1997A&A...324..699T

We present new observations of the Paschen series hydrogen recombination lines in the circumstellar shell of MWC 349. We find the Paschen decrement to be characteristic of a very high density plasma, n_e∼10⁸cm^–3, indicating the origin of the higher quantum number Paschen lines in the dense corona of the circumstellar disk. The strength of the masing hydrogen recombination α-lines detected previously in the high density plasma of this source is strongly dependent on n_e, since the n_e for maximum amplification increases systematically for decreasing quantum numbers n. The absence of gas with n_e significantly higher than 10⁸cm^–3 implies that the strength of recombination α-line masers may peak near n=20 (λ=400µm) and that the shorter wavelength transitions may mase only at reduced efficiency. The good agreement over a large range of n between the observed and the theoretical Paschen decrement shows that any departure from standard recombination theory, possibly due to the strong IR radiation field of the circumstellar disk, must be small.