We investigate the impact of radiation pressure on spatial dust distribution inside \ion Hii 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 \ion Hii region , a dust cavity region is formed by radiation pressure .
Resulting dust cavity sizes ( \sim 0.2 pc ) agree with observational estimates reasonably well .
Since dust inside an \ion Hii 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 _ { \sun } , we find that the radiation pressure changes the grain size distribution inside \ion Hii regions .
Since large ( 0.1 \micron ) dust grains are accelerated more efficiently than small ( 0.01 \micron ) 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 .