On Abel Maps of Stable Curves

We construct Abel maps for a stable curve X. Namely, for each one-parameter deformation of X to a smooth curve, having regular total space, and each integer d ≥ 1, we construct by specialization a map αdX : Ẋ d → P d X , where Ẋ ⊆ X is the smooth locus, and P d X is the moduli scheme for balanced line bundles on semistable curves over X. For d = 1, we show that αX extends to a map α 1 X : X → P 1 X , and does not depend on the choice of the deformation. Finally, we give a precise description of when αX is injective.