Detailed Numerical Simulations on the Formation of Pillars around
  HII-regions
  We study the structural evolution of turbulent molecular clouds under the
influence of ionizing radiation emitted from a nearby massive star by
performing a high resolution parameter study with the iVINE code. The
temperature is taken to be 10K or 100K, the mean number density is either
100cm^3 or 300cm^3. Besides, the turbulence is varied between Mach 1.5 and Mach
12.5 and the main driving scale between 1pc and 8pc. We vary the ionizing flux
by an order of magnitude. In our simulations the ionizing radiation enhances
the initial turbulent density distribution and thus leads to the formation of
pillar-like structures observed adjacent to HII regions in a natural way.
Gravitational collapse occurs regularly at the tips of the structures. We find
a clear correlation between the initial state of the turbulent cold cloud and
the final morphology and physical properties of the structures formed. The most
favorable regime for the formation of pillars is Mach 4-10. Structures and
therefore stars only form if the initial density contrast between the high
density unionized gas and the gas that is going to be ionized is lower than the
temperature contrast between the hot and the cold gas. The density of the
resulting pillars is determined by a pressure equilibrium between the hot and
the cold gas. A thorough analysis of the simulations shows that the complex
kinematical and geometrical structure of the formed elongated filaments
reflects that of observed pillars to an impressive level of detail. In
addition, we find that the observed line-of sight velocities allow for a
distinct determination of different formation mechanisms. Comparing the current
simulations to previous results and recent observations we conclude that e.g.
the pillars of creation in M16 formed by the mechanism proposed here and not by
the radiation driven implosion of pre-existing clumps.
