Hyperons, deconfinement and the speed of sound in neutron stars

TL;DR

This paper investigates how hyperons and deconfined quark matter affect the speed of sound in neutron stars, using a combined hadron-quark model, and compares predictions with observational data, finding hyperons increase the sound speed beyond the conformal limit.

Contribution

It introduces a composite covariant model to study the impact of hyperons and phase transitions on neutron star properties, highlighting the conditions for the speed of sound exceeding the conformal limit.

Findings

Hyperons cause the speed of sound to surpass the conformal limit.

Only one out of six models matches observational data.

The results align qualitatively with neutron star oscillation studies.

Abstract

The effects of the presence of hyperons and a phase transition to deconfined quark matter on the speed of sound in neutron stars is investigated. For this purpose a composite description consisting of a model of the covariant field theory of hadrons and one for unbound quarks are used. A phase transition with continuous and monotonous variation of the equation of state is assumed. The predictions 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. Qualitative agreement with recent work about the influence of the speed of sound on the g-modes of oscillation in neutron stars is obtained.