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Paper: On the Puzzle of the Anomalous Oxygen Recombination Lines in Planetary Nebulae
Volume: 199, Asymmetrical Planetary Nebulae II: From Origins to Microstructures
Page: 301
Authors: Dinerstein, Harriet L.; Lafon, Christine E.; Garnett, Donald R.
Abstract: We discuss the conundrum of the O II recombination lines in planetary nebulae (PNe). The strengths of these lines in a few previously observed PNe indicated implausibly high O/H abundances. In a survey of ten PNe, we find several for which the O/H values from O II recombination lines agree with those derived from collisionally excited [O III] lines, and others in which the recombination lines yield abundances that are higher by up to factors of 4-5. Certain transitions tend to yield higher abundances than other lines. We interpret this as a line enhancement effect due to an unidentified physical mechanism which is more significant for some lines and nebulae than others. We speculate that elevated dielectronic recombination rates, perhaps in hot gas, or time-dependent effects such as ionization or recombination fronts, may be responsible.
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