The effect of radiation pressure on spatial distribution of dust inside
  HII regions
  We investigate the impact of radiation pressure on spatial dust distribution
inside H$_\mathrm{II}$ regions using one-dimensional radiation hydrodynamic
simulations, which include absorption and re-emission of photons by dust. In
order to investigate grain size effects as well, we introduce two additional
fluid components describing large and small dust grains in the simulations.
Relative velocity between dust and gas strongly depends on the drag force. We
include collisional drag force and coulomb drag force. We find that, in a
compact H$_\mathrm{II}$ region, a dust cavity region is formed by radiation
pressure. Resulting dust cavity sizes (~0.2 pc) agree with observational
estimates reasonably well. Since dust inside an H$_\mathrm{II}$ region is
strongly charged, relative velocity between dust and gas is mainly determined
by the coulomb drag force. Strength of the coulomb drag force is about 2-order
of magnitude larger than that of the collisional drag force. In addition, in a
cloud of mass $10^5$ $M_{\odot}$, we find that the radiation pressure changes
the grain size distribution inside H$_\mathrm{II}$ regions. Since large (0.1
$\mathrm{\mu m}$) dust grains are accelerated more efficiently than small (0.01
$\mathrm{\mu m}$) grains, the large to small grain mass ratio becomes smaller
by an order of magnitude compared with the initial one. Resulting dust size
distributions depend on the luminosity of the radiation source. The large and
small grain segregation becomes weaker when we assume stronger radiation
source, since dust grain charges become larger under stronger radiation and
hence coulomb drag force becomes stronger.
