Heavy Element Abundances in Ionized Nebulae

Details Heavy Element Abundances in Ionized Nebulae Heavy Element Abundances in Ionized Nebulae

Mar 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........19t&link_type=abstract

PhD Thesis, University College London

Physics

7

Ism, Abundances, Planetary Nebulae, H Ii Regions, Line Identification, 30 Doradus, Lmc N11B, Smc N66, M17, Ngc 3576

Scientific paper

In this thesis I present a study of the physical conditions and elemental abundances for a sample of planetary nebulae (PNe) and H II regions, based on optical long-slit spectroscopy and complemented by ultraviolet (IAU) and infrared (IRAS, ISO) spectra. Optical recombination-line (ORL) abundances for carbon, nitrogen, oxygen, and neon have been measured for 12 Galactic-disk and three Magellanic Cloud PNe (LMC N66, LMC N141, SMC N87). A detailed comparison with the corresponding abundances derived from ultraviolet, optical and infrared collisionally-excited lines (CELs) was made. In total about 2000 optical emission lines were measured on ESO1.5m and NTT3.6m spectral tracings. Two extreme cases of nebulae have been found, NGC 2022 and LMC N66, that exhibit very high ORL versus CEL ionic abundance enhancements: factors of >10 for the O2+ ion. The type I nebula NGC 2440 is shown to exhibit ORL/CEL discrepancy factors for total C, N, O abundances of 4.4, 5.6 and 4.2, i.e. similar to those observed in the previously studied NGC 7009 and M 2-36. The abundance discrepancy factors (ADFs) for various ions differ among nebulae, spanning a range from 1 to 36.
The ADFs for doubly ionized C, N and O are positively correlated with the difference (Delta T) between the Balmer discontinuity and [O III] forbidden-line temperatures, suggesting that temperature variations, real or due to biased diagnostics, are implicated in the discrepancies. The relative uniformity, however, of the overabundance patterns and lack of consistent correlation with the excitation energies of the various UV, optical and IR CELs point away from temperature fluctuations in a chemically homogeneous medium as the cause of the problem. Instead, it is argued that the main cause of the abundance anomaly is enhanced ORL emission from cold ionized gas located in hydrogen-deficient clumps inside the main body of the nebulae. An empirical model of NGC 5882 is built incorporating fully ionized, relatively oxygen-rich clumps with a volume filling factor of 1E-5 embedded in a rarefied medium, with a density contrast a factor of 218 between the two nebular components. In this model, about 50 per cent of the high order Balmer lines (n=11 to 24) and Balmer discontinuity emission originate from the dense clumps.
The abundance discrepancies are inversely correlated with the intrinsic nebular surface brightness, and positively correlated with PN absolute radii, i.e. young, bright nebulae display smaller ADFs than older, more extended ones. It is further shown that very similar correlations exist between Delta T and the nebular radii and surface brightness, suggesting that fainter, more extended objects are more likely to have temperature discrepancies than brighter, smaller ones. These findings strongly indicate that the ORL/CEL abundance anomaly problem is associated with the evolution of PN.
Optical spectra of the H II regions M17 and NGC 3576 in the Galaxy, and 30 Doradus, LMC N11B and SMC N66 in the Magellanic Clouds have been analyzed. In total, about 600 emission lines were detected and measured on tracings obtained with the NTT3.6m, ESO1.5m and AAT3.9m telescopes. The computed ORL/CEL ADFs for O2+ are in the range of two to five thus placing these objects in the abundance discrepancy regime of PN; the problem is documented for the fist time in extragalactic H II regions. The relative intensities of O II V1 multiplet 3s-3p ORLs are observed to deviate significantly from their expected LS coupling values probably due to the breakdown of thermal equilibrium among the fine-structure levels of the parent triplet P0,1,2 ground term of recombining O2+. This effect is more pronounced in the lower density Magellanic Cloud nebulae and suggests that the O II ORLs are mostly formed under relatively low density conditions. Temperature-insensitive ORL C2+/O2+ ratios have been derived for all five nebulae showing remarkable agreement within each galactic system.

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