Stable degenerations of symmetric squares of curves

Abstract

The main result of this article is that the component of the Alexeev-Kollár-Shepherd-Barron moduli space of stable surfaces parameterizing stable degenerations of symmetric squares of curves is isomorphic to the moduli space of stable curves. The moduli space of stable curves is used to ensure that any degeneration can be replaced by a degeneration to the symmetric square of a stable curve. These degenerations are not stable, however, but are partially resolved by the relative Hilbert scheme of length two subschemes in the fibers of a family of stable curves. The article gives a geometric description of the relative minimal models of these Hilbert schemes.