The silicon chemistry of IRC+10 degrees 216

We have modelled the circumstellar chemistry of SiC, SiC2, SiC4, SiH2, SiH4 , HNSi and SiN in the carbon-rich envelope of the variable star IRC + 10 de grees 216. We find, in agreement with previous work, that SiC2 must be a pa rent and that neutral-neutral reactions are necessary to form SiC4. Our inc lusion of many more neutral-neutral reactions has a limited influence on th e calculated distributions of the Si-C compounds. In particular, the sugges tion of Lucas et al. - that the SiC2 distribution peaks by almost an order of magnitude in the outer envelope relative to its inner envelope abundance - cannot be explained in any of our models. This enhancement is most likel y to arise from a density peak or peaks along the line of sight. We show th at the presence of SiH2 and the Si-N chemistry require that SiH4 supplies t he bulk of available silicon in the photochemical region. The column densit ies and radial distributions of all the observed Si species are compared wi th their observed values, and column densities are given for several molecu les predicted to be detectable in IRC + 10 degrees 216 - Sill, SiH3, SiC2H, HCSi, HNSi and SiC3.