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 nonmetastable inversion lines. ^The most outstanding is intense maser emission from the (J,K) = (9,6) inversion line. ^Only in W51D are any masering nonmetastable transitions of para-NH3 to be found. ^Since the masering levels are more than 500 K 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- encompassing ammonia maser excitation model is still going on. ^(Author (revised))