The polymerization of acetylene, hydrogen cyanide, and carbon chains in the neutral layers of carbon-rich proto-planetary nebulae

I present chemical evolution models for the neutral layers of proto-planetary nebulae in order to explain the high abundance of polyynes, cyanopolyynes, methyl-polyynes, benzene, and other C-bearing species found in CRL 618 by Cernicharo et al. In the neutral regions behind the H II region, the radiation field produces a rich photochemistry on timescales shorter than the dynamical evolution of the central H II region, leading to the formation of large carbon-rich molecules C-n H-2, HC(2n+1)N, and C-n. Reactions between radicals H and H-2 play a crucial role in maintaining a high abundance for these species in spite of the strong radiation field emerging from the central star. The models predict that proto-planetary nebulae in the early stages of evolution will have high abundances for species such as HC(2n+1)N, C-n H-4, and CnH3N. The role of reactions between neutrals and radicals in the growth of aromatic molecules is discussed. The large abundance derived for small cumulenes suggests that the growth of small carbon grains with C/H much greater than 1 could be dominated by reactions between polyynes and cumulenes. The models also explain the observed abundances of H2CO, OH, H2O, CO2, and [O I] in the region where CO is photodissociated.