Infrared lasers in the circumstellar disk of MWC 349

Abstract

Observations of hydrogen recombination lines were made with the Infrared Space Observatory (ISO) towards the star MWC349, where the alpha -transitions at short millimeter and submillimeter wavelengths have been found to be masers. All alpha -line s in the ISO wavelength range from 2.4 to 190mu m (quantum numbers n=4 to 15) were detected. We find that amplification persists down to 19mu 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 laser/maser phenomenon, and suggest a peak amplification near n=19 in the unexplored region around 300mu m. The entire laser/maser phenomenon can be described quantitatively by case B recombination in a plasma 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 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.