The peculiar distribution of CH3CN in IRC+10216 seen by ALMA

Abstract

IRC+10216 is a circumstellar envelope around a carbon-rich evolved star which contains a large variety of molecules. According to interferometric observations, molecules are distributed either concentrated around the central star or as a hollow shell with a radius of 15". We present ALMA Cycle 0 band 6 observations of the J=14-13 rotational transition of CH3CN in IRC+10216, obtained with an angular resolution of 0.76x0.61". The bulk of the emission is distributed as a hollow shell located at just 2" from the star, with a void of emission in the central region up to a radius of 1". This spatial distribution is markedly different from those found to date in this source for other molecules. Our analysis indicate that methyl cyanide is not formed neither in the stellar photosphere nor far in the outer envelope, but at radial distances as short as 1-2", reaching a maximum abundance of 0.02 molecules cm-3 at 2" from the star. Standard chemical models of IRC+10216 predict that the bulk of CH3CN molecules should be present at a radius of 15", where other species such as polyyne radicals and cyanopolyynes are observed, with an additional inner component within 1" from the star. The non-uniform structure of the circumstellar envelope and grain surface processes are discussed as possible causes of the peculiar distribution of methyl cyanide in IRC+10216.

Accepted for publication in ApJ. Revised version includes updates following the recommendations of our referee

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• Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
• Discovery of the first Ca-bearing molecule in space: CaNC
• The impact of stellar companion UV photons on the chemistry of the circumstellar environments of AGB stars
• The Role of Internal Photons on the Chemistry of the Circumstellar Envelopes of AGB Stars
• Methyl cyanide (CH3CN) and propyne (CH3CCH) in the low mass protostar IRAS 16293-2422
• Investigating Anomalous Photochemistry in the Inner Wind of IRC+10216 Through Interferometric Observations of HC${}_3$N
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• Clues to NaCN formation
• Chemistry in AGB stars: successes and challenges
• The Role of Ultraviolet Photons in Circumstellar Astrochemistry
• Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS