  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 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 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 ~ 45 km/s, well separated from the quasi-thermal emission
near 60 km/s. These features were absent 25 years ago and show a velocity drift
of about +0.2 km/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 57 km/s component of the (9,6) maser line is found to be strongly linearly
polarized. 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.
