  We study evolution of hydrogenated amorphous carbon (HAC) grains under harsh
UV radiation in photo-dissociation regions (PDRs) near young massive stars. Our
aim is to evaluate the impact of the HAC grains on formation of observed small
hydrocarbons: C$_2$H, C$_2$H$_2$, C$_3$H$^+$, C$_3$H, C$_3$H$_2$, C$_4$H, in
PDRs. We developed a microscopic model of the HAC grains based on available
experimental results. The model includes processes of photo- and
thermodesorption, accretion of hydrogen and carbon atoms and subsequent
formation of carbonaceous mantle on dust surface. H$_2$, CH$_4$, C$_2$H$_2$,
C$_2$H$_4$, C$_2$H$_6$, C$_3$H$_4$, C$_3$H$_6$, C$_3$H$_8$ are considered as
the main fragments of the HAC photo-destruction. We simulated evolution of the
HAC grains under the physical conditions of two PDRs, the Orion Bar and the
Horsehead nebula. We estimated the production rates of the HAC fragments in gas
phase chemical reactions and compared them with the production rates of
fragments due to the HAC destruction. The latter rates may dominate under some
conditions, namely, at A$_V$=0.1 in both PDRs. We coupled our model with the
gas-grain chemical model MONACO and calculated abundances of observed small
hydrocarbons. We conclude that the contribution of the HAC destruction
fragments to chemistry is not enough to match the observed abundances, although
it increases the abundances by several orders of magnitude in the Orion Bar at
A$_V$=0.1. Additionally, we found that the process of carbonaceous mantle
formation on dust surface can be an inhibitor for the formation of observed
small hydrocarbons in PDRs.

Astrophysics of Galaxies;Solar and Stellar Astrophysics