Limits of stable pairs

Abstract

Let (X_0,B_0) be the canonical limit of a one-parameter family of stable pairs, provided by the log Minimal Model Program. We prove that X_0 is S2 and that [B_0] is S_1, as an application of a general local statement: if (X,B+\epsilon D) is log canonical and D is Q-Cartier then D is S2 and [B] \cap D is S1, i.e. has no embedded components. When B has coefficients smaller than 1, examples due to Hacking and Hassett show that B_0 may indeed have embedded primes. We resolve this problem by introducing a category of stable branchpairs. We prove that the corresponding moduli functor is proper for families with normal generic fiber.