Differential operators on an affine curve: ideal classes and Picard groups

Let X be a smooth complex affine curve, and let R be the space of right ideal classes in the ring D of differential operators on X. We introduce and study a fibration \gamma : R \to Pic(X). We relate this fibration to the corresponding one in the classical limit, and derive an integer invariant $n$ which indexes the decomposition of the fibres of \gamma into Calogero-Moser spaces (see [BC]). We also study the action of the group Pic(D) on our fibration; and we explain how to define \gamma in the framework of the Grassmannian description of R due to Cannings and Holland.