THE FORMATION OF CARBON-CHAIN MOLECULES IN IRC+10216

A schematic model for the synthesis of chain molecules (both cyanopolynes and polyacetylenes) is developed for the C-rich AGB star IRC + 10216. The key processes are neutral reactions of photoproduced CN and C2H radicals with acetylene and other hydrocarbons, shown to be fast in recent low-temperature laboratory experiments. Abundance calculations based on our photochemical model of the envelope demonstrate that the chains are efficiently produced in the outer circumstellar envelope, in accord with abundance distributions deduced from maps made at millimeter wavelengths. The results extend the scope of the photochemical model to an important class of species observed in the envelope of IRC + 10216.