Anticanonical geometry of the blow-up of $\mathbb{P}^4$ in $8$ points and its Fano model

Abstract

Building on the work of Casagrande-Codogni-Fanelli, we develop our study on the birational geometry of the Fano fourfold $Y=M_{S,-K_S}$ which is the moduli space of semi-stable rank-two torsion-free sheaves with $c_1=-K_S$ and $c_2=2$ on a polarised degree-one del Pezzo surface $(S,-K_S)$. Based on the relation between $Y$ and the blow-up of $\mathbb{P}^4$ in $8$ points, we describe completely the base scheme of the anticanonical system $|{-}K_Y|$. We also prove that the Bertini involution $\iota_Y$ of $Y$, induced by the Bertini involution $\iota_S$ of $S$, preserves every member in $|{-}K_Y|$. In particular, we establish the relation between $\iota_Y$ and the anticanonical map of $Y$, and we describe the action of $\iota_Y$ by analogy with the action of $\iota_S$ on $S$.