Radio Recombination Lines at Millimeter Wavelengths in HII Regions
Radio recombination lines from HII regions involve much smaller principal quantum numbers at millimeter than at centimeter wavelengths. In this regime pressure broadening and partial maser effects are small and can usually be neglected. Free of these complications, millimeter-wave lines have been used to determine the electron temperatures of HII regions and to explore the atomic population processes for principal quantum numbers between 25 and 50. In general, observations appear to fit the transfer theory suggested by Goldberg (1966) with typical departure coefficients such as those calculated by Salem and Brocklehurst (1979).
This review is intended to cover the salient features of radio recombination lines (RRLs) at millimeter wavelengths but excludes discussion of the mm-wave maser lines from the star MWC349, which is described in a separate article in this volume.
KeywordsElectron Temperature Oscillator Strength Line Intensity Principal Quantum Number Departure Coefficient
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