Magnetic fields at the periphery of UCH ii regions from carbon recombination line observations

Several indirect evidences indicate a magnetic origin for the non-thermal width of spectral lines observed toward molecular clouds. In this letter, I suggest that the origin of the non-thermal width of carbon recombination lines (CRLs) observed from photo-dissociation regions (PDRs) near ultra-compact H ii regions is magnetic and that the magnitude of the line width is an estimate of the Alfv́en speed. The magnetic field strengths estimated based on this suggestion compare well with those measured toward molecular clouds with densities similar to PDR densities. I conclude that multi-frequency CRL observations have the potential to form a new tool to determine the field strength near star forming regions. Subject headings: ISM:magnetic fields – radio lines:ISM – ISM:lines and bands – ISM:molecules – MHD – H ii regions