  For a right-angled Artin group $A_\Gamma$, the untwisted outer automorphism
group $U(A_\Gamma)$ is the subgroup of $Out(A_\Gamma)$ generated by all of the
Laurence-Servatius generators except twists (where a {\em twist} is an
automorphisms of the form $v\mapsto vw$ with $vw=wv$). We define a space
$\Sigma_\Gamma$ on which $U(A_\Gamma)$ acts properly and prove that
$\Sigma_\Gamma$ is contractible, providing a geometric model for $U(A_\Gamma)$
and its subgroups. We also propose a geometric model for all of $Out(A_\Gamma)$
defined by allowing more general markings and metrics on points of
$\Sigma_\Gamma$.
