Electron collisional excitation cross sections for Fe III and Fe VI and iron abundances in gaseous nebulae

Abstract

We have calculated electron impact excitation cross sections between most of the low metastable levels of Fe iii and Fe vi, using the close-coupling method. In addition, we have recalculated the electric quadrupole and magnetic dipole transition probabilities for these transitions in Fe vi. The statistical-equilibrium equations appropriate to a collisional/radiative model are solved to obtain level populations for each ion. Line emissivities have been calculated for the forbidden lines [Fe iii] and [Fe vi]. Comparison is made with intensities in several nebulae and peculiar stars. The iron abundances in the Orion Nebula, NGC 6720, N0C 7009, and NGC 7662 are discussed. NGC 7662 seems to have an iron abundance an order of magnitude smaller than the solar abundance. Subject headings: atomic processes - nebulae: abundances - transition probabilities