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[74.02] Recombination Line vs. Forbidden Line Abundances in Planetary Nebulae

M. Robertson-Tessi, D. R. Garnett (University of Arizona)

Recombination lines (RLs) of C II, N II, and O II in planetary nebulae (PNe) have been found to give abundances that are much larger in some cases than abundances from collisionally-excited forbidden lines (CELs). The origins of this abundance discrepancy are highly debated. We present new spectroscopic observations of O~II and C~II recombination lines for six planetary nebulae. With these data we compare the abundances derived from the optical recombination lines with those determined from collisionally-excited lines. Combining our new data with published results on RLs in other PNe, we examine the discrepancy in abundances derived from RLs and CELs. We find that there is a wide range in the measured abundance discrepancy \Delta(O⁺²) = log O⁺²(RL) - log O⁺²(CEL), ranging from approximately 0.1 dex (within the 1\sigma measurement errors) up to 1.3 dex. This tends to rule out errors in the recombination coefficients as a source of the discrepancy. Most RLs yield similar abundances, with the notable exception of O II multiplet V15, known to arise primarily from dielectronic recombination, which gives abundances averaging 0.6 dex higher than other O II RLs. We compare \Delta(O⁺²) against a variety of physical properties of the PNe to look for clues as to the mechanism responsible for the abundance discrepancy. The strongest correlation is found with Balmer surface brightness; high surface brightness, compact PNe show small values of \Delta(O⁺²), while large low surface brightness PNe show the largest discrepancies. Rougher correlations of \Delta(O⁺²) are found with He⁺²/He⁺ and with the expansion velocity. No correlations are seen with electron temperature, electron density, central star effective temperature and luminosity, stellar mass loss rate, or nebular morphology. Similar results are found for carbon, comparing C~II RL abundances with ultraviolet measurements of C~III].

This work is supported by NSF grant AST-0203905.

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Bulletin of the American Astronomical Society, 36 #2
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