Radiation-MHD simulations of pillars and globules in HII regions

TL;DR

This paper presents radiation-MHD simulations of pillar and globule formation in HII regions, comparing implicit and explicit raytracing algorithms, and explores magnetic field effects on their evolution and structure.

Contribution

It introduces and compares efficient implicit and explicit algorithms for radiation-MHD simulations and investigates magnetic field influences on pillar formation in HII regions.

Findings

Implicit algorithm is more efficient for fast ionisation fronts.

Weak and medium magnetic fields align with pillars during evolution.

Strong magnetic fields confine flows and alter pillar morphology.

Abstract

Implicit and explicit raytracing-photoionisation algorithms have been implemented in the author's radiation-magnetohydrodynamics code. The algorithms are described briefly and their efficiency and parallel scaling are investigated. The implicit algorithm is more efficient for calculations where ionisation fronts have very supersonic velocities, and the explicit algorithm is favoured in the opposite limit because of its better parallel scaling. The implicit method is used to investigate the effects of initially uniform magnetic fields on the formation and evolution of dense pillars and cometary globules at the boundaries of HII regions. It is shown that for weak and medium field strengths an initially perpendicular field is swept into alignment with the pillar during its dynamical evolution, matching magnetic field observations of the`Pillars of Creation' in M16. A strong perpendicular magnetic field remains in its initial configuration and also confines the photoevaporation flow into a bar-shaped, dense, ionised ribbon which partially shields the ionisation front.