Clump collisions in molecular clouds - Gravitational instability and coalescence

Collisions between clumps in molecular clouds are studied in a series of numerical experiments employing a two-dimensional, time-dependent hydrodynamics code that includes self-gravity and an accurate cooling function. It is found that nonlinear effects are critical in the generation of gravitational instability and that linearized, normal mode analyses of compressed gas layers are inadequate to describe this phenomenon. The density contrast between the collision pair is recognized to be a key parameter in determining the stability and coalescence properties of the collision. Collisions between clumps are distinguished by the symmetry that exists in the gas motions; collisions between identical clumps have a plane of mirror symmetry that promotes gravitational instability, while collisions between clumps of different sizes and densities are inherently asymmetric and tend to be disruptive. These results are discussed with respect to current models of molecular cloud formation and clumping with molecular clouds.