An analysis of Born-Infeld boson compact objects

TL;DR

This paper investigates Born-Infeld scalar field-based boson stars, revealing their potential for highly compact, horizon-encased structures with unique properties and complex solution branches, differing significantly from canonical boson stars.

Contribution

It provides a detailed analysis of the structure, junction conditions, and boundary conditions of Born-Infeld boson stars, uncovering their rich and peculiar solution space.

Findings

Existence of multiple solution branches depending on junction conditions

Objects can have surface compactness arbitrarily close to the black hole limit

Interior geometries resemble 'bag of gold' spacetimes and may hide behind event horizons

Abstract

In this paper we study gravitationally bound compact objects sourced by a string theory inspired Born-Infeld scalar field. Unlike many of their canonical scalar field counterparts, these ``boson stars'' do not have to extend out to infinity and may generate compact bodies. We analyze in detail both the junction conditions at the surface as well as the boundary conditions at the center which are required in order to have a smooth structure throughout the object and into the exterior vacuum region. These junction conditions, although involved, turn out to be relatively easy to satisfy. Analysis reveals that these compact objects have a richer structure than the canonical boson stars and some of these properties turn out to be physically peculiar: There are several branches of solutions depending on how the junction conditions are realized. Further analysis illustrates that in practice the junction conditions tend to require interior geometries reminiscent of ``bag of gold'' spacetimes, and also hide the star behind an event horizon in its exterior. The surface compactness of such objects, defined here as the ratio $2M/r$, can be made arbitrarily close to unity indicating the absence of a Buchdahl bound. Some comments on the stability of these objects is provided to find possible stable and unstable regimes. However, we argue that even in the possibly stable regime the event horizon in the vacuum region shielding the object is potentially unstable, and would cut off the star from the rest of the universe.