Herschel/HIFI deepens the circumstellar NH3enigma

K M Menten,E De Beck,A De Koter,M Salez,P Planesas,L B F M Waters,A P Marston,J Cernicharo,Gary Melnick ,E Caux,F Wyrowski,C Dominik,J Alcolea,K Edwards,M Olberg,D Teyssier,H Olofsson,P Morris,R Szczerba,L Decin,T G Phillips,K Justtanont,M Schmidt,David A Neufeld ,F L Schöier

doi:10.1051/0004-6361/201015108

Abstract

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. 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. 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 latter 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 obtained for the ground state H2O line. The NH3 abundances relative to H2 are very high and range from 2 x 10-7 to 3 x 10-6 for the objects we have studied. 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 leads 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.

Highlights

Ammonia (NH3) was the first polyatomic molecule detected in an astronomical object (Cheung et al 1968)
NH3 has been detected toward a still limited, but diverse number of Circumstellar envelopes (CSEs) around evolved stars, first using infrared (IR) heterodyne absorption spectroscopy toward the high mass-loss asymptotic giant branch (AGB), extreme carbon star CW Leo (= IRC+10216; Betz et al 1979)
The observations were made with the two orthogonal HIFI receivers available for each band, which in all cases work in double side-band (DSB) mode

Summary

Introduction

Ammonia (NH3) was the first polyatomic molecule detected in an astronomical object (Cheung et al 1968). As to bona fide shocked regions like PPN outflows, Morris et al (1987) suggested that, for OH 231.8+4.2, N2, which binds most of the nitrogen, might be dissociated in the high-velocity gas and that the high NH3 abundance might be the result of a series of hydrogenation reactions Whether this can be confirmed by detailed chemical models remains to be explored. As described, we observed the NH3 JK = 10−00 ortho ground-state transition in O-rich stars of widely different natures and mass loss rates: the high mass-loss LPV IK Tau, the peculiar red supergiant VY CMa, the archetypical OH/IR star OH 26.5+0.6, and the hypergiant IRC+10420. 3.2, we present radiative transfer calculations conducted to model the observed line profiles, taking advantage of the constraints from the VLA imaging

Observations

Ammonia spectroscopy and astrophysics

VLA observations of inversion lines

NH3 versus H2O emission

Radiative transfer modeling and constraints on abundances

Summary and outlook