2015ApJ...814..143A

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 CH₃ CN 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 indicates that methyl cyanide is not formed in either the stellar photosphere or far in the outer envelope, but at radial distances as short as 1"-2", reaching a maximum abundance of ∼0.02 molecules/cm3 at 2" from the star. Standard chemical models of IRC +10216 predict that the bulk of CH₃ CN 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.