Wesson, Roger; (2004) Heavy element abundances in emission line nebulae. Doctoral thesis (Ph.D.), UCL (University College London).

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Abstract

This thesis contains analysis of observations of ionized nebula in the optical, infrared, ultraviolet and radio wavelength regions. The aim of the thesis is to consider the well-known problem in nebular studies that heavy element abundances measured from optical recombination lines (ORLs) are invariably higher than those measured from collisionally excited lines (CELs), by factors of up to eighty. A detailed study of the hydrogen-deficient knots in the planetary nebula Abell 30 shows that the ORL emission from the knots is very strong, and abundances from ORLs are found to be several hundred times higher than those derived from CELs. Temperatures derived from weakly temperature-sensitive O II recombination line ratios show that the knots contain some extremely hydrogen-deficient material at very low temperatures (2500 K). The knots are found to be oxygen-rich, in contradiction to the predictions of the 'born-again' scenario in which the knots were previously assumed to have formed. Images and spectra of the Cat's Eye Nebula NGC 6543 are analysed, and they show that temperature fluctuations in this nebula are very small. ORL abundances are higher than CEL abundances by a factor of three. It is found that temperature fluctuations cannot account for this discrepancy, and instead the evidence points towards the existence in this nebula of a cold hydrogen-deficient component, perhaps similar in origin to the knots of Abell 30. Hubble Space Telescope images and spectra of NGC 6153 are analysed. This nebula is known to show a large abundance discrepancy. We find that, while the data supports previous studies suggesting the presence of H-deficient knots, we cannot constrain their size and distribution. High-resolution images suggest that any knots present must be less that 160AU in diameter. The high-excitation planetary nebula NGC 1501 is studied, and very large abundance discrepancy factors (~30) are found. It is shown that temperature fluctuations are ruled out as an explanation for this discrepancy; the magnitude of the fluctuations required is implausibly high and is not supported by the observed temperatures from CELs, the hydrogen Balmer jump and He I line ratios. Instead, H-deficient knots once again appear to be the most plausible explanation for the observed abundance and temperature discrepancies. Finally, abundances are derived from ORLs and CELs for a large sample of northern planetary nebulae, and abundance discrepancies ranging from factors of 1.5 to 18 are found. In 22 out of 23 cases, temperature fluctuations can be definitely ruled out as causes of the discrepancies, and are even unlikely in the one ambiguous case. Cold hydrogen-deficient seem the most likely explanation for the observed discrepancies.

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