The enhancon, black holes, and the second law - art. no. 106002

We reexamine the physics of five-dimensional black holes, constructed from D5- and D1-branes and momentum modes in type IIB string theory compactified on K3. Since these black holes incorporate D5-branes wrapped on K3, an enh ancon locus appears in the spacetime geometry. With a "small" number of D1- branes, the entropy of a black hole is maximized by including precisely hal f as many D5-branes as there are D1-branes in the black hole. Any attempts to introduce more D5-branes, and so reduce the entropy, are thwarted by the appearance of the enhancon locus above the horizon, which then prevents th eir approach. The enhancon mechanism thereby acts to uphold the second law of thermodynamics. This result generalizes: For each type of bound state ob ject which can be made of both types of brane, we show that a new type of e nhancon exists at successively smaller radii in the geometry, again acting to prevent any reduction of the entropy just when needed. We briefly explor e the appearance of the enhancon locus in the black hole interior.