Dense hadronic matter in neutron stars

TL;DR

This paper explores the possibility of two distinct families of compact stars—hadronic and quark stars—based on the hadronic equation of state, delta isobars, and recent nuclear matter experiments, reconciling observational constraints.

Contribution

It proposes a dual-star model with distinct compositions and highlights the role of delta isobars and recent experimental data in understanding neutron star matter.

Findings

Existence of two star families: hadronic and quark stars.

Delta isobars influence the hadronic equation of state and star properties.

Recent nuclear experiments support early delta isobar appearance.

Abstract

The existence of stars with masses up to $2 M_{\odot}$ and the hints of the existence of stars with radii smaller than $\sim 11$ km seem to require, at the same time, a stiff and a soft hadronic equation of state at large densities. We argue that these two apparently contradicting constraints are actually an indication of the existence of two families of compact stars: hadronic stars which could be very compact and quark stars which could be very massive. In this respect, a crucial role is played, in the hadronic equation of state, by the delta isobars whose early appearance shifts to large densities the formation of hyperons. We also discuss how recent experimental information on the symmetry energy of nuclear matter at saturation indicate, indirectly, an early appearance of delta isobars in neutron star matter.