Hyperelliptic Stable Curves

Abstract

We provide an intrinsic characterization of hyperelliptic stable curves of genus $g \geq 2$, independent of admissible covers or auxiliary moduli data. A stable curve is hyperelliptic if it admits an involution yielding a rational tree quotient, subject to a characteristic-dependent condition. By analyzing the action of this involution on the nodes and decomposing the curve based on its connectivity, we obtain an explicit structural description of hyperellipticity and prove that the hyperelliptic involution is unique. Furthermore, we explain the connection to the very ampleness of the dualizing sheaf. This framework applies in arbitrary characteristic, explicitly capturing the divergent geometric and combinatorial behavior in characteristic 2. We verify that this formulation precisely captures the geometric points of the moduli stack of hyperelliptic stable curves $\overline{\mathcal{H}}_g$, defined as the scheme-theoretic closure of the smooth hyperelliptic locus $\mathcal{H}_g$ within the moduli stack of stable curves $\overline{\mathcal{M}}_g$. Extending this definition to flat families yields an explicit modular description of $\overline{\mathcal{H}}_g$ over $\operatorname{Spec} \mathbb{Z}[1/2]$.