Herschel/HIFI deepens the circumstellar NH3 enigma

摘要

Context. Circumstellar envelopes (CSEs) of a varietyof evolved stars have been found to contain ammonia (NH3)in amounts that exceed predictions from conventional chemical models bymany orders of magnitude. Aims. The observations reported here were performedin order to better constrain the NH3 abundancein the CSEs of four, quite diverse, oxygen-rich stars using the NH3ortho JK= 10 - 00 ground-stateline. Methods. We used the Heterodyne Instrument for theFar Infrared aboard Herschel to observe the NH3JK = 10-00transition near 572.5GHz, simultaneously with the ortho-H2OJKa,Kc= 11,0 - 10,1 transition,toward VY CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTEradiative transfer modeling with the goal to derive the NH3abundance in these objects' CSEs. For the last two stars, Very LargeArray imaging of NH3 radio-wavelength inversionlines were used to provide further constraints, particularly on thespatial extent of the NH3-emitting regions. Results. We find remarkably strong NH3emission in all of our objects with the NH3 lineintensities rivaling those of the ground state H2Oline. The NH3 abundances relative to H2are very high and range from to for the objects we have studied. Conclusions. Our observations confirm and evendeepen the circumstellar NH3 enigma. While ourradiative transfer modeling does not yield satisfactory fits to theobserved line profiles, it does lead to abundance estimates thatconfirm the very high values found in earlier studies. New ways totackle this mystery will include further Herschelobservations of more NH3 lines and imaging withthe Expanded Very Large Array. Key words: stars: AGB and post-AGB -supergiants - circumstellar matter