Angularly excited and interacting boson stars and Q-balls

TL;DR

This paper explores angularly excited and interacting boson stars and Q-balls in 3+1 dimensions, revealing new phenomena such as the absorption of non-rotating stars by rotating ones and potential instabilities linked to ergoregions.

Contribution

It introduces the first explicit examples of rotating boson stars interacting with non-rotating ones and studies their stability and energy properties in a gravitational setting.

Findings

Rotating boson stars tend to absorb non-rotating ones at small gravitational coupling.

Systems can become unstable due to ergoregion formation when interaction terms are large.

New phenomena differ from flat space-time behavior, highlighting gravity's role in soliton interactions.

Abstract

We study angularly excited as well as interacting non-topological solitons, so-called Q-balls and their gravitating counterparts, so-called boson stars in 3+1 dimensions. Q-balls and boson stars carry a non-vanishing Noether charge and arise as solutions of complex scalar field models in a flat space-time background and coupled minimally to gravity, respectively. We present examples of interacting Q-balls that arise due to angular excitations, which are closely related to the spherical harmonics. We also construct explicit examples of rotating boson stars that interact with non-rotating boson stars. We observe that rotating boson stars tend to absorb the non-rotating ones for increasing, but reasonably small gravitational coupling. This is a new phenomenon as compared to the flat space-time limit and is related to the negative contribution of the rotation term to the energy density of the solutions. In addition, our results indicate that a system of a rotating and non-rotating boson star can become unstable if the direct interaction term in the potential is large enough. This instability is related to the appearance of ergoregions.