Probing ammonia formation in oxygen-rich circumstellar envelopes

Ka Tat Wong, Karl M. Menten, Tomasz Kaminski, Friedrich Wyrowski & Helmut Wiesemeyer

Max Planck Institute for Radio Astronomy, Germany

The ammonia (NH₃) chemistry in circumstellar envelopes (CSEs) of evolved stars is poorly understood. Thermodynamic equilibrium chemistry predicts an abundance (relative to H₂) of 10^-12 for cool giants, orders of magnitude below observational results (>10^-7). To date, there is no consistent model for the origin of the high NH₃ abundances. Pulsation-driven shocks in the inner CSEs and general enhancement in nitrogen production from nucleosynthesis have been speculated to explain the enigma. To better understand the physical conditions necessary for the formation of NH₃ molecules and constrain the spatial distribution of its abundance, we observed the NH₃ rotational and inversion emission lines from multiple oxygen-rich stars with the Herschel/HIFI and the Very Large Array. Our 1-D radiative transfer modelling with RATRAN (1) confirms the high NH₃ abundance in the CSEs and (2) suggests that radiative pumping via the vibrationally excited states (v₂=1) has to be included in order to get consistent fitting of the spectral lines.

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