Exploring the limits of nucleonic metamodelling using different relativistic density functionals

TL;DR

This study investigates two classes of relativistic density functionals for neutron star modeling, revealing similar macroscopic predictions but differing in internal composition, emphasizing the need for non-equilibrium observations.

Contribution

It applies a metamodelling approach to relativistic density functionals, generating extensive model ensembles constrained by nuclear physics and astrophysics, to analyze their predictions and compositions.

Findings

Both models predict similar neutron star mass-radius relations.

Proton fractions differ significantly between models despite similar equations of state.

Composition information is masked in $eta$-equilibrium, requiring additional observations.

Abstract

In this work, we explore two classes of density dependent relativistic mean-field models, their predictions of proton fractions at high densities and neutron star structure. We have used a metamodelling approach to these relativistic density functionals. We have generated a large ensemble of models with these classes and then applied constraints from theoretical and experimental nuclear physics and astrophysical observations. We find that both models produce similar equations of state and neutron star mass-radius sequences. But, their underlying compositions, denoted by the proton fraction in this case, are vastly different. This reinstates previous findings that information on composition gets masqueraded in $\beta$-equilibrium. Additional observations of non-equilibrium phenomena are necessary to pin it down.