  Let $\mathcal{G}$ be a Grothendieck category, let
$\mathbf{t}=(\mathcal{T},\mathcal{F})$ be a torsion pair in $\mathcal{G}$ and
let $(\mathcal{U}_\mathbf{t},\mathcal{W}_\mathbf{t})$ be the associated
Happel-Reiten-Smal$\o$ t-structure in the derived category
$\mathcal{D}(\mathcal{G})$. We prove that the heart of this t-structure is a
Grothendieck category if, and only if, the torsionfree class $\mathcal{F}$ is
closed under taking direct limits in $\mathcal{G}$.
