Mori dream Jacobian elliptic surfaces of Kodaira dimension one

Abstract

Let $\pi :X\to {\mathbb{P}}^1$ be a Jacobian elliptic surface over $\mathbb{C}$, and set $\chi =\chi ({\mathcal{O}}_X)\ge 3$, so that $\kappa (X)=1$. Assume that the Mordell--Weil group of $\pi$ is finite and that $\pi$ has at least one reducible fiber, the reducible fibers being of types $I_{n_1},\dots ,I_{n_s}$. We prove that the zero section and the components of the reducible fibers generate the closed Mori cone if and only if \[ \sum_{i=1}^s\frac{\lfloor n_i^2/4\rfloor}{n_i}\leχ. \] If ${\sum }_i{n}_i\le 2\chi +3$, then $X$ is a Mori dream surface. The proof combines an explicit description of the facets of the cone generated by the curves visible in the fibration with Artin's criterion applied to the null loci of the dual nef rays. We also show that, in Kodaira dimension one, finiteness of both the Mordell--Weil group and the automorphism group does not mply polyhedrality of the Mori cone. In the polyhedral range, we construct a Jacobian elliptic surface with $(\chi ,n)=(3,11)$, Picard number $12$, and a big and nef divisor which is not semiample; in particular, this surface is not a Mori dream surface. Finally, for every integer $\rho \ge 2$, we construct a Jacobian elliptic surface of Kodaira dimension one and Picard number $\rho$ that is a Mori dream surface.

26 pages, revised version, generalizing the original Theorem 1.1 to semistable fibrations, comments are welcome!