Collision between Molecular Clouds I. The effect of the cloud virial
  ratio in head-on collisions
  In a series of papers we investigate the effect of collisions between
turbulent molecular clouds on their structure, evolution and star formation
activity. In this paper we look into the role of the clouds' initial virial
ratios. Three different scenarios were examined: both clouds initially bound,
one cloud bound and one unbound, and both clouds initially unbound. Models in
which one or both clouds are bound generate filamentary structures aligned
along the collision axis and discernible in position-position and
position--velocity space. If neither cloud is bound, no filaments result.
Unlike in previous simulations of collisions between smooth clouds, owing to
the substructure created in the clouds by turbulence before the collisions,
dissipation of kinetic energy by the collision is very inefficient and in none
of our simulations is sufficient bulk kinetic energy lost to render the clouds
bound. Simulations where both clouds are bound created twice as much stellar
mass than the bound-unbound model, and both these scenarios produced much more
stellar mass than the simulation in which both clouds are unbound. Each
simulation was also compared with a control run in which the clouds do not
collide. We find the bound-bound collision increases the overall star formation
efficiency by a factor of approximately two relative to the control, but that
the bound-unbound collision produces a much smaller increase, and the collision
has very little effect on the unbound--unbound cloud collision.
