Tracing the chemistry in the clumpy shells around carbon-rich AGB-stars with the VLA

Denise Keller, K.M. Menten, T. Kaminski & M.J. Claussen

Max Planck Institute for Radioastronomy, Germany

Asymptotic giant branch (AGB) stars are a unique site for a rich nucleosynthesis. Through mixing and strong mass loss the processed material from the interior of the star, mainly carbon, is brought to the surface. Driven by stellar pulsation, the material gets elevated away from the stellar surface to lower temperature regions in which molecules form. At temperatures below about 1500 K dust starts to arise and a circumstellar outflow is driven, which builds up a circumstellar envelope (CSE). In many cases the molecular spatial distribution in CSEs departs from spherical symmetry and is clumpy. There is no global view on the morphology of CSEs on different scales, not even for the nearby (~ 130 pc), high mass-loss and carbon-rich AGB-star IRC+10216. This object is one of the most important chemical laboratories in astronomy, still with many enigmas. Carbon-bearing molecules in the CSE of IRC+10216 are distributed in clumpy shells similar to the dust which indicates a gas-dust coupling. It is not well understood to what extent these density-enhanced shells influence the chemistry of carbon molecules in carbon-rich CSEs in general. Chemical models and observations are inconsistent. Observing many molecules at multiple transitions improves our understanding of the chemical network, the influence of shocks and UV-radiation and constrains the structure and dynamics in CSEs. This is crucial knowledge for determining the star's mass loss rate and thermal structure, and subsequently chemical abundances. We have obtained a new key dataset using the expanded Jansky Very Large Array. It consists of a spectral and imaging survey of IRC+10216 which covers 18-50 GHz with a sensitivity on a mJy level and a resolution of 1 arcsec. This survey traces the molecular distribution in unprecedented detail and strongly constrains the chemistry in these clumpy shells. So far line emission from over 20 spieces has been identified and imaged, for several lines from different energy levels, isotopologues and vibrationally excited states. I will present our new dataset and highlight first results important for understanding the chemistry and origin of circumstellar shells and their link to the interior of AGB stars.

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