Bending Mode Instabilities and Fragmentation in Interstellar Cloud Collisions: A Mechanism for Complex Structure

Abstract

A fundamental problem in interstellar gasdynamics is the collision between two interstellar clouds. We present high-resolution two-dimensional results of this interaction using Adaptive mesh refinement (AMR) hydrodynamics with a Godunov scheme for accurate shock tracking in multidimensions. These results are at a resolution that is significantly higher than has been previously achieved by other methodologies such as smoothed particle hydrodynamics. We have studied the collisions between homogeneous clouds with an adiabatic equation of state, isothermal clouds, radiatively cooling clouds, and clouds with initial surface perturbations. In all instances, the collision is complex, resulting in flows that are strongly influenced by Kelvin-Helmholtz and nonlinear thin shell bending mode instabilities. In particular we find that the early evolution of homogeneous cloud collisions initially produces a cold dense disk in the collision midplane. A low mass jet propagates outward with characteristics of dense protostellar jets in a low-density medium. 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 interior of the merged cloud system and a collapse back onto the symmetry axis. If the colliding clouds are initially smooth, the end result of the collision is a large aspect ratio filament with a homogeneous interior and an irregular surface. If the clouds have finite surface perturbations, a bending mode instability renders the merged cloud system asymmetrical and highly inhomogeneous with islands of high density surrounded by low density regions throughout the interior. These results have implications for coelescence models of star formation. The appearance of the merged system is that of a clumpy filamentary structure with a large aspect ratio. This instability is shown to occur for both isothermal shocks, as well as shocks with radiative cooling. The instability occurs in adiabatic shocks for compressions greater than 10. The bending mode instability increases the vorticity of the merged cloud system, resulting in an axial velocity that is twice as large as in the smooth cloud case. Recent observations show an abundance of elongated clumpy filaments in the Orion Molecular Cloud (OMC-1). Our calculations 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.