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With the 100-m telescope at Effelsberg, 19 ammonia (NH3) maser lines havebeen detected toward the prominent massive star forming region W51-IRS2. Elevenof 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 masersfor the first time in outer space. All detected masers are related to highlyexcited inversion doublets. The (5,4) maser originates from an inversiondoublet 340 K above the ground state, while the (12,12) transition, at 1450 K,is the most highly excited NH3 maser line so far known. Strong variability isseen not only in ortho- but also in para-NH3 transitions. Bright narrowemission features are observed, for the first time, in (mostly) ortho-ammoniatransitions, at V ~ 45 km/s, well separated from the quasi-thermal emissionnear 60 km/s. These features were absent 25 years ago and show a velocity driftof about +0.2 km/s/yr. The component is likely related to the SiO maser sourcein W51-IRS2 and a possible scenario explaining the velocity drift is outlined.The 57 km/s component of the (9,6) maser line is found to be strongly linearlypolarized. Maser emission in the (J,K) to (J+1,K) inversion doublets isstrictly forbidden by selection rules for electric dipole transitions in theground vibrational state. However, such pairs (and even triplets with (J+2,K))are common toward W51-IRS2. Similarities in line widths and velocities indicatethat such groups of maser lines arise from the same regions, which can beexplained by pumping through vibrational excitation. The large number of NH3maser lines in W51-IRS2 is most likely related to the exceptionally highkinetic temperature and NH3 column density of this young massive star formingregion.

Ammonia in the hot core W51-IRS2: 12 new maser lines and a maser component with a velocity drift