The effect of radiation pressure on spatial distribution of dust inside HII regions

TL;DR

This study uses radiation hydrodynamic simulations to explore how radiation pressure influences dust distribution and grain size segregation within HII regions, aligning well with observational data.

Contribution

It introduces a detailed simulation model including dust grain size effects and Coulomb drag, revealing how radiation pressure shapes dust cavities and alters grain size distributions.

Findings

Dust cavity sizes (~0.2 pc) match observations.

Large grains are accelerated more efficiently, reducing the large-to-small grain ratio.

Stronger radiation sources weaken grain segregation due to increased Coulomb drag.

Abstract

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.