Synthetic polarization observations of magnetized pillars in HII regions: The limitations of the Davis-Chandrasekhar-Fermi method

TL;DR

This study uses simulations and synthetic polarimetric observations to evaluate the effectiveness of the Davis-Chandrasekhar-Fermi method in measuring magnetic fields in HII region pillars, revealing systematic overestimations due to external pressure effects.

Contribution

It demonstrates that the DCF method overestimates magnetic field strengths in externally compressed pillars, highlighting limitations of classical techniques in such environments.

Findings

DCF overestimates magnetic field strength by factors > 2.

Polarimetry reliably traces magnetic field morphology.

External pressure dominates field alignment, violating DCF assumptions.

Abstract

We investigated the morphology and strength of magnetic fields in pillar-shaped structures at the boundaries of HII regions by combining three-dimensional radiation-magnetohydrodynamic, R-MHD, simulations with synthetic polarimetric observations. Our analysis focuses on the first pillar formed self-consistently in the simulation and is used as a proof of concept to test the applicability of the Davis-Chandrasekhar-Fermi (DCF) method under conditions dominated by external agents. The pillar arises as the ionization front compresses a dense clump, producing a magnetically aligned, elongated structure whose morphology and field configuration closely resemble those observed in real systems such as the pillars of M16. Synthetic dust-polarization maps at 850 $\mu$m reproduce the large-scale magnetic morphology of the simulated pillar, confirming that polarimetry is a reliable tracer of magnetic field geometry. However, magnetic field strengths inferred using DCF-type methods systematically overestimate intrinsic values from the simulations by factors $\gtrsim 2$. We attribute this discrepancy to the fact that field alignment and amplification are primarily driven by external gas pressure from the expanding HII region rather than internal turbulence, thus violating key assumptions of the DCF method. Our results highlight the need for caution when applying classical DCF-based analysis to pillars or other structures shaped by external compressions.