Infrared Observations of Neutron-Capture Elements in Planetary Nebulae

We present results from an ongoing survey of the infrared emission lines [Kr III] 2.199 and [Se IV] 2.287 micron in Galactic planetary nebulae (PNe). Krypton and selenium may be produced by slow neutron-capture nucleosynthesis (the ``s-process'') during the asymptotic giant branch (AGB) phase of PN progenitor stars, and brought to the surface by convective dredge-up before PN ejection. We detect emission from Se and Kr in 65 of 114 Galactic PNe, and use the line fluxes to derive ionic abundances. We employ ionization correction factors based on coincidences of ionization potentials to calculate total elemental abundances, and discuss photoionization models designed to test the veracity of these corrections. The derived Se and Kr abundances of our targets range from approximately solar to enriched by a factor of 5, which indicates varying degrees of dredge-up and s-process efficiencies in the progenitor stars. In PNe exhibiting emission from both Se and Kr, we find that the relative abundances of these elements are generally in agreement with predictions from theoretical models of s-process nucleosynthesis. We examine our results for correlations between s-process enrichments and other nebular properties, such as CNO abundances, morphology, and characteristics of the central stars. PNe with Wolf-Rayet central stars tend to exhibit more elevated Se and Kr abundances than other nebulae. Bipolar nebulae, believed to arise from the most massive of PN progenitors, may have lower n-capture abundances than elliptical PNe.