The Launching of Cold Clouds by Galaxy Outflows. IV. Cosmic-Ray-Driven
  Acceleration
  We carry out a suite of simulations of the evolution of cosmic-ray (CR)
driven, radiatively-cooled cold clouds embedded in hot material, as found in
galactic outflows. In such interactions, CRs stream towards the cloud at the
Alfv\'en speed, which decreases dramatically at the cloud boundary, leading to
a bottleneck in which pressure builds up in front of the cloud. At the same
time, CRs stream along the sides of the cloud, forming a boundary layer where
large filaments develop. Shear in this boundary layer is the primary mode of
cloud destruction, which is relatively slow in all cases, but slowest in the
cases with the lowest Alfv\'en speeds. Thus the CR ray pressure in the
bottleneck region has sufficient time to accelerate the cold clouds
efficiently. Furthermore, radiative cooling has relatively little impact on
these interactions. Our simulations are two-dimensional and limited by a
simplified treatment of cosmic-ray dynamics, the neglect of CR heating, and an
idealized magnetic field geometry. Nevertheless, our results suggest that
cosmic rays, when acting as the primary source of momentum input, are capable
of accelerating clouds to velocities comparable to those observed in galaxy
outflows.
