Radiative transfer effects in hydrogen recombination masers

Using a simple on-the-spot approximation, we explore the parameter range in the (log(R), log(ne)) plane for which maser action in atomic hydrogen is present and briefly discuss how the borderlines of this region of strong population inversion arise. In particular we address the influence of radiative transfer on the population structure. We find that in normal HII regions no strong maser activity occurs, while for ultra compact HII regions in a certain parameter range strong hydrogen masers should be found. Our fit to the available observational data for MWC 349 leads to a density estimate that is compatible with previous work (Thum & Greve 1997). Furthermore, we derive for the first time a reasonable representation of the variation of the flux ratio of Hnβ and Hnα lines measured towards MWC 349.