Magnetic Field Strengths in Photodissociation Regions

We measure carbon radio recombination line (RRL) emission at 5.3 GHz toward four H ii regions with the Green Bank Telescope to determine the magnetic field strength in the photodissociation region (PDR) that surrounds the ionized gas. Roshi suggests that the non-thermal line widths of carbon RRLs from PDRs are predominantly due to magneto-hydrodynamic 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 µG in W3 and NGC 6334A. Our results for W49 and NGC 6334D are less well constrained with total magnetic field strengths between B ~ 200-1000 µG. H i 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.