Abstract : 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
2O J
Ka , Kc = 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
2O line. The NH
3 abundances relative to H
2 are very high and range from 2×10
-7 to 3×10
-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.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendix A (page 5) is only available in electronic form at
http://www.aanda.org