(Non-)Extendability of Abel-Jacobi Maps

We investigate the “natural” locus of definition of Abel-Jacobi maps. In particular, we show that, for a proper, geometrically reduced curve C – not necessarily smooth – the Abel-Jacobi map from the smooth locus C^{sm} into the Jacobian of C does not extend to any larger (separated, geometrically reduced) curve containing C^{sm} except under certain particular circumstances which we describe explicitly. As a consequence, we deduce that the Abel-Jacobi map has closed image except in certain explicitly described circumstances, and that it is always a closed embedding for irreducible curves not isomorphic to P^1.