The Abundances of Light Neutron‐Capture Elements in Planetary Nebulae. II. s ‐Process Enrichments and Interpretation1

Abstract

We present the results of a large-scale survey of neutron(n)-capture elements in Galactic planetary nebulae (PNe), undertaken to study enrichments from s-process nucleosynthesis in their progenitor stars. From new K-band obser- vations of over 100PNe supplemented by data fromthe literature,we have detected theemission lines [Kr iii]2.199?m and/or [Se iv]2.287?min 81 of 120 objects.We determine Se and Kr elemental abundances,employing ionization cor- rection formulae derived in the first paper of this series.We find a significant range in Se andKr abundances, from near solar (no enrichment) to enhanced by >1.0 dex relative to solar, which we interpret as self-enrichment due to in situ s-process nucleosynthesis. Kr tends to be more strongly enriched than Se; in 18 objects exhibiting both Se and Kr emission,we find that ½Kr/Se?¼ 0:5 ? 0:2. Our survey has increased the number of PNewith n-capture element abun- dance determinations by a factor of 10, enabling us for the first time to search for correlations with other nebular prop- erties. As expected,we find a positive correlation between s-process enrichments and theC/O ratio.Type I and bipolar PNe, which arise from intermediate-mass progenitors (>3–4M?), exhibit little to no s-process enrichments. Finally, PNe with H-deficientWolf-Rayet central stars do not exhibit systematically larger s-process enrichments than objects with H-rich nuclei. Overall,44%of thePNe in our sample display significant s-process enrichments (>0.3 dex). Using an empirical PN luminosity function to correct for incompleteness, we estimate that the true fraction of s-process en- riched Galactic PNe is at least 20%