Grain charging in HII regions

Equilibrium grain potentials have been calculated as a function of radial position and for a wide range of electron densities in HII regions ionized by stars of spectral type O5 and B0. Results are presented for both graphite (low yield photoemitter) and aluminium oxide (high yield photoemitter) for which laboratory photoemission data has been obtained. The results for aluminium oxide should approximate the behaviour expected of dielectric grains — e.g., silicates which may be present in HII regions. The importance of charging is discussed in relation to the growth and motion of grains in these regions.