GMC Collisions As Triggers of Star Formation. IV. The Role of Ambipolar
  Diffusion
  We investigate the role of ambipolar diffusion (AD) in collisions between
magnetized giant molecular clouds (GMCs), which may be an important mechanism
for triggering star cluster formation. Three dimensional simulations of GMC
collisions are performed using a version of the Enzo magnetohydrodynamics code
that has been extended to include AD. The resistivities are calculated using
the 31-species chemical model of Wu et al. (2015). We find that in the
weak-field, $10\:{\rm \mu G}$ case, AD has only a modest effect on the
dynamical evolution during the collision. However, for the stronger-field,
$30\:{\rm \mu G}$ case involving near-critical clouds, AD results in formation
of dense cores in regions where collapse is otherwise inhibited. The overall
efficiency of formation of cores with $n_{\rm H}\geq10^{6}\:{\rm cm}^{-3}$ in
these simulations is increases from about 0.2% to 2% once AD is included,
comparable to observed values in star-forming GMCs. The gas around these cores
typically has relatively slow infall at speeds that are a modest fraction of
the free-fall speed.
