  A new chemical model is presented for the carbon-rich circumstellar envelope
of the AGB star IRC+10216. The model includes shells of matter with densities
that are enhanced relative to the surrounding circumstellar medium. The
chemical model uses an updated reaction network including reactions from the
RATE06 database and a more detailed anion chemistry. In particular, new
mechanisms are considered for the formation of CN-, C3N- and C2H-, and for the
reactions of hydrocarbon anions with atomic nitrogen and with the most abundant
cations in the circumstellar envelope. New reactions involving H- are included
which result in the production of significant amounts of C2H- and CN- in the
inner envelope. The calculated radial molecular abundance profiles for the
hydrocarbons C2H, C4H and C6H and the cyanopolyynes HC3N and HC5N show narrow
peaks which are in better agreement with observations than previous models.
Thus, the narrow rings observed in molecular microwave emission surrounding
IRC+10216 are interpreted as arising in regions of the envelope where the gas
and dust densities are greater than the surrounding circumstellar medium. Our
models show that CN- and C2H- may be detectable in IRC+10216 despite the very
low theorised radiative electron attachment rates of their parent neutral
species. We also show that magnesium isocyanide (MgNC) can be formed in the
outer envelope through radiative association involving Mg+ and the cyanopolyyne
species.
