  Let $p$ be a Lie subalgebra of a semisimple Lie algebra $g$ and $(G,P)$ be
the corresponding pair of connected Lie groups. A Cartan geometry of type
$(G,P)$ associates to a smooth manifold $M$ a principal $P$-bundle and a Cartan
connection, and a parabolic geometry is a Cartan geometry where $P$ is
parabolic. We show that if $P$ is parabolic, the adjoint tractor bundle of a
Cartan geometry, which is isomorphic to the Atiyah algebroid of the principal
$P$-bundle, admits the structure of a (pre-)Courant algebroid, and we identify
the topological obstruction to the bracket being a Courant bracket. For
semisimple $G$, the Atiyah algebroid of the principal $P$-bundle associated to
the Cartan geometry of $(G,P)$ admits a pre-Courant algebroid structure if and
only if $P$ is parabolic.
