Letter to the Editor
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: email@example.com
2 Observatorio Astronómico Nacional (IGN), Alfonso XII No 3, 28014 Madrid, Spain
3 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
4 Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
5 European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
6 Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Spain
7 CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
8 Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands
9 Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
10 Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
11 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
12 The Johns Hopkins University, 3400 North Charles St, Baltimore, MD 21218, USA
13 Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
14 Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
15 N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
16 Department of Physics and Astronomy, University of Waterloo, Waterloo, ON Canada N2L 3G1, Canada
17 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
18 California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
19 Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
20 Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
21 LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
22 Institute Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
23 CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
Accepted: 29 June 2010
Context. Circumstellar envelopes (CSEs) of a variety of evolved stars have been found to contain ammonia (NH3) 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 NH3 ortho JK = 10–00 ground-state line.
Methods. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH3 JK = 10–00 transition near 572.5 GHz, simultaneously with the ortho-H2O JKa, Kc = 11,0 - 10,1 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 NH3 abundance in these objects' CSEs. For the last two stars, Very Large Array imaging of NH3 radio-wavelength inversion lines were used to provide further constraints, particularly on the spatial extent of the NH3-emitting regions.
Results. We find remarkably strong NH3 emission in all of our objects with the NH3 line intensities rivaling those of the ground state H2O 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 NH3 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 NH3 lines and imaging with the Expanded Very Large Array.
Key words: stars: AGB and post-AGB / supergiants / circumstellar matter
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
© ESO, 2010