Herschel/HIFI deepens the circumstellar NH_3 enigma

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 NH3 abundance in the CSEs of four, quite diverse, oxygen rich stars using the NH_3 ortho JK = 1_0−0_0 ground-state line. Methods. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH_3 JK = 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 NH3 emission in all of our objects with the NH_3 line intensities rivaling those of the ground state H_2O line. The NH3 abundances relative to H2 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.