2013A&A...549A..90H

With the 100-m telescope at Effelsberg, 19 ammonia (NH₃) maser lines have been detected toward the prominent massive star forming region W51-IRS2. Eleven of these inversion lines, the (J, K) = (6, 2), (5, 3), (7, 4), (8, 5), (7, 6), (7, 7), (9, 7), (10, 7), (9, 9), (10, 9), and (12, 12) transitions, are classified as masers for the first time in outer space. All detected masers are related to highly excited inversion doublets. The (5, 4) maser originates from an inversion doublet ∼340K above the ground state, while the (12, 12) transition, at∼1450K, is the most highly excited NH₃ maser line so far known. Strong variability is seen not only in ortho- but also in para-NH₃ transitions. Bright narrow emission features are observed, for the first time, in (mostly) ortho-ammonia transitions, at V_LSR∼45km/s, well separated from the quasi-thermal emission near 60km/s. These features were absent ∼25 years ago and show a velocity drift of about +0.2km/s/yr. The component is likely related to the SiO maser source in W51-IRS2 and a possible scenario explaining the velocity drift is outlined. The 57km/s component of the (9, 6) maser line is found to be strongly linearly polarised. Maser emission in the (J, K) to (J+1, K) inversion doublets is strictly forbidden by selection rules for electric dipole transitions in the ground vibrational state. However, such pairs (and even triplets with (J+2, K)) are common toward W51-IRS2. Similarities in line widths and velocities indicate that such groups of maser lines arise from the same regions, which can be explained by pumping through vibrational excitation. The large number of NH₃ maser lines in W51-IRS2 is most likely related to the exceptionally high kinetic temperature and NH₃ column density of this young massive star forming region.