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J.P. Simpson, S.W.J. Colgan, R.H. Rubin, E.F. Erickson, M.R. Haas (NASA/Ames)
Numerous studies have shown that the H II regions in the inner parts of our Galaxy are of low excitation, meaning that singly ionized ions generally predominate over doubly ionized ions. This is true of other massive galaxies as well, and is usually attributed to the H II regions being of higher than solar metallicity. The inner parts of the Galaxy cannot be studied at optical wavelengths because of the high extinction in the plane of the Galaxy. Consequently, the ratio of secondary to primary nucleosynthesis has in the past been determined from the reliably measured far-infrared N++ and O++ lines with corrections for unobservable ionization states like O+. These corrections, estimated from H II region model calculations, can be as large as an order of magnitude, depending on stellar effective temperature and elemental abundances. In order to make the corrections smaller and hence the metallicities more reliable, we discuss here a different proxy for the secondary/primary nucleosynthesis, namely the (N+ + N++)/S++ ratio (all three species have reasonably bright FIR lines). This diagnostic ratio has the advantage that it covers the H II region ionization states well and the maximum correction for S+ or S+++ is rarely more than a factor of two in low excitation H II regions. These points will be illustrated with data from ISO and the Kuiper Airborne Observatory, with suggestions for future observations by SIRTF and SOFIA.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.