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We consider dust drift under the influence of stellar radiation pressureduring the pressure-driven expansion of an HII region using the chemo-dynamicalmodel MARION. Dust size distribution is represented by four dust types:conventional polycyclic aromatic hydrocarbons (PAHs), very small grains (VSGs),big grains (BGs) and also intermediate-sized grains (ISGs), which are largerthan VSGs and smaller than BGs. The dust is assumed to move at terminalvelocity determined locally from the balance between the radiation pressure andgas drag. As Coulomb drag is an important contribution to the overall gas drag,we evaluate a grain charge evolution within the HII region for each dust type.BGs are effectively swept out of the HII region. The spatial distribution ofISGs within the HII region has a double peak structure, with a smaller innerpeak and a higher outer peak. PAHs and VSGs are mostly coupled to the gas. Themean charge of PAHs is close to zero, so they can become neutral from time totime because of charge fluctuations. These periods of neutrality occur oftenenough to cause the removal of PAHs from the very interior of the HII region.For VSGs, the effect of charge fluctuations is less pronounced but stillsignificant. We conclude that accounting for charge dispersion is necessary todescribe the dynamics of small grains.

Dust dynamics and evolution in expanding HII regions. I. Radiative drift of neutral and charged grains