Herschel/HIFI deepens the circumstellar NH3 enigma
Journal article, 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.
stars
supergiants
circumstellar matter
inversion lines
emission
stars: AGB and post-AGB
infrared observations
molecules
water
interstellar ammonia
vy canis majoris
irc+10216
mass-loss
bearing
Author
K. M. Menten
Max Planck Society
F. Wyrowski
Max Planck Society
J. Alcolea
Elvire De Beck
KU Leuven
L. Decin
KU Leuven
University of Amsterdam
A. P. Marston
European Space Agency (ESA)
V. Bujarrabal
J. Cernicharo
Centro de Astrobiologia (CAB)
C. Dominik
European Space Agency (ESA)
Radboud University
Kay Justtanont
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
A. de Koter
Utrecht University
European Space Agency (ESA)
G. J. Melnick
Harvard-Smithsonian Center for Astrophysics
D. A. Neufeld
Johns Hopkins University
Hans Olofsson
Chalmers, Earth and Space Sciences, Onsala Space Observatory
P. Planesas
Atacama Large Millimeter-submillimeter Array (ALMA)
M. Schmidt
Polish Academy of Sciences
Fredrik Schöier
Chalmers, Earth and Space Sciences, Onsala Space Observatory
R. Szczerba
Polish Academy of Sciences
D. Teyssier
European Space Agency (ESA)
Lbfm Waters
KU Leuven
University of Amsterdam
K. Edwards
University of Waterloo
Netherlands Institute for Space Research (SRON)
Michael Olberg
Chalmers, Earth and Space Sciences, Onsala Space Observatory
T. G. Phillips
California Institute of Technology (Caltech)
P. Morris
California Institute of Technology (Caltech)
M. Salez
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
Ecole Normale Superieure (ENS)
E. Caux
Institut National des Sciences de l'Univers
University of Toulouse
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 521 1 Article Number: L7- L7Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/201015108