Sheets, filaments, and clumps – high-resolution simulations of how the thermal instability can form molecular clouds

The hydrodynamic version of the adaptive mesh refinement (AMR) code, MG, has been employed to perform 3D simulations of the formation of collapsing cold clumps on the scale of a few parsecs, inside a larger cloud complex. The diffuse atomic initial condition consists of a stationary, thermally unstable, 200pc diameter spherical cloud in pressure equilibrium with low density surroundings. The diffuse atomic cloud was seeded with 10% density perturbations at the finest initial grid level (0.29pc) around n_H=1.1cm^-3 and evolved with self-gravity included from the outset. No magnetic field was imposed. Resimulation of a region of this simulation at higher resolution (down to 0.039pc), shows that the thermal instability dynamically forms sheets and filaments. The natural width of the filaments is 0.1-0.3pc. Following this, clumps grow at the intersections of filaments with size-scales of around 5pc and aspect ratios around unity. Imposed on the entire potential well of the cloud, the FellWalker routine finds 21 distinct clumps. The properties of these clumps are in agreement with clumps observed in molecular clouds. Given their positions, at the interconnections of the filamentary network, many show evidence of infalling linear structure. Not all are gravitationally bound, but the convergent nature of the infall and increasing central density in each suggest they are highly-likely to form stars. Further simulation of the most massive clump reveals the final gravitational collapse of the clump, to resolved levels of density six orders of magnitude higher than the initial condition (i.e. to n_H>10^6cm^-3). The clumps within the cloud provide a realistic initial condition that can be used to study feedback in 1) individual clumps, 2) interacting clumps and 3) across the entire molecular cloud complex. Future work will consider the effects of cluster feedback in each of these three scenarios.