2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/63362Here we focus on the geometry of $\pdgbar$, the compactification of the universal Picard variety constructed by L. Caporaso. In particular, we show that the moduli space of spin curves constructed by M. Cornalba naturally injects into $\pdgbar$ and we give generators and relations of the rational divisor class group of $\pdgbar$, extending previous work by A. Kouvidakis.Revised version (modified statement of Theorem 2 and other minor changes)Algebraic Geometrytext