The Abundances of Light Neutron-Capture Elements in Planetary Nebulae II. s-process Enrichments and Interpretation

TL;DR

This study surveys neutron-capture element abundances in Galactic planetary nebulae, revealing s-process enrichment patterns, correlations with nebular properties, and estimating the prevalence of such enrichments in the galaxy.

Contribution

It provides the first large-scale, systematic analysis of n-capture element enrichments in PNe, expanding the sample size and enabling correlation studies with nebular characteristics.

Findings

Significant variation in Se and Kr abundances, indicating in situ s-process enrichment.

Kr is generally more enriched than Se in PNe.

Approximately 44% of PNe show notable s-process enrichment, with an estimated 20% in the galaxy.

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 observations of over 100 PNe supplemented by data from the literature, we have detected the emission lines [Kr III] 2.199 and/or [Se IV] 2.287 $\mu$m in 81 of 120 objects. We determine Se and Kr elemental abundances, employing ionization correction formulae derived in the first paper of this series. We find a significant range in Se and Kr 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 [Kr/Se] = 0.5$\pm$0.2. Our survey has increased the number of PNe with n-capture element abundance determinations by a factor of ten, enabling us for the first time to search for correlations with other nebular properties. As expected, we find a positive correlation between s-process enrichments and the C/O ratio. Type I and bipolar PNe, which arise from intermediate-mass progenitors (>3-4 solar masses), exhibit little to no s-process enrichments. Finally, PNe with H-deficient Wolf-Rayet central stars do not exhibit systematically larger s-process enrichments than objects with H-rich nuclei. Overall, 44% of the PNe 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 enriched Galactic PNe is at least 20%.