Moduli of curves, Gr\"obner bases, and the Krichever map

Abstract

We study moduli spaces of (possibly non-nodal) curves (C,p_1,\ldots,p_n) of arithmetic genus g with n smooth marked points, equipped with nonzero tangent vectors, such that ${\mathcal O}_C(p_1+\ldots+p_n)$ is ample and $H^1({\mathcal O}_C(a_1p_1+\ldots+a_np_n))=0$ for given weights ${\bf a}=(a_1,\ldots,a_n)$ such that $a_i\ge 0$ and $\sum a_i=g$. We show that each such moduli space $\widetilde{{\mathcal U}}^{ns}_{g,n}({\bf a})$ is an affine scheme of finite type, and the Krichever map identifies it with the quotient of an explicit locally closed subscheme of the Sato Grassmannian by the free action of the group of changes of formal parameters. We study the GIT quotients of $\widetilde{{\mathcal U}}^{ns}_{g,n}({\bf a})$ by the natural torus action and show that some of the corresponding stack quotients give modular compactifications of ${\mathcal M}_{g,n}$ with projective coarse moduli spaces. More generally, using similar techniques, we construct moduli spaces of curves with chains of divisors supported at marked points, with prescribed number of sections, which in the case n=1 corresponds to specifying the Weierstrass gap sequence at the marked point.