Tsamis, Ioannis; (2002) Heavy element abundances in ionized nebulae. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Optical recombination-line (ORL) abundances for carbon, nitrogen, and oxygen, relative to hydrogen, are presented for 12 Galactic planetary nebulae (PN) and 3 Magellanic Cloud PN (LMC N141, LMC N66 & SMC N87) and an extensive comparison with the corresponding abundances derived from UV (IUE), optical and IR (IRAS, ISO) collisionally-excited lines (CELs) is performed. Two extreme nebulae, NGC2022 and LMC N66, are exposed that exhibit very discordant - ORL versus CEL - abundances; by a factor of >10 for the O2+ ion. The Type I PN NGC2440 is revealed to have discrepancy factors similar to those of the previously studied NGC7009 and M2-36. Along with previous results it is shown that ORL/CEL ionic abundance enhancements differ among nebulae, spanning a range from ~ 1.5 to > 20. There are indications that the C/O, N/O elemental ratios derived from ORLs may be larger that the corresponding CEL ratios, hinting at an origin for the two types of line from nebular material of dissimilar chemical history. The abundance enhancements of doubly ionized C, N and O are positively correlated with the difference (AT) between the Balmer recombination continuum and [O III] forbidden-line temperatures, suggesting that temperature variations, real or induced, are partly to blame for the discrepancies. The relative uniformity, however, of the overabundance patterns and lack of consistent correlation with CEL excitation energies point away from Peimbert-type, simple temperature fluctuations as the cause of the problem. Recent results by Garnett & Dinerstein (2000) are confirmed, showing that the abundance discrepancies are: i) anticorrelated with the intrinsic nebular surface brightness, and ii) positively correlated with PN absolute radii, i.e. young, bright nebulae display less abundance discrepancies than older, more extended ones. It is further shown however, that very similar correlations exist between [delta]T and the nebular radii and surface brightnesses, suggesting that fainter, more extended objects are more likely to have temperature discrepancies than brighter, smaller ones. These findings strongly indicate that the long-standing ORL/CEL abundance discrepancy problem is associated with the evolution of PN. Optical spectra of five H II regions, two within the Galaxy (M 17 and NGC 3576) and three in the Magellanic Clouds (30 Doradus, LMC N11B and SMC N66) were analyzed. The ORL/CEL discrepancy factors for the O2+ ion are found for the first time to be in the range of 2-5, thus placing these objects in the abundance discrepancy regime of PN. For the first time also ORL/CEL discrepancies are documented for extragalactic H II regions. Accurate, temperature-insensitive ORL C2+/O2+ ratios are derived for all five nebulae, showing remarkable agreement within each galactic system.

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