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N. C. Sterling (University of Texas-Austin)
I present results of the first large-scale survey of neutron(n)-capture elements (Z > 30) in planetary nebulae (PNe). I have observed the near-infrared emission lines [Kr III] 2.199 and [Se IV] 2.287 microns in over 100 PNe, using the CoolSpec spectrometer on the 2.7-m Harlan J. Smith Telescope at McDonald Observatory. Se (Z = 34) and Kr (Z = 36) may be enriched in PNe if their progenitor stars experienced slow n-capture nucleosynthesis (the ``s-process'') and convective dredge-up during the AGB. This survey provides the first insight into s-process nucleosynthesis in PN progenitor stars as a population. Se and/or Kr have been detected in 65 of 114 PNe, for a detection rate of nearly 60%. I have added Se and Kr to the atomic databases of the publicly available photoionization codes CLOUDY and XSTAR, which I use to correct for unseen ionization stages and thereby derive elemental abundances. Our analysis shows that Se and Kr abundances are enhanced relative to solar in about 40% of the PNe exhibiting [Se IV] and [Kr III] emission. This provides evidence that the s-process occurred in the progenitor stars, with varying efficiencies as implied by the range of enhancements (1--5 times solar). Kr tends to be more enriched than Se, in agreement with theoretical models of s-process nucleosynthesis. I find that Se and Kr tend to be more strongly enhanced in PNe with hydrogen-deficient central stars exhibiting Wolf-Rayet emission features, as well as objects with lower mass (M < 4 Msun) progenitors. These results provide important information for models of s-process nucleosynthesis, and our understanding of the evolution of low- and intermediate-mass stars and their impact on the chemical enrichment of the Galaxy.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.