On the interaction of a thin, supersonic shell with a molecular cloud

Molecular Clouds (MCs) are stellar nurseries; however, formation of stars within MCs depends on the ambient physical conditions. MCs, over a free-fall time, are exposed to numerous dynamical phenomena, of which, the interaction with a thin, dense shell of gas is one. Below we present results from self-gravitating, 3D smoothed particle hydrodynamics (SPH) simulations of the problem; seven realizations of the problem have been performed by varying the pre-collision density within the cloud, the nature of the post-collision shock and the spatial resolution in the computational domain. Irrespective of the type of shock, a complex network of dense filaments, seeded by numerical noise, readily appears in the shocked cloud. Segregation of the dense and rarefied gas phases also manifests itself in a bimodal distribution of gas density. We demonstrate that the power spectrum for rarefied gas is Kolomogorov like, while that for the denser gas is considerably steeper. As a corollary to the main problem, we also look into the possibly degenerative effect of the SPH artificial viscosity on the impact of the incident shell. It is observed that stronger viscosity leads to greater post-shock dissipation that strongly decelerates the incident shock front and promotes formation of contiguous structure, albeit on a much longer time-scale. We conclude that too much viscosity is likely to enhance the proclivity towards gravitational boundedness of structure, leading to unphysical fragmentation, while insufficient resolution appears to suppress fragmentation. Convergence of results is tested at both extremes, first by repeating the test case with more than a million particles and then with only half the number of particles in the original test case.