Minimal boson stars in 5 dimensions: classical instability and existence of ergoregions

TL;DR

This paper demonstrates that minimal boson stars in five dimensions are inherently classically unstable, contrasting with four-dimensional cases, and explores the conditions under which ergoregions form in rotating boson stars within Einstein and Gauss-Bonnet gravity.

Contribution

It establishes the classical instability of minimal boson stars in 5D across various gravity theories and analyzes ergoregion formation in rotating cases.

Findings

Minimal boson stars in 5D are always classically unstable.

Ergoregions in rotating boson stars appear only within specific parameter ranges.

Gauss-Bonnet gravity influences the near-axis behavior of rotating solutions.

Abstract

We show that minimal boson stars, i.e. boson stars made out of scalar fields without self-interaction, are always classically unstable in 5 space-time dimensions. This is true for the non-rotating as well as rotating case with two equal angular momenta and in both Einstein and Gauss-Bonnet gravity, respectively, and contrasts with the 4-dimensional case, where classically stable minimal boson stars exist. We also discuss the appearance of ergoregions for rotating boson stars with two equal angular momenta. While rotating black holes typically possess an ergoregion, rotating compact objects without horizons such as boson stars have ergoregions only in a limited range of the parameter space. In this paper, we show for which values of the parameters these ergoregions appear and compare this with the case of standard Einstein gravity. We also point out that the interplay between Gauss-Bonnet gravity and rotation puts constraints on the behaviour of the space-time close to the rotation axis.