The Constant-Sound-Speed parameterization for NJL models of quark matter in hybrid stars

TL;DR

This paper investigates hybrid stars with quark cores modeled by NJL models, showing that high transition pressures and strong first-order transitions limit the existence of stable hybrid stars.

Contribution

It introduces the Constant-Sound-Speed parameterization to analyze the impact of quark matter transitions on hybrid star stability in NJL models.

Findings

High transition pressures hinder stable hybrid star formation.

Non-local NJL models tend to destabilize stars immediately.

Local NJL models produce very short hybrid star branches.

Abstract

The discovery of pulsars as heavy as 2 solar masses has led astrophysicists to rethink the core compositions of neutron stars, ruling out many models for the nuclear equations of state (EoS). We explore the hybrid stars that occur when hadronic matter is treated in a relativistic mean-field approximation and quark matter is modeled by three-flavor local and non-local Nambu Jona-Lasinio (NJL) models with repulsive vector interactions. The NJL models typically yield equations of state that feature a first order transition to quark matter. Assuming that the quark-hadron surface tension is high enough to disfavour mixed phases, and restricting to EoSes that allow stars to reach 2 solar masses, we find that the appearance of the quark matter core either destabilizes the star immediately (this is typical for non-local NJL models) or leads to a very short hybrid star branch in the mass-radius relation (this is typical for local NJL models). Using the Constant-Sound-Speed parametrization we can see that the reason for the near-absence of hybrid stars is that the transition pressure is fairly high and the transition is strongly first order.