Radio Recombination Lines

Radio recombination lines (RRLs) are important diagnostic tools in the determination of the physical conditions in HII regions. Temperatures in the HII regions, rms electron densities, the excitation temperature of the exciting stars are examples of the possible properties that can be determined. Below is a plot of the O/H abundance ratio gradient in the Galaxy inferred from the electron temperatures determined from the line-to-continuum ratio. (See Quireza et al. 2006)

H and He RRLs can be used to determine the He/H abundance in the HII region and estimate the enrichment of He with metallicity. C RRLs can be used to probe the physical conditions in the photo-dissociation regions.

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