The silicon chemistry of IRC+10°216

Abstract

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°216. We find, in agreement with previous work, that SiC2 must be a parent and that neutral-neutral reactions are necessary to form SiC4. Our inclusion of many more neutral-neutral reactions has a limited influence on the calculated distributions of the Si-C compounds. In particular, the suggestion 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 likely to arise from a density peak or peaks along the line of sight. We show that the presence of SiH2 and the Si-N chemistry require that SiH4 supplies the bulk of available silicon in the photochemical region. The column densities and radial distributions of all the observed Si species are compared with their observed values, and column densities are given for several molecules predicted to be detectable in IRC + 10°216 - SiH, SiH3, SiC2H, HCSi, HNSi and SiC3.