Impact of PAH photodissociation on the formation of small hydrocarbons
  in the Orion Bar and the Horsehead PDRs
  We study whether polycyclic aromatic hydrocarbons (PAHs) can be a weighty
source of small hydrocarbons in photo-dissociation regions (PDRs). We modeled
the evolution of 20 specific PAH molecules in terms of dehydrogenation and
destruction of the carbon skeleton under the physical conditions of two
well-studied PDRs, the Orion Bar and the Horsehead nebula which represent
prototypical examples of PDRs irradiated by "high" and "low" ultraviolet
radiation field. PAHs are described as microcanonical systems. The acetylene
molecule is considered as the main carbonaceous fragment of the PAH
dissociation as it follows from laboratory experiments and theory. We estimated
the rates of acetylene production in gas phase chemical reactions and compared
them with the rates of the acetylene production through the PAH dissociation.
It is found that the latter rates can be higher than the former rates in the
Orion Bar at $A_{\rm V}<1$ and also at $A_{\rm V}>3.5$. In the Horsehead
nebula, the chemical reactions provide more acetylene than the PAH
dissociation. The produced acetylene participate in the reactions of the
formation of small hydrocarbons (C$_2$H, C$_3$H, C$_3$H$^{+}$, C$_3$H$_2$,
C$_4$H). Acetylene production via the PAH destruction may increase the
abundances of small hydrocarbons produced in gas phase chemical reactions in
the Orion Bar only at $A_{\rm V}>3.5$. In the Horsehead nebula, the
contribution of PAHs to the abundances of the small hydrocarbons is negligible.
We conclude that the PAHs are not a major source of small hydrocarbons in both
PDRs except some locations in the Orion Bar.
