Infrared lasers in the circumstellar disk of MWC 349 ?

Observations of hydrogen recombination lines were made with the Infrared Space Observatory (ISO) towards the star MWC 349, where the α–transitions at short millimeter and submillimeter wavelengths have been found to be masers. Allα– lines in the ISO wavelength range from 2.4 to 190μm (quantum numbers n = 4 to 15) were detected. We find that amplification persists down to 19μm (n = 7), showing that these lines are infrared lasers. Our measurements permit for the first time a global view of the quantum number range of the recombination line laser/maser phenomenon, and suggest a peak amplification near n = 19 in the unexplored region around 300μm. The entire laser/maser phenomenon can be described quantitatively by case B recombination in a plasma with electron densities peaking at 108 cm−3, as expected in the corona of the circumstellar disk. In this model, the maser turns off at long wavelengths because the disk dimensions do not permit coherent gain paths longer than ∼ 250 a.u. At short wavelengths, inversion ceases probably because the level populations are thermalized as a consequence of trapping of line photons for n ≤ 6.