Horndeski stars

TL;DR

This paper demonstrates the existence of Horndeski stars, stable scalar field configurations in modified gravity theories, with potential astrophysical signatures that could distinguish them from traditional compact objects.

Contribution

It introduces time-dependent solitons in a Horndeski framework, exploring their properties, stability, and astrophysical implications, extending the concept of boson stars within a modified gravity context.

Findings

Existence of stable, finite-energy Horndeski star solutions.

Mass-radius relation and stability analysis performed.

Potential observable signatures like lensing and gravitational waves.

Abstract

We establish the existence of time-dependent solitons in a modified gravity framework, which is defined by the low energy limit of theories with a weakly broken galileon symmetry and a mass term. These are regular vacuum configurations of finite energy characterized by a single continuous parameter representing the amplitude of the scalar degree of freedom at the origin. When the central field amplitude is small the objects are indistinguishable from boson stars. In contrast, increasing the central value of the amplitude triggers the effect of higher derivative operators in the effective theory, leading to departures from the previous solutions, until the theory becomes strongly coupled and model-dependent. The higher order operators are part of the (beyond) Horndeski theory, hence the name of the compact objects. Moreover, a remnant of the galileon non-renormalization theorem guarantees that the existence and properties of these solutions are not affected by quantum corrections. Finally, we discuss the linear stability under small radial perturbations, the mass-radius relation, the compactness, the appearance of innermost stable circular orbits and photon spheres, and some astrophysical signatures (accretion disks, gravitational radiation and lensing) that may be relevant to falsify the model.