Neutron-Capture Element Abundances in Planetary Nebulae

N C Sterling

doi:10.3390/galaxies8020050

Abstract

Nebular spectroscopy is a valuable tool for assessing the production of heavy elements by slow neutron(n)-capture nucleosynthesis (the s-process). Several transitions of n-capture elements have been identified in planetary nebulae (PNe) in the last few years, with the aid of sensitive, high-resolution, near-infrared spectrometers. Combined with optical spectroscopy, the newly discovered near-infrared lines enable more accurate abundance determinations than previously possible, and provide access to elements that had not previously been studied in PNe or their progenitors. Neutron-capture elements have also been detected in PNe in the Sagittarius Dwarf galaxy and in the Magellanic Clouds. In this brief review, I discuss developments in observational studies of s-process enrichments in PNe, with an emphasis on the last five years, and note some open questions and preliminary trends.

Highlights

Neutron(n)-capture elements are produced by slow and rapid n-capture nucleosynthesis
The r-process is known to occur in high-mass stars [1], and the discovery of lanthanide-rich material in the kilonova AT 2017gfo [4] established that neutron star mergers produce r-process nuclei
Recent observations using large (>6-m) telescopes demonstrate that n-capture elements can be detected in planetary nebulae (PNe) belonging to nearby galaxies

Summary

Introduction

Neutron(n)-capture elements (atomic number Z > 30) are produced by slow and rapid n-capture nucleosynthesis (the s and r-processes, respectively) These processes are distinguished by the relative time scales of successive n-captures and β-decays of the participating isotopes, and produce distinct enrichment patterns, with the r-process leading to more neutron-rich nuclei [1]. Despite their cosmic rarity, n-capture element abundances can reveal valuable details of the chemical evolution and star formation histories of galaxies (e.g., [2,3]), and it is critical to understand their sites of origin and chemical yields in stars of different mass and metallicity. Newly-identified emission lines have spurred investigations into the atomic data needed to interpret these features, work that has developed concurrently with observations; see [19] for a review

Near-Infrared Observations

Optical Spectroscopy

Extragalactic Planetary Nebulae

Discussion

Summary