Three-dimensional simulations of globule and pillar formation around HII regions: turbulence and shock curvature

Aims: We investigate the interplay between the ionization radiation from massive stars and the turbulence inside the surrounding molecular gas using three-dimensional (3D) numerical simulations.
Methods: We used the 3D hydrodynamical code HERACLES to model an initial turbulent medium that is ionized and heated by an ionizing source. Three different simulations were performed with different mean Mach numbers (1, 2, and 4). A non-equilibrium model for the ionization and the associated thermal processes was chosen. This turned out to be crucial when turbulent ram pressure is on the same order as the ionized-gas pressure.
Results: The density structures initiated by the turbulence cause local curvatures of the dense shell formed by the ionization compression. When the curvature of the shell is sufficient, the shell collapses in on itself to form a pillar, while a smaller curvature leads to the formation of dense clumps that are accelerated with the shell and therefore remain in the shell during the simulation. When the turbulent ram pressure of the cold gas is sufficient to balance the ionized-gas pressure, some dense-gas bubbles have enough kinetic energy to penetrate the ionized medium, forming cometary globules. This suggests that there is a direct relation in the observations between the presence of globules and the relative significance of the turbulence compared to the ionized-gas pressure. The probability density functions present a double peak structure when the turbulence is low relative to the ionized-gas pressure. This could be interpreted in observations as an indication of the turbulence inside molecular clouds.