Simpson Jacobians of generalized tree-like curves

Abstract

The compactified Jacobian of any projective curve $X$ is defined as the Simpson moduli space of torsion free rank one degree $d$ sheaves that are semistable with respect to a fixed polarization $H$ on $X$. In this paper we give explicitly the structure of this compactified Simpson Jacobian in the case where $X$ is a generalized tree-like curve, i.e., a projective, reduced and connected curve such that the intersection points of its irreducible components are disconnecting ordinary double points. We prove that it is isomorphic to the product of the compactified Jacobians of a certain degree $d_i$ of its components $C_i$, where the degrees $d_i$ depend on $d$, $H$ and on the particular structure of the curve. We find also necessary and sufficient conditions for the existence of stable points which allow us to study the variation of these Simpson Jacobians as the polarization $H$ changes.