Herschel/HIFI deepens the circumstellar NH3 enigma
Artikel i vetenskaplig tidskrift, 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 J(K) = 1(0)-0(0) ground-state line. Methods. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH3 J(K) = 1(0)-0(0) 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 H-2 are very high and range from 2 x 10(-7) to 3 x 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.



inversion lines


interstellar ammonia


vy canis majoris

circumstellar matter



infrared observations



stars: AGB and post-AGB


K. M. Menten

Max Planck-institutet

F. Wyrowski

Max Planck-institutet

J. Alcolea

Elvire De Beck

KU Leuven

L. Decin

Universiteit Van Amsterdam

KU Leuven

A. P. Marston

European Space Astronomy Centre

V. Bujarrabal

J. Cernicharo

CSIC-INTA - Centro de Astrobiologia (CAB)

C. Dominik

Radboud University Nijmegen

European Space Astronomy Centre

Kay Justtanont

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

A. de Koter

Sterrekundig Instituut Utrecht

European Space Astronomy Centre

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

D. A. Neufeld

Johns Hopkins University

Hans Olofsson

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

P. Planesas

Atacama Large Millimeter-submillimeter Array

M. Schmidt

Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

Fredrik Schöier

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

R. Szczerba

Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

D. Teyssier

European Space Astronomy Centre

Lbfm Waters

Universiteit Van Amsterdam

KU Leuven

K. Edwards

University of Waterloo

SRON Netherlands Institute for Space Research

Michael Olberg

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

T. G. Phillips

California Institute of Technology

P. Morris

California Institute of Technology

M. Salez

École Normale Supérieure

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

E. Caux

Universite de Toulouse

Institut National des Sciences de l'Univers

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 521 Article Number: L7- L7


Astronomi, astrofysik och kosmologi