3D simulations of globules and pillars formation around HII regions: turbulence and shock curvature

TL;DR

This study uses 3D simulations to explore how turbulence and shock curvature influence the formation of globules and pillars around HII regions, revealing the roles of pressure balance and shell curvature in structure formation.

Contribution

It introduces a detailed 3D hydrodynamical simulation approach to understand the impact of turbulence and shock curvature on globule and pillar formation around HII regions, highlighting the importance of non-equilibrium ionization modeling.

Findings

Shell curvature determines pillar formation versus dense clumps.

Turbulent ram pressure influences globule penetration into ionized regions.

Double-peaked density PDFs indicate turbulence levels in molecular clouds.

Abstract

We investigate the interplay between the ionization radiation from massive stars and the turbulence inside the surrounding molecular gas thanks to 3D numerical simulations. 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 are performed with different mean Mach numbers (1, 2 and 4). A non-equilibrium model for the ionization and the associated thermal processes was used. This revealed to be crucial when turbulent ram pressure is of the same order as the ionized-gas pressure. 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 collapse 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 inside the ionized medium, forming cometary globules. This suggests a direct relation in the observations between the presence of globules and the relative importance 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 used in observations as an indication of the turbulence inside molecular clouds.