Ammonia masers in the interstellar medium

Abstract

Since 1986, 10 ammonia masers have been found. Most of these are found in the sources W51D, W51e1/e2, NGC7538, DR21 and W33. Among them are a few masers or maser candidates, arising from metastable inversion lines. There are many more masers arising from non-metastable inversion lines. The most outstanding is intense maser emission from the (J,K)=(9,6) inversion line. Only in W51D are any masering non-metastable transitions of para-NH₃ to be found. Since the masering levels are more than 500K above ground state, there are a large number of levels populated, and the excitation scheme must be complex. It is likely that there is no unique excitation scheme for all types of ammonia masers. Although there have been a few attempts to model the ammonia maser excitation, including excitation involving vibrationally excited levels, the quest for an all-encompasing ammonia maser excitation model is still going on.