  A model of key processes influencing the evolution of a hydrocarbon grain of
an arbitrary size under astrophysical conditions corresponding to ionized
hydrogen regions (HII regions) and supernova remnants is presented. The
considered processes include aromatization and photodestruction, sputtering by
electrons and ions, and shattering due to collisions between grains. The model
can be used to simulate the grain size distribution and the aromatization
degree during the evolution of HII regions and supernova remnants for a
specified radiation field, relative velocity of gas and dust, etc. The
contribution of various processes to the evolution of hydrocarbon dust grains
for parameters typical for the interstellar medium of our Galaxy is presented.
Small grains (less than 50 carbon atoms) should be fully aromatized in the
general interstellar medium. If larger grains initially have an aliphatic
structure, it is preserved to a substantial extent. Variations in the size
distribution of the grains due to their mutual collisions depend appreciably on
the adopted initial size distribution. For the MRN initial distribution a
significant redistribution of grain sizes is obtained, which increases the mass
fraction of smaller grains. Characteristic for an initial distribution from the
work of Jones et al. (2013), with high initial fraction of small grains, is a
general decrease in the number of grains of all sizes.
