If $M$ is a compact Riemannian manifold, is the space of $C^{\infty}$ divergence free-free vector fields dense in the space of $C^r$ divergence free-free vector fields, in the $C^r$ topology (r\geq 1$r\geq 1$)? How about if we consider divergence free-free vector fields compactly supported on $\mathbb R^n$?