Equation-of-State Independent Relations in Rapidly Rotating Hybrid Stars

TL;DR

This study demonstrates that certain universal relations like I-Love-Q and I-C-Q hold approximately across a wide range of equations of state, including hybrid and rapidly rotating neutron stars, with some deviations.

Contribution

It extends the understanding of EoS-insensitive relations to include hybrid stars with exotic matter and rapid rotation, confirming their robustness and limitations.

Findings

Universal relations are approximately EoS-insensitive within 10-20%.

Relations hold for both slowly and rapidly rotating neutron stars.

Applicability extends to stars with exotic core compositions and low masses.

Abstract

We study 22 hadronic and 100 hybrid equations of state (EoSs) that allow for heavy baryons and deconfined quark matter at high densities, aiming to establish quasi-universal relations for both slowly and rapidly rotating neutron stars. All EoSs are consistent with current observational constraints, including NICER and GW170817. Our results confirm that the I-Love-Q and I-C-Q relations remain approximately EoS-insensitive across this broad EoS set, with deviations typically within 10\% for hadronic stars and up to 20\% for those with complex core compositions. These relations are extended to include low-mass neutron stars such as HESS J1731-347, and to stars with general core compositions--nucleonic, hyperonic (full baryon octet), and quark matter. The analysis underscores both the robustness and the limitations of universal relations when applied to compact stars with exotic degrees of freedom and rapid rotation.