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, 200pc diameter
spherical cloud in pressure equilibrium with low density surroundings. This 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. No magnetic
field was imposed. Resimulations at a higher resolution of a region extracted
from this simulation (down to 0.039pc), show that the thermal instability forms
sheets, then filaments and finally clumps. The width of the filaments increases
over time, in one particular case from 0.26 to 0.56pc. Thereafter clumps with
sizes of around 5pc grow at the intersections of filaments. 21 distinct clumps,
with properties similar to those observed in molecular clouds, are found by
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 >10^6 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.
