Highly compact neutron stars and screening mechanisms. I. Equilibrium and stability

TL;DR

This paper investigates how modified gravity theories with screening mechanisms can produce highly compact neutron stars with unscreened cores, revealing new stable solutions and scalarization effects in such dense objects.

Contribution

It demonstrates the existence of stable neutron star solutions with unscreened cores in scalar-tensor theories, expanding understanding of their phenomenology beyond trivial screening expectations.

Findings

Stable solutions with unscreened cores in chameleon models.

Discovery of scalarized equilibrium solutions in dilaton models.

Potential for observable deviations in neutron star properties.

Abstract

Modified theories of gravity that offer viable models for dark energy often rely on mechanisms that screen their effects in high density environments. From this perspective, it would appear that, once solar system constraints are satisfied, these theories would predict a trivial phenomenology for (much denser) neutron stars. In this work we explore the fact that in scalar-tensor theories the scalar degree of freedom does not couple to the mass density alone, but to the trace of the energy-momentum tensor - which can increase and eventually change sign as density and pressure build up in the core of neutron stars -, and investigate whether there could be a partial unscreening of the scalar field inside the most compact stars found in Nature. For this purpose, we construct neutron star solutions with realistic equations of state in theories with screening mechanisms and study their stability under radial perturbations. In particular, we find that stable solutions with a unscreened core can exist in chameleon models, while for the environmentally-dependent dilaton model a wealth of new, scalarized equilibrium solutions are found, some of which can be stable.