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We have carried out an investigation of the chemical evolution of gas indifferent carbon-rich circumstellar environments. Previous studies have tendedto invoke terrestrial flame chemistries, based on acetylene (C2H2) combustionto model the formation of carbon dust, via Polycyclic Aromatic Hydrocarbons(PAHs). In this work we pay careful attention to the accurate calculation ofthe molecular photoreaction rate coefficients to ascertain whether there is auniversal formation mechanism for carbon dust in strongly irradiatedastrophysical environments. A large number of possible chemical channels mayexist for the formation of PAHs, so we have concentrated on the viability ofthe formation of the smallest building block species, C2H2, in a variety ofcarbon-rich stellar outflows. C2H2 is very sensitive to dissociation by UVradiation. This sensitivity is tested, using models of the time-dependentchemistry. We find that C2H2 formation is sensitive to some of the physicalparameters and that in some known sources of dust-formation it can never attainappreciable abundances. Therefore multiple (and currently ill-defined)dust-formation channels must exist.

Is acetylene essential for carbon dust formation?