Heavy element abundances in emission line nebulae

Details Heavy element abundances in emission line nebulae Heavy element abundances in emission line nebulae

Jul 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........14w&link_type=abstract

PhD Thesis, University of London

Computer Science

1

Scientific paper

This thesis considers 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). A detailed study of the knots in the planetary nebula Abell 30 shows that the ORL abundances are several hundred times higher than the CEL abundances. Temperatures derived from O II recombination line ratios show that the knots contain 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.
Analysis of spectra of NGC 6543 show that ORL abundances exceed CEL abundances by a factor of three. A temperature map is derived, and shows that temperature fluctuations cannot account for this discrepancy. 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. While the data supports previous studies suggesting the presence of H-deficient knots, their size and distribution are still unconstrained. High-resolution images suggest that any knots present must be less that 160AU in diameter.
Analysis of spectra of NGC 1501 reveals very large abundance discrepancy factors (˜30). Temperature fluctuations cannot cause this discrepancy; implausibly large fluctuations would be required, and are not indicated by the observed temperatures from CELs, 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 12 are found. In 22 out of 23 cases, temperature fluctuations are ruled out as causes of the discrepancies, and are even unlikely in the one ambiguous case. Cold hydrogen-deficient knots seem the most likely explanation for the observed discrepancies.

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