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

This paper describes 3D simulations of the formation of collapsing cold clumps via thermal instability inside a larger cloud complex. The initial condition was a diffuse atomic, stationary, thermally unstable, 200 pc diameter spherical cloud in pressure equilibrium with low-density surroundings. This was seeded with 10 per cent density perturbations at the finest initial grid level (0.29 pc) around nH= 1.1 cm−3 and evolved with self-gravity included. No magnetic field was imposed. Resimulations at a higher resolution of a region extracted from this simulation (down to 0.039 pc) show that the thermal instability forms sheets, then filaments, and finally clumps. The width of the filaments increases overtime, in one particular case from 0.26 to 0.56 pc. Thereafter, clumps with sizes of around 5 pc grow at the intersections of filaments. 21 distinct clumps, with properties similar to those observed in molecular clouds, are found using the FellWalker algorithm to find minima in the gravitational potential. Not all of these are gravitationally bound, but the convergent nature of the flow and increasing central density suggest they are likely to form stars. Further simulation of the most massive clump shows the gravitational collapse to a density >106 cm−3. These results provide realistic initial conditions that can be used to study feedback in individual clumps, interacting clumps, and the entire molecular cloud complex.