GMC Collisions as Triggers of Star Formation. I. Parameter Space
  Exploration with 2D Simulations
  We utilize magnetohydrodynamic (MHD) simulations to develop a numerical model
for GMC-GMC collisions between nearly magnetically critical clouds. The goal is
to determine if, and under what circumstances, cloud collisions can cause
pre-existing magnetically subcritical clumps to become supercritical and
undergo gravitational collapse. We first develop and implement new
photodissociation region (PDR) based heating and cooling functions that span
the atomic to molecular transition, creating a multiphase ISM and allowing
modeling of non-equilibrium temperature structures. Then in 2D and with ideal
MHD, we explore a wide parameter space of magnetic field strength, magnetic
field geometry, collision velocity, and impact parameter, and compare isolated
versus colliding clouds. We find factors of ~2-3 increase in mean clump density
from typical collisions, with strong dependence on collision velocity and
magnetic field strength, but ultimately limited by flux-freezing in 2D
geometries. For geometries enabling flow along magnetic field lines, greater
degrees of collapse are seen. We discuss observational diagnostics of cloud
collisions, focussing on 13CO(J=2-1), 13CO(J=3-2), and 12CO(J=8-7) integrated
intensity maps and spectra, which we synthesize from our simulation outputs. We
find the ratio of J=8-7 to lower-J emission is a powerful diagnostic probe of
GMC collisions.
