TY - JOUR
T1 - Herschel/HIFI deepens the circumstellar NH3 enigma
AU - Menten, K. M.
AU - Wyrowski, F.
AU - Alcolea, J.
AU - De Beck, E.
AU - Decin, L.
AU - Marston, A.P.
AU - Bujarrabal, V.
AU - Cernicharo, J.
AU - Dominik, C.
AU - Justtanont, K.
AU - de Koter, A.
AU - Melnick, G.
AU - Neufeld, D. A.
AU - Olofsson, H.
AU - Planesas, P.
AU - Schmidt, M.
AU - Schöier, F. L.
AU - Szczerba, R.
AU - Teyssier, D.
AU - Waters, L.B.F.M.
AU - Edwards, K.
AU - Olberg, M.
AU - Phillips, T.G.
AU - Morris, P.
AU - Salez, M.
AU - Caux, E.
PY - 2010
Y1 - 2010
N2 - 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, oxygenrich
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.
AB - 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, oxygenrich
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.
U2 - 10.1051/0004-6361/201015108
DO - 10.1051/0004-6361/201015108
M3 - Article
SN - 0004-6361
VL - 521
SP - L7/1-L7/5
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
IS - 1
ER -