Magnetic fields at the periphery of ultracompact H II regions from carbon recombination line observations

Abstract

Several indirect evidences indicate a magnetic origin for the nonthermal width of spectral lines observed toward molecular clouds. In this Letter, I suggest that the origin of the nonthermal width of carbon recombination lines (CRLs) observed from photodissociation regions (PDRs) near ultracompact H II regions is magnetic and that the magnitude of the line width is an estimate of the Alfvén 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 multifrequency CRL observations have the potential to form a new tool to determine the field strength near star-forming regions.