Interpretation of HII Region Radio Recombination Lines

Abstract

There have been several recent reviews of the subject of radio recombination line from HII regions (Brown et al., 1978; Viner et al. 197 9) and I see no need for another review in the traditional sense. There does however seem to be a certain amount of confusion, at least among observers, as to whether or not theory and observations are consistent with one another. One problem is that simple models of the recombination line transfer do not appear to explain the observed run of line-to-continuumratio (T_L/T_C) with frequency. In fact, the observed values of T_L/T_C seem to vary only slightly from those which one would expect in the ideal situation of an optically thin L.T.E. gas without impact broadening. This has led some observers (see e.g. Chaisson and Dopita (1977)) to claim that, for practical purposes, L.T.E. may be assumed and electron temperatures derived on this basis. The claim is however hotly contested by Lockman and Brown (1978), who point out that there are difficulties fitting low frequency data (n >85) on the basis of an “L.T.E.” model. More fundamentally, it is clear that the lines are simply not in L.T.E. in the sense that, unless we completely misunderstand the physics of highly excited atoms in HII regions, the b_n factors are not equal to unity and the line absorption coefficients are negative. Hence stimulated emission should occur and the question remains as to why its effects are not more apparent.