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

TL;DR

This study uses ALMA observations to reveal that methyl cyanide in IRC+10216 is distributed in a hollow shell close to the star, challenging existing chemical models and suggesting complex formation processes.

Contribution

First high-resolution ALMA imaging of CH3CN in IRC+10216 showing a unique shell distribution not predicted by standard models.

Findings

CH3CN forms in a shell at 1-2" from the star

Distribution differs from other molecules like polyyne radicals

Standard models predict a different spatial distribution

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.