Relativistic stars in vector-tensor theories

TL;DR

This paper investigates how second-order generalized Proca theories with derivative couplings affect the structure and stability of relativistic stars, revealing increased mass and radius under certain conditions and identifying solutions with no deviation from general relativity.

Contribution

It provides new insights into relativistic star solutions within vector-tensor theories, especially regarding the effects of derivative couplings on star properties and stability.

Findings

Stars with cubic and quartic derivative couplings have larger mass and radius than in GR.

Negative derivative coupling constants lead to increased star size.

Intrinsic vector-mode couplings can produce GR-like solutions with no modifications.

Abstract

We study relativistic star solutions in second-order generalized Proca theories characterized by a $U(1)$-breaking vector field with derivative couplings. In the models with cubic and quartic derivative coupling, the mass and radius of stars become larger than those in general relativity for negative derivative coupling constants. This phenomenon is mostly attributed to the increase of star radius induced by a slower decrease of the matter pressure compared to general relativity. There is a tendency that the relativistic star with a smaller mass is not gravitationally bound for a low central density and hence dynamically unstable, but that with a larger mass is gravitationally bound. On the other hand, we show that the intrinsic vector-mode couplings give rise to general relativistic solutions with a trivial field profile, so the mass and radius are not modified from those in general relativity.