Charged Scalar-Tensor Boson Stars: Equilibrium, Stability and Evolution

TL;DR

This paper investigates charged boson stars within scalar-tensor theories, analyzing their stability, maximum charge-to-mass ratios, and evolution, revealing how scalarization and conformal frame choices impact their physical properties.

Contribution

It provides the first detailed analysis of charged boson stars in scalar-tensor theories, including stability limits, numerical solutions, and cosmological evolution insights.

Findings

Maximum charge-to-mass ratio can exceed GR limits in scalar-tensor theories.

Charge limit decreases with increasing central boson density.

Star's energy and density decrease over cosmological time.

Abstract

We study charged boson stars in scalar-tensor (ST) gravitational theories. We analyse the weak field limit of the solutions and analytically show that there is a maximum charge to mass ratio for the bosons above which the weak field solutions are not stable. This charge limit can be greater than the GR limit for a wide class of ST theories. We numerically investigate strong field solutions in both the Brans Dicke and power law ST theories. We find that the charge limit decreases with increasing central boson density. We discuss the gravitational evolution of charged and uncharged boson stars in a cosmological setting and show how, at any point in its evolution, the physical properties of the star may be calculated by a rescaling of a solution whose asymptotic value of the scalar field is equal to its initial asymptotic value. We focus on evolution in which the particle number of the star is conserved and we find that the energy and central density of the star decreases as the cosmological time increases. We also analyse the appearance of the scalarization phenomenon recently discovered for neutron stars configurations and, finally, we give a short discussion on how making the correct choice of mass influences the argument over which conformal frame, the Einstein frame or the Jordan frame, is physical.