Formation of pillar-like structures
The formation process and structure of pillar-like structures at the border of HII-regions like the famous Pillars of Creation are of great interest to many astronomers as these dense molecular clouds (MCs) are the birthplaces for new stars. Recent polarimetric observations of the Pillars of Creation [Pattle et al. 2018] give direct insight into the magnetic field morphology within the dense gas. The observations show that the field lines are mostly aligned with the pillar. Therefore, the paper proposes an evolutionary scenario (Fig. 1) in which the approaching shock bends the initially parallel magnetic field around an existing over-density. The compressed magnetic field inside the pillar then supports the pillar against radial collapse.
We run magnetohydrodynamic simulations with the adaptive mesh renement (AMR) code FLASH 4, where an OB-type star ionizes its turbulent surrounding. The refinement criterion refines the blocks at the ionization front to level 7, which corresponds to a resolution of ~ 0.03 pc/cell. The mean magnetic field strength is 3 μG and the simulation is run with self-gravity.
Fig. 2 shows the slice of a pillar-like structure. The pillar is approximately 1000 times denser than the ionized gas surrounding it. The magnetic field lines, which were aligned mostly parallel to the incoming ionization front, are bending and follow the form of the pillar. The halo around the tip of the pillar results from fast gas streaming away with up to 25 km/s from the high ablation pressure region.