Orientifolds, M theory, and the ABCD’s of the enhançon

Supergravity solutions related to large NSU(N) 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 SO(2N+1), USp(2N) and SO(2N) 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 Z₂ 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.