The Enhancon and the Consistency of Excision

TL;DR

The paper demonstrates that the enhançon mechanism, which eliminates a class of repulson singularities by excising the interior with a shell of branes, is consistent at the level of classical gravity through explicit junction-condition analysis. It shows that the shell’s stress-energy and field sources precisely match those of wrapped D6-branes, with the brane tension vanishing at the enhançon radius, and extends the construction to include D2-branes and non-extremal generalizations that can harbor horizons. The non-extremal analysis reveals a rich interior structure with potential bound states and horizons, though some parameters (e.g., the interior non-extremality scale $r_0'$) remain undetermined within pure supergravity, indicating a need for deeper microscopic treatment of the thermal shell dynamics. Overall, the work clarifies how stringy phenomena enforce a consistent gravitational excision and outlines directions for further understanding the interior physics and thermodynamics of enhançon-like configurations.

Abstract

The enhancon mechanism removes a family of time-like singularities from certain supergravity spacetimes by forming a shell of branes on which the exterior geometry terminates. The problematic interior geometry is replaced by a new spacetime, which in the prototype extremal case is simply flat. We show that this excision process, made inevitable by stringy phenomena such as enhanced gauge symmetry and the vanishing of certain D-branes' tension at the shell, is also consistent at the purely gravitational level. The source introduced at the excision surface between the interior and exterior geometries behaves exactly as a shell of wrapped D6-branes, and in particular, the tension vanishes at precisely the enhancon radius. These observations can be generalised, and we present the case for non-extremal generalisations of the geometry, showing that the procedure allows for the possibility that the interior geometry contains an horizon. Further knowledge of the dynamics of the enhancon shell itself is needed to determine the precise position of the horizon, and to uncover a complete physical interpretation of the solutions.

Figures (3)

• Figure 1: The effective potential for increasing amounts of non--extremality, $r_0$. The top curve is the extremal case.
• Figure 2: Some special radii for increasing non--extremality, $r_0$, in the case $M=0$. The solid curves are as follows: The enhançon radius is top, the repulson radius is bottom, and the minimal radius for which excision would remove all repulsiveness, ${\hat{r}}_{\rm d}$, (which also deserves a name) is the middle curve. Notice that the latter two coalesce into the would--be horizon (dotted line at very bottom) at large $r_0$.
• Figure 3: The two branches (solid lines) of supergravity solutions for varying values of the ratio $M/N$. There is a crucial discontinuity at $N=M$. (See text.)