Magnetic Field Strengths in Photodissociation Regions

TL;DR

This study measures magnetic field strengths in photodissociation regions around HII regions using carbon radio recombination lines, providing estimates that support the magnetic origin of non-thermal line widths.

Contribution

It introduces a method to derive magnetic field strengths in PDRs from carbon RRL observations and models, aligning with Zeeman measurements.

Findings

Magnetic fields in PDRs are estimated to be 100-300 micro Gauss.

Results are consistent with Zeeman measurements of line-of-sight magnetic fields.

Non-thermal line widths are primarily due to MHD waves.

Abstract

We measure carbon radio recombination line (RRL) emission at 5.3 GHz toward four HII regions with the Green Bank Telescope (GBT) to determine the magnetic field strength in the photodissociation region (PDR) that surrounds the ionized gas. Roshi (2007) suggests that the non-thermal line widths of carbon RRLs from PDRs are predominantly due to magneto-hydrodynamic (MHD) waves, thus allowing the magnetic field strength to be derived. We model the PDR with a simple geometry and perform the non-LTE radiative transfer of the carbon RRL emission to solve for the PDR physical properties. Using the PDR mass density from these models and the carbon RRL non-thermal line width we estimate total magnetic field strengths of B ~ 100-300 micro Gauss in W3 and NGC6334A. Our results for W49 and NGC6334D are less well constrained with total magnetic field strengths between B ~ 200-1000 micro Gauss. HI and OH Zeeman measurements of the line-of-sight magnetic field strength (B_los), taken from the literature, are between a factor of ~0.5-1 of the lower bound of our carbon RRL magnetic field strength estimates. Since |B_los| <= B, our results are consistent with the magnetic origin of the non-thermal component of carbon RRL widths.