A Large-Scale Survey of Neutron-Capture Element Abundances in Planetary Nebulae

We present results from the first large-scale survey of neutron(n)capture element abundances in planetary nebulae (PNe). This survey was motivated by the fact that a PN may be enriched in n-capture elements if its progenitor star experienced s-process nucleosynthesis during the thermally-pulsing asymptotic giant branch (AGB) phase. We have measured emission from Se and Kr in over 100 PNe, and use literature data to expand our sample to 120 objects. [Kr III] 2.199 and/or [Se IV] 2.287 μm were detected in 81 PNe, for a detection rate of nearly 70%. We derive Se and Kr abundances or upper limits using ionization correction factors derived from photoionization models. A significant range is found in the Se and Kr abundances, from near solar (no enrichment), to enriched by a factor of ten. Overall, 41 of the 94 PNe with derived Se and/or Kr abundances or meaningful upper limits exhibit s-process enrichments. Our survey has increased the number of PNe with known n-capture element abundances by an order of magnitude, enabling us to explore correlations between s-process enrichments and other nebular and central star properties. In particular, the Se and Kr enrichments display a positive correlation with nebular C/O ratios, as theoretically expected. Peimbert Type I PNe and bipolar PNe, whose progenitors are believed to be intermediate-mass stars (> 3– 4 M⊙), exhibit little or no s-process enrichment. Interestingly, PNe with H-deficient [WC] central stars do not exhibit systematically larger s-process enrichments than other PNe, despite the fact that their central stars are enriched in C and probably ncapture elements. Finally, the few PNe in our sample with known or probable binary central star systems exhibit little s-process enrichment, which may be explained if binary interactions truncated their AGB phases. We also briefly discuss a new observational program to detect optical emission lines of n-capture elements, and new atomic data calculations that will greatly improve the accuracy of n-capture element abundance determinations in PNe.