  The generation of infrared (IR) radiation and the observed IR intensity
distribution at wavelengths of 8, 24, and 100 micron in the ionized hydrogen
region around a young, massive star is investigated. The evolution of the HII
region is treated using a self-consistent chemical-dynamical model in which
three dust populations are included -- large silicate grains, small graphite
grains, and polycyclic, aromatic hydrocarbons (PAHs). A radiative transfer
model taking into account stochastic heating of small grains and macromolecules
is used to model the IR spectral energy distribution. The computational results
are compared with Spitzer and Herschel observations of the RCW 120 nebula. The
contributions of collisions with gas particles and the radiation field of the
star to stochastic heating of small grains are investigated. It is shown that a
model with a homogeneous PAH content cannot reproduce the ring-like
IR-intensity distribution at 8 micron. A model in which PAHs are destroyed in
the ionized region provides a means to explain this intensity distribution.
This model is in agreement with observations for realistic characteristic
destruction times for the PAHs.

Astrophysics of Galaxies