SIMBAD references

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. All α-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 10⁸cm^–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 ∼250a.u. At short wavelengths, inversion ceases probably because the level populations are thermalized as a consequence of trapping of line photons for n≤6.