Nicholas Craig Sterling

Nucleosynthesis Predictions for Intermediate-Mass Asymptotic Giant Branch Stars: Comparison to Observations of Type I Planetary Nebulae

Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of ∼3–8 M� . These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light trans-iron elements Se and Kr, in order to constrain convective mixing and the s-process in these stars. A partial mixing zone is included in selected models to explore the effect of a 13 C pocket on the s-process yields. The solar-metallicity models produce enrichments of [(Se, Kr)/Fe] 0.6, consistent with Galactic Type I PNe where the observed enhancements are typically 0.3 dex, while lower metallicity models predict larger enrichments of C, N, Se, and Kr. O destruction occurs in the most massive models but it is not efficient enough to account for the 0.3 dex O depletions observed in some Type I PNe. It is not possible to reach firm conclusions regarding the neutron source operating in massive AGB stars from Se and Kr abundances in Type I PNe; abundances for mores-process elements may help to distinguish between the two neutron sources. We predict that only the most massive (M 5 M� ) models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3–4 M� ), if these stars are to evolve into Type I PNe.

Improved Neutron-Capture Element Abundances in Planetary Nebulae

Spectroscopy of planetary nebulae (PNe) provides the means to investigate s-process enrichments of neutron(n)-capture elements that cannot be detected in Asymptotic Giant Branch (AGB) stars. However, accurate abundance determinations of these elements present a challenge. Corrections for unobserved ions can be large and uncertain, since in many PNe only one ion of a given n-capture element has been detected. Furthermore, the atomic data governing the ionization balance of these species are not well-determined, inhibiting the derivation of accurate ionization corrections. We present initial results of a program that addresses these challenges. Deep high-resolution optical spectroscopy of ~20 PNe has been performed to detect emission lines from trans-iron species including Se, Br, Kr, Rb and Xe. The optical spectral region provides access to multiple ions of these elements, which reduces the magnitude and importance of uncertainties in the ionization corrections. In addition, experimental and theoretical efforts are providing determinations of the photoionization cross sections and recombination rate coefficients of Se, Kr and Xe ions. These new atomic data will make it possible to derive robust ionization corrections for these elements. Together, our observational and atomic data results will enable n-capture element abundances to be determined with unprecedented accuracy in ionized nebulae.

Experimental and theoretical photoionization cross sections for Se + from 18 eV to 31 eV

Absolute Se photoionization cross-section measurements and Dirac-Coulomb R-matrix calculations are reported for the photon energy range 18.0 eV – 31.0 eV, which spans the ionization thresholds of the 4S03/2 ground state and the low-lying 2D03/2,5/2 and 2P01/2,3/2 metastable states. The determination of the photoionization and recombination properties of n-capture element ions is motivated by their astrophysical detection and the importance of their elemental abundances in testing theories of nucleosynthesis and stellar structure.