Radiation-MHD models of elephant trunks and globules in H II regions

TL;DR

This study uses 3D radiation-MHD simulations to explore how magnetic fields influence the formation and evolution of elephant trunks and globules in H II regions, aligning with observations and revealing magnetic suppression effects.

Contribution

It provides new insights into magnetic field effects on pillar formation in H II regions through detailed 3D simulations, extending previous models.

Findings

Magnetic fields are swept into alignment with pillars during evolution.

Weaker magnetic fields enhance alignment effects.

Stronger magnetic fields prevent pillar evolution altogether.

Abstract

We study the formation and evolution of pillars of dense gas, known as elephant trunks, at the boundaries of H II regions, formed by shadowing of ionising radiation by dense clumps. The effects of magnetic fields on this process are investigated using 3D radiation-magnetohydrodynamics simulations. For a simulation in which an initially uniform magnetic field of strength |B|=50 uG is oriented perpendicular to the radiation propagation direction, the field is swept into alignment with the pillar during its dynamical evolution, in agreement with observations of the "Pillars of Creation" in M16, and of some cometary globules. This effect is significantly enhanced when the simulation is re-run with a weaker field of 18 uG. A stronger field with |B|=160 uG is sufficient to prevent this evolution completely, also significantly affecting the photoionisation process. Using a larger simulation domain it is seen that the pillar formation models studied in Mackey & Lim (2010) ultimately evolve to cometary structures in the absence of dense gas further from the star.