Chemical Abundances of Planetary Nebulae: A UV to Far-IR Case Study of NGC 6153

Abstract

An outstanding problem in nebular abundance studies is that the C, N and O abundances derived from optical recombination lines are systematically higher than those derived from UV and optical collisionally excited forbidden lines, which has been attributed to the presence of significant temperature and/or density fluctuations, yielding apparent lower forbidden line abundances. The physics leading to such large temperature fluctuations is unknown (Peimbert 1994; Barlow & Liu 1996). The difficulty in separating the effects arising from nebular thermal structure and those from density inhomogeneity when analyzing UV and optical forbidden lines leads to an ambiguity in interpreting the observations. Such an ambiguity can in principle be removed by observing IR fine structure lines such as those of [N III], [O III] and [Ne III], observable with the ISO SWS and LWS instruments. With E ex < 1100K, ionic abundances derived from these lines are insensitive to temperature and thus provide a critical test of the interpretation of temperature fluctuations as the cause of the discrepancy. In addition, these IR lines cover a wide range of critical densities and can be used to quantify density inhomogeneity and its effects on abundance determinations, without complications from thermal and ionization stratifications.