Predictions from $s$-process AGB models of the isotopic variations of zirconium and neodymium for comparison to bulk meteorites

TL;DR

This study compares isotopic variations of zirconium and neodymium in meteorites to predictions from AGB star models, suggesting super-solar metallicity AGB stars as the source of observed s-process isotopic anomalies.

Contribution

It demonstrates that super-solar metallicity AGB star models can explain the isotopic variability differences between Zr and Nd in meteorites.

Findings

AGB models match Zr and Nd isotopic variability

Super-solar metallicity stars are likely sources

Deeper convective overshoot improves model fit

Abstract

Bulk meteoritic data show isotopic variability of $slow$-neutron-capture ($s$-process) origin in a several elements heavier than Fe. One peculiar feature is that the lighter $s$-process elements (e.g., Zr and Mo) present larger anomalies than the heavier $s$-process elements (e.g., Nd and W). To address this observation, we compared Zr and Nd data to model predictions of the s-process abundances at the surface of low-mass asymptotic giant branch (AGB) stars of initial metallicity from solar to twice solar. We found that the relative magnitude of the isotopic variability between these two elements can be matched by models of AGB stars of super-solar metallicity. The match is favoured by stronger convective overshoot, leading to a deeper dredge-up of the H-rich envelope into the He-rich region, and/or a smaller (~ half than standard) mass of the region rich in the $^{13}$C nuclei that produce free neutrons via the $^{13}$C($\alpha$,n)$^{16}$O reaction. We conclude that nucleosynthesis in AGB stars can match the difference in the magnitude of the bulk meteoritic variations in Zr and Nd, provided that super-solar metallicity stars are the original site of these signatures. The AGB stars that produced such variations could have belonged to the current population of old, super-solar metallicity stars seen in the galactic solar neighbourhood.