Boundary of equisymmetric loci of Riemann surfaces with abelian symmetry

Abstract

Let ${\mathcal M}_g$ be the moduli space of compact connected Riemann surfaces of genus $g\geq 2$ and let $\widehat{{\mathcal M}_g}$ be its Deligne-Mumford compactification, which is stratified by the topological type of the stable Riemann surfaces. We consider the equisymmetric loci in $\mathcal M_g$ corresponding to Riemann surfaces whose automorphism group is abelian and determine the topological type of the maximal dimension strata at their boundary. For the particular cases of the hyperelliptic and the cyclic $p$-gonal actions, we describe all the topological strata at the boundary in terms of trees with a fixed number of edges.