Quarkyonic stars with isospin-flavor asymmetry

TL;DR

This paper extends the quarkyonic model to isospin asymmetric matter, enabling the construction of quarkyonic stars with unique properties such as larger size, higher maximum mass, and lower proton fraction, impacting neutron star cooling.

Contribution

It introduces a novel extension of the quarkyonic model to isospin asymmetric matter, incorporating $eta$-equilibrium and isospin-flavor asymmetry, and applies it to construct quarkyonic star models.

Findings

Quarkyonic stars are larger and have higher maximum mass than neutron stars.

The model predicts a bump in the sound speed within quarkyonic stars.

Lower proton fractions at $eta$-equilibrium may suppress fast cooling.

Abstract

We suggest an extension to isospin asymmetric matter of the quarkyonic model from McLerran and Reddy. This extension allows us to construct the $\beta$-equilibrium between quarks, nucleons and leptons. The concept of the quarkyonic matter originates from the large number of color limit for which nucleons are the correct degrees of freedom near the Fermi surface -- reflecting the confining forces -- while deep inside the Fermi sea quarks naturally appear. In isospin asymmetric matter, we suggest that this new concept can be implemented within a global isoscalar relation between the shell gaps differentiating the nucleon and the quark sectors. In addition, we impose the conservation of the isospin-flavor asymmetry in the nucleon and the quark phases. Within this model, several quarkyonic stars are constructed on top of the SLy4 model for the nucleon sector, producing a bump in the sound speed, which implies that quarkyonic stars are systematically bigger and have a larger maximum mass than the associated neutron stars. They also predict lower proton fraction at $\beta$-equilibrium, which potentially quenches fast cooling in massive compact stars.