MIDINFRARED HYDROGEN RECOMBINATION LINE EMISSION FROM THE MASER STAR

We have detected and spectrally resolved the mid-IR hydrogen recombina tion lines H6 alpha (12.372 mu m), H7 alpha (19.062 mu m), H7 beta (11 .309 mu m), and H8 gamma (12.385 mu m) from the star MWC 349A. This ob ject has strong hydrogen maser emission reported in the millimeter and submillimeter hydrogen recombination lines from H36 alpha to H21 alph a, and laser emission seen in the H15 alpha, H12 alpha and H10 alpha l ines. The lasers/masers are thought to arise predominantly in a Kepler ian disk around the star. The mid-IR lines we measure do not show evid ent signs of lasing, and can be adequately modeled as arising from the strong stellar wind, with a component arising from a quasi-static atm osphere around the disk, similar to what is hypothesized for the near- IR (less than or equal to 4 mu m) recombination lines, Since populatio n inversions are expected to exist in the levels producing these mid-I R transitions, even at electron densities where they could produce sig nificant amplification (from 10(8) even up to 10(11) cm(-3)), our resu lts imply that the disk does not contain such high-density ionized gas over long enough path lengths to produce a gain greater than or equal to 1. More modest laser emission might be present from smaller zones, for example, from high-density clumps within the disk, but is minor c ompared to the spontaneous background Line emission from the rest of t he source. The results reinforce the interpretation of the three repor ted far-IR lines as true lasers. (C) 1997 American Astronomical Societ y.