Is acetylene essential for carbon dust formation?
  We have carried out an investigation of the chemical evolution of gas in
different carbon-rich circumstellar environments. Previous studies have tended
to invoke terrestrial flame chemistries, based on acetylene (C2H2) combustion
to model the formation of carbon dust, via Polycyclic Aromatic Hydrocarbons
(PAHs). In this work we pay careful attention to the accurate calculation of
the molecular photoreaction rate coefficients to ascertain whether there is a
universal formation mechanism for carbon dust in strongly irradiated
astrophysical environments. A large number of possible chemical channels may
exist for the formation of PAHs, so we have concentrated on the viability of
the formation of the smallest building block species, C2H2, in a variety of
carbon-rich stellar outflows. C2H2 is very sensitive to dissociation by UV
radiation. This sensitivity is tested, using models of the time-dependent
chemistry. We find that C2H2 formation is sensitive to some of the physical
parameters and that in some known sources of dust-formation it can never attain
appreciable abundances. Therefore multiple (and currently ill-defined)
dust-formation channels must exist.
