Hyperons, deconfinement, and the speed of sound in neutron stars

Abstract

The effects on the speed of sound in neutron stars due to the presence of hyperons and a phase transition to deconfined quark matter is investigated. For this purpose a composite description within the covariant field theory is used; it consists of different models for the hadronic and for the unbound quark configurations. A phase transition with continuous and monotonous variation of the equation of state is assumed. The predictions obtained are contrasted with recent observational data on isolated neutron stars as well as on binary systems. Only one candidate is finally obtained from six different descriptions. According to the present calculations, the onset of the hyperons causes the equilibrium speed of sound to exceed the conformal limit. Based on recent works on the propagation of g-modes in neutron stars, different definitions of the speed of sound are considered.