Iron abundance in HII regions

Optical CCD spectra are used to determine the Fe abundances at several positions inside seven bright Galactic H II regions. The observed [Fe III] line ratios are compared with the predictions of different sets of collision strengths and transition probabilities for this ion to select the atomic data providing the best fit to the observations. The values found for the Fe++ and Fe+ abundances, along with ionization correction factors for the contribution of Fe3+, obtained from available grids of photoionized models, imply that the Fe/O ratio in the ionized gas is between 2% and 30% of solar. The Fe abundances derived for each area are correlated both with the degree of ionization and the colour excess. A possible explanation is suggested, namely the presence of a population of small grains, probably originating from the fragmentation of larger grains. These small grains would release Fe atoms into the gas after the absorption of energetic photons; the small grains surviving this destruction process would be swept out of the ionized region by the action of radiation pressure or stellar winds. An indication of a further and more efficient destruction agent is given by the high Fe abundance derived for a position sampling the optical jet H 399 in M 20, where dust destruction due to shock waves has presumably taken place.