Context : Circumstellar envelopes ( CSEs ) of a variety of evolved stars have been found to contain ammonia ( NH _ { 3 } ) in amounts that exceed predictions from conventional chemical models by many orders of magnitude .

Aims : The observations reported here were performed in order to better constrain the NH _ { 3 } abundance in the CSEs of four , quite diverse , oxygen-rich stars using the NH _ { 3 } ortho J _ { K } = 1 _ { 0 } -0 _ { 0 } ground-state line .

Methods : We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH _ { 3 } J _ { K } = 1 _ { 0 } -0 _ { 0 } transition near 572.5 GHz , simultaneously with the ortho-H _ { 2 } O J _ { K _ { a } ,K _ { c } } = 1 _ { 1 , 0 } -1 _ { 0 , 1 } transition , toward VY CMa , OH 26.5+0.6 , IRC+10420 , and IK Tau .
We conducted non-LTE radiative transfer modeling with the goal to derive the NH _ { 3 } abundance in these objects ’ CSEs .
For the last two stars , Very Large Array imaging of NH _ { 3 } radio-wavelength inversion lines were used to provide further constraints , particularly on the spatial extent of the NH _ { 3 } -emitting regions .

Results : We find remarkably strong NH _ { 3 } emission in all of our objects with the NH _ { 3 } line intensities rivaling those of the ground state H _ { 2 } O line .
The NH _ { 3 } abundances relative to H _ { 2 } are very high and range from 2 \times 10 ^ { -7 } to 3 ~ { } \times 10 ^ { -6 } for the objects we have studied .

Conclusions : Our observations confirm and even deepen the circumstellar NH _ { 3 } enigma .
While our radiative transfer modeling does not yield satisfactory fits to the observed line profiles , it does lead to abundance estimates that confirm the very high values found in earlier studies .
New ways to tackle this mystery will include further Herschel observations of more NH _ { 3 } lines and imaging with the Expanded Very Large Array .