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A new chemical model is presented for the carbon-rich circumstellar envelopeof the AGB star IRC+10216. The model includes shells of matter with densitiesthat are enhanced relative to the surrounding circumstellar medium. Thechemical model uses an updated reaction network including reactions from theRATE06 database and a more detailed anion chemistry. In particular, newmechanisms are considered for the formation of CN-, C3N- and C2H-, and for thereactions of hydrocarbon anions with atomic nitrogen and with the most abundantcations in the circumstellar envelope. New reactions involving H- are includedwhich result in the production of significant amounts of C2H- and CN- in theinner envelope. The calculated radial molecular abundance profiles for thehydrocarbons C2H, C4H and C6H and the cyanopolyynes HC3N and HC5N show narrowpeaks which are in better agreement with observations than previous models.Thus, the narrow rings observed in molecular microwave emission surroundingIRC+10216 are interpreted as arising in regions of the envelope where the gasand dust densities are greater than the surrounding circumstellar medium. Ourmodels show that CN- and C2H- may be detectable in IRC+10216 despite the verylow theorised radiative electron attachment rates of their parent neutralspecies. We also show that magnesium isocyanide (MgNC) can be formed in theouter envelope through radiative association involving Mg+ and the cyanopolyynespecies.

Density-enhanced gas and dust shells in a new chemical model for IRC+10216