Magnetic Field Strengths in Photodissociation Regions
  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.
