We review what has been learnt and what remains unknown about the physics of hot enhancons following studies in supergravity. We recall a rather general family of static, spherically symmetric, non-extremal enhancon solutions describing D4 branes wrapped on K3 and discuss physical aspects of the solutions. We embed these solutions in the six dimensional supergravity describing type IIA strings on K3 and generalize them to have arbitrary charge vector. This allows us to demonstrate the equivalence with a known family of hot fractional D0 brane solutions, to widen the class of solutions of this second type and to carry much of the discussion across from the D4 brane analysis. In particular we argue for the existence of a horizon branch for these branes.
Surprisingly, when the system is near extremality and the asymptotic volume of the K3 is large, the first two terms combine into a dominant, negative, contribution. Thus the shell branch violates the WEC. It was argued @cite_7 that the shell branch should therefore be regarded as unphysical. Accordingly, the horizon branch should be considered the dominant, valid, supergravity solution for non-BPS , for the range of parameters admitting it. For the region of parameter space in which no horizon branch exists, other solutions, more general than those yet considered, might be valid @cite_7 .
@cite_7
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We study the supergravity solutions describing nonextremal enhan c c ons. There are two branches of solutions: a shell branch'' connected to the extremal solution, and a horizon branch'' which connects to the Schwarzschild black hole at large mass. We show that the shell branch solutions violate the weak energy condition, and are hence unphysical. We investigate linearized perturbations of the horizon branch and the extremal solution numerically, completing an investigation initiated in a previous paper. We show that these solutions are stable against the perturbations we consider. This provides further evidence that these latter supergravity solutions are capturing some of the true physics of the enhan c c on.