http://arxiv.org/abs/1506.06951
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 0.76×0.61. 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 5e-8 with respect to H2 at about 6″ from the star. Standard chemical models of IRC+10216 predict that CH3CN should form farther out in the envelope, at a radius of 15″, as other species such as polyyne radicals and cyanopolyynes. We discuss possible mechanims able to bring the region of formation of CH3CN to shorter radii.
M. Agundez, J. Cernicharo, G. Quintana-Lacaci, et. al.
Wed, 24 Jun 15
44/54
Comments: Submitted to ApJ
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