Einstein locally conformal calibrated $G_2$-structures

Abstract

We study locally conformal calibrated $G_2$-structures whose underlying Riemannian metric is Einstein, showing that in the compact case the scalar curvature cannot be positive. As a consequence, a compact homogeneous $7$-manifold cannot admit an invariant Einstein locally conformal calibrated $G_2$-structure unless the underlying metric is flat. In contrast to the compact case, we provide a non-compact example of homogeneous manifold endowed with a locally conformal calibrated $G_2$-structure whose associated Riemannian metric is Einstein and non Ricci-flat. The homogeneous Einstein metric is a rank-one extension of a Ricci soliton on the $3$-dimensional complex Heisenberg group endowed with a left-invariant coupled ${\rm SU}(3)$-structure $(\omega, \Psi)$, i.e., such that $d \omega = c {\rm Re}(\Psi)$, with $c \in \mathbb{R} - \{ 0 \}$. Nilpotent Lie algebras admitting a coupled ${\rm SU}(3)$-structure are also classified.