BENDING MODE-INSTABILITIES AND FRAGMENTATION IN INTERSTELLAR CLOUD COLLISIONS - A MECHANISM FOR COMPLEX STRUCTURE

A fundamental problem in interstellar gasdynamics is the collision bet ween two interstellar clouds. We present high-resolution two-dimension al results of this interaction using Adaptive mesh refinement (AMR) hy drodynamics with a Godunov scheme for accurate shock tracking in multi dimensions. These results are at a resolution that is significantly hi gher than has been previously achieved by other methodologies such as smoothed particle hydrodynamics. We have studied the collisions betwee n homogeneous clouds with an adiabatic equation of state, isothermal c louds, radiatively cooling clouds, and clouds with initial surface per turbations. In all instances, the collision is complex, resulting in f lows that are strongly influenced by Kelvin-Helmholtz and nonlinear th in shell bending mode instabilities. In particular we find that the ea rly evolution of homogeneous cloud collisions initially produces a col d dense disk in the collision midplane. A low mass jet propagates outw ard with characteristics of dense protostellar jets in a low-density m edium. Once the clouds have been compressed by strong shocks, pressure gradients drive the dense disk to re-expand along the symmetry axis. This reexpansion overshoots, resulting in a pressure deficit in the in terior of the merged cloud system and a collapse back onto the symmetr y axis. If the colliding clouds are initially smooth, the end result o f the collision is a large aspect ratio filament with a homogeneous in terior and an irregular surface. If the clouds have finite surface per turbations, a bending mode instability renders the merged cloud system asymmetrical and highly inhomogeneous with islands of high density su rrounded by low density regions throughout the interior. These results have implications for coelescence models of star formation. The appea rance of the merged system is that of a clumpy filamentary structure w ith a large aspect ratio. This instability is shown to occur for both isothermal shocks, as well as shocks with radiative cooling. The insta bility occurs in adiabatic shocks for compressions greater than 10. Th e bending mode instability increases the vorticity of the merged cloud system, resulting in an axial velocity that is twice as large as in t he smooth cloud case. Recent observations show an abundance of elongat ed clumpy filaments in the Orion Molecular Cloud (OMC-1). Our calculat ions of cloud-cloud collisions undergoing the bending mode instability provide a new mechanism for for generating inhomogeneous filamentary structures which appear to be common in the interstellar medium.