INFRARED-LASERS IN THE CIRCUMSTELLAR DISK OF MWC-349

Observations of hydrogen recombination lines were made with the Infrar ed Space Observatory (ISO) towards the stat MWC 349, where the alpha-t ransitions at short millimeter and submillimeter wavelengths have been found to be masers. All alpha-lines in the ISO wavelength range from 2.4 to 190 mu m (quantum numbers n = 4 to 15) were detected. We find t hat amplification persists down to 19 mu m (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 l aser/maser phenomenon, and suggest a peak amplification near n = 19 in the unexplored region around 300 mu m. The entire laser/maser phenome non can be described quantitatively by case B recombination in a plasm a with electron densities peaking at 10(8) cm(-3), as expected in the corona of the circumstellar disk. In this model, the maser turns off a t long wavelengths because the disk dimensions do not permit coherent gain paths longer than similar to 250 a.u. At short wavelengths, inver sion ceases probably because the level populations are thermalized as a consequence of trapping of line photons for n less than or equal to 6.