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