neutron star matter at about twice the nuclear saturation density, and emphasis is placed on effects that can be attributed to the general multi-species composition of the matter, hence being only weakly dependent on the specific modeling of strong interactions. Our analysis indicates that hyperon formation not only softens the equation of state but also severely constrains its values at high densities. Correspondingly, the valid range for the maximum neutron star mass is limited to about 1.5-1.8 $M_\odot$, which is a much narrower range than available when hyperon formation is ignored. Effects concerning neutron star radii and rotational evolution are suggested, and we demonstrate that the effect of hyperons on the equation of state allows a reconciliation of observed pulsar glitches with a low neutron star maximum mass. We discuss the effects hyperons may have on neutron star cooling rates, including recent results which indicate that hyperons may also couple to a superfluid state in high density matter. We compare nuclear matter to matter with hyperons and show that once hyperons accumulate in neutron star matter they reduce the likelihood of a meson condensate, but increase the susceptibility to baryon deconfinement, which could result in a mixed baryon-quark matter phase.