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

TL;DR

This study tests whether s-process isotopic variations observed in bulk meteorites, notably the larger Zr than Nd anomalies, can be reproduced by AGB star models of metallicity higher than solar. Using 68 Monash-based post-processing AGB models with varied convective overshoot and $^{13}$C-pocket sizes, the authors translate surface abundances into diluted $\varepsilon$ values for $^{96}$Zr/$^{90}$Zr and $^{148}$Nd/$^{144}$Nd, and compare their ratio $\varepsilon^{96}$Zr/$\varepsilon^{148}$Nd to meteoritic data. They find that the observed Zr–Nd contrast is best matched by super-solar metallicity AGB stars with strong third-dredge-up overshoot and/or small $M_{\rm mixed}$, suggesting that the presolar material contributing to the Solar System came from an old, high-metallicity stellar population. This work supports a scenario where bulk meteoritic s-process signatures originate from specific AGB progenitors rather than requiring multiple dust carriers, with implications for Galactic chemical evolution and the early Solar System’s stellar environment. Future work will refine neutron-source rates and cross sections, extend the analysis to other isotopes and meteorite components, and probe the impact of varying metallicities and stellar masses on broader s-process signatures.

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($α$,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.

Figures (5)

• Figure 1: Selected sections of the nuclide chart showing the stable isotopes (black boxes, with the contribution of each isotope to the elemental abundance in the Solar System) of Zr (top) and of Nd (bottom) and their unstable isotopes (blue boxes, with their half-lives) potentially located on the $s$-process path of neutron captures (which moves from left to right to higher masses by adding neutrons). In the case of Zr, $^{93}$Zr has a temperature-dependent half-life in any case long enough to make it behave like a stable isotope during the $s$ process, while $^{96}$Zr has a constant half-life and is a branching point with a typically higher probability of decaying than capturing a neutron. In the case of Nd, both $^{147}$Nd and $^{149}$Nd have almost constant half-lives, and live short enough that they typically decay before capturing a neutron.
• Figure 2: The predicted $\varepsilon$$^{96}$Zr to $\varepsilon$$^{148}$Nd ratios calculated from the final surface abundances of the AGB star models of metallicities $Z$ indicated on the x-axis, initial stellar masses between 2 and 3.5 M$_\odot$ ($M$, different symbols, as indicated in the legend box), variable values of the $N_\mathrm{ov}$ parameter (different colors, as indicated in the legend box) and fixed mass of the mixed region $M_{\rm mixed} = 2 \times 10^{-3}$ M$_\odot$ (top) and $1 \times 10^{-3}$ M$_\odot$ (bottom). The full symbols correspond to the SL models plotted in Figure \ref{['fig:choice']}. The observed ratio of the order of $\sim$10 is indicated as a dashed horizontal line.
• Figure 3: Same as Figure \ref{['fig:ratios']}, but for the selected SL models only, i.e., with one value of $N_\mathrm{ov}$ for each given mass, metallicity, and $M_{\rm mixed}$.
• Figure 4: The $\varepsilon$$^{96}$Zr/$\varepsilon$$^{148}$Nd ratio as function of the C/O ratio calculated using the Zr and Nd isotopic abundances as they evolve at the surface of the 3 M$_\odot$, $Z=0.02$ models with $M_{\rm mixed}= 1 \times 10^{-3}$ M$_\odot$ (top) and the 3 M$_\odot$, $Z=0.03$ models with $M_{\rm mixed}= 2 \times 10^{-3}$ M$_\odot$ (bottom).
• Figure 5: Abundances of Zr (black circles) and Ce (red triangles) in giant Ba stars decastro16pereira11 with metallicities (represented by the Fe abundance) comparable to the stellar models considered here. The square bracket notation indicates ratios relative to solar, in Log$_{10}$ scale (so that zero is solar). The black and red squares represent the average of the [Zr/Fe] and [Ce/Fe] values, respectively, in 0.05 dex bins in [Fe/H] (with the right edge points included in the averaging). The error bars are the standard deviations, note that the black square at [Fe/H]=0.225 has 0 error, since the two Zr dots have the same value. Overplotted are the [Zr/Fe] and [Ce/Fe] (solid and dashed lines, respectively) predicted at the three metallicities ([Fe/H]=0, 0.15, and 0.33) by the SL 3 M$_\odot$ models plotted in Figure \ref{['fig:choice']} for the three different choices of $M_{\rm mixed}$= 0.5, 1, and 2 $\times 10^{-3}$ M$_\odot$.