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N. C. Sterling, H. L. Dinerstein (University of Texas)
We present results from an ongoing survey of the infrared emission lines [Kr III] 2.199 and [Se IV] 2.287 \mum, first identified by Dinerstein (2001, ApJ, 550, L223), in planetary nebulae (PNe). Isotopes of Se (Z=34) and Kr (Z=36) are created by the slow neutron-capture process (\emph{s}-process) during the asymptotic giant branch (AGB) phase, and may be brought to the stellar envelope by third dredge-up (TDU). Therefore, PNe will be enriched in \emph{n}-capture elements if efficient TDU occurred in the progenitor star. Since intermediate-mass stars are the source of a significant fraction of the \emph{s}-process isotopes in the Galaxy (via TDU), measurement of \emph{n}-capture element abundances in PNe is of vital importance for models of Galactic chemical evolution. Using CoolSpec (Lester et al. 2000, PASP, 112, 384) on the 2.7-m Harlan J. Smith telescope at McDonald Observatory, we have observed 49 PNe and detected [Kr III] and/or [Se IV] in 27. The high detection rate demonstrates that these two lines are measurable in a significant fraction of Galactic PNe. We use the ionization correction factors Se3+/Se~\approx~Ar++/Ar and Kr++/Kr~\approx~S++/S to derive total elemental abundances, and find that the Kr and Se abundances vary widely among the studied PNe, from roughly solar to overabundant by a factor of 10. This indicates that the efficiency of TDU differed significantly among our sample's progenitor stars. We compare the derived Kr and Se abundances to other nebular properties to search for correlations. Our initial results lend support to the suggestion of Sterling, Dinerstein, & Bowers (2002, ApJ, 578, L55) that \emph{n}-capture element abundances are larger in PNe with Wolf-Rayet ([W-R]) central stars than those without. This is believed to be due to the heavy mass loss and/or deep mixing which these stars experienced as they evolved into H-poor, C-rich [W-R] stars.
This research is supported by NSF grant AST 97-31156.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.