Dust dynamics and evolution in HII regions - II. Effects of dynamical
  coupling between dust and gas
  In this paper, we extend the study initiated in PaperI by modelling grain
ensemble evolution in a dynamical model of an expanding HII region and checking
the effects of momentum transfer from dust to gas. The radiation pressure on
the dust, the dust drift, and the lug on the gas by the dust are all important
process that should be considered simultaneously to describe the dynamics of
HII regions. With accounting for the momentum transfer from the dust to the
gas, the expansion time of the HII region is notably reduced (for our model of
RCW120, the time to reach the observed radius of the HII region is reduced by a
factor of 1.5). Under the common approximation of frozen dust, where there is
no relative drift between the dust and gas, the radiation pressure from the
ionizing star drives the formation of the very deep gas cavity near the star.
Such a cavity is much less pronounced when the dust drift is taken into
account. The dust drift leads to the two-peak morphology of the dust density
distribution and significantly reduces the dust-to-gas ratio in the ionized
region (by a factor of 2 to 10). The dust-to-gas ratio is larger for higher
temperatures of the ionizing star since the dust grains have a larger electric
charge and are more strongly coupled to the gas.
