Orientifolds, M theory, and the $\mathrm{ABCD}$’s of the enhançon

Supergravity solutions related to large $N\mathrm{SU}\left(N\right)\mathrm{}$ pure gauge theories with eight supercharges have recently been shown to give rise to an “enhançon,” a new type of hypersurface made of D-branes. We show that enhançons also arise in similar situations pertaining to $\mathrm{SO}(2N+1),$ $\mathrm{USp}\left(2N\right)\mathrm{}$ and $\mathrm{SO}\left(2N\right)\mathrm{}$ gauge theories, using orientifolds. Enhançons therefore appear to come in types A, B, C, and D. The latter three differ globally from type A by having an extra ${\mathbf{Z}}_2$ identification, and are distinguished locally by their subleading behavior in large N. We focus mainly on (2+1)-dimensional gauge theory, where a relation to M theory and the Atiyah-Hitchin and Taub-Newman-Unti-Tamburino (NUT) manifolds enables the construction of the smooth supergravity solution and the study of some of the $1/N$ corrections. The role of the enhançon in 11-dimensional supergravity is also uncovered. There is a close relation to certain multimonopole moduli space problems.