Title: Heavy Element Abundances in Ionized Nebulae
Author(s):  Y. Tsamis (Oral)

ABSTRACT
The chemical composition of ionized nebulae has been investigated employing emission line spectroscopy, using deep optical spectra obtained at several southern hemisphere telescopes. Gas phase elemental abundances have been derived for fifteen Planetary Nebulae and five H II regions, in the Galaxy and the Magellanic Clouds. The prime aim of this project was to derive accurate C, N, O abundances, measuring their weak optical recombination line (ORL) spectra, aided by atomic data that have recently become available. Heavy element abundances relative to hydrogen from ionic ORLs should be intrinsically secure, since they do not depend on the adopted temperature structure of the nebulae under study. In contrast, the classical approach makes use of collisionally excited lines (CELs) which are subject to severe temperature and density dependence. The two methods yield substantially different results, with ORL abundances being significantly larger than CEL abundances, for a wide range of nebulae. This phenomenon, which was known from previous studies of a few galactic PN, is established here to be a common occurrence in many more objects, including Magellanic Cloud PN; more importantly it is shown to be present in H II regions as well. These findings may have important implications in the light of current stellar and galactic evolution theory and are investigated using semi-empirical modeling in an attempt to determine the true heavy element gaseous abundances of these objects.