Light Neutron-capture Element Abundances in Planetary Nebulae

This dissertation presents a spectroscopic study of light neutron(n)-capture elements in Galactic planetary nebulae (PNe). The abundances of n-capture elements may be enhanced in PNe if their progenitor stars experienced s-process nucleosynthesis and third dredge-up (TDU) during the asymptotic giant branch (AGB) phase. We derive the abundances of Ge, Se, and Kr in PNe from ultraviolet (UV) and near-infrared (NIR) spectroscopic observations. We detect Ge III in absorption against the central star UV continua of six PNe, and derive Ge abundances for five of these objects. Four of these PNe exhibit Ge abundances enriched by a factor of ≥3--10, depending on the level of Ge depletion into dust, which provides evidence for the occurrence of the s-process in their progenitor stars. We have observed 103 PNe in the NIR with the CoolSpec spectrometer at McDonald Observatory, to search for emission lines of [Se IV] and [Kr III]. Including NIR, spectra of 17 PNe from the literature, Se and/or Kr are detected in 81 of 120 objects. We construct a grid of photoionization models to derive widely applicable formulae that can be used to correct for the abundances of unobserved Se and Kr ions. These correction factors are used to determine the Se and Kr elemental abundances of PNe in our sample. We find that Se and Kr are enriched relative to solar in 35 of the 73 PNe with determined Se and Kr abundances, and hence their progenitor stars experienced the s-process and TDU during the AGB phase. Type I and bipolar PNe, which have intermediate-mass (> 3-4 M⊙ ) progenitors, are far less enriched in Se and Kr than other PNe, indicating that intermediate-mass AGB stars do not experience significant s-process enrichments. The Se and Kr abundances are correlated with nebular C/O ratios, as theoretically expected. We estimate that at least 20% of Galactic PNe experienced s-process nucleosynthesis and TDU, by constructing a PN luminosity function for our sample and correcting it for completeness at faint luminosities. This study comprises the first large-scale survey of n-capture elements in PNe, and significantly increases the number of PNe with known n-capture element abundances.