Letter to the Editor

Herschel/HIFI deepens the circumstellar NH₃ enigma ^*,**

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: kmenten@mpifr.de
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

Received: 31 May 2010
Accepted: 29 June 2010

Abstract

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 J_K = 1₀–0₀ ground-state line.

Methods. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH₃ J_K = 1₀–0₀ transition near 572.5 GHz, simultaneously with the ortho-H₂O 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 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.