Constraints for Nuclear Astrophysics from an Unusual Presolar Silicate-Oxide Aggregate Grain Found in Primitive Ordinary Chondrite Meteorite Hills 00526

TL;DR

This study analyzes an unusual presolar grain's isotopic data to infer its stellar origin and provides insights into galactic chemical evolution relevant to nuclear astrophysics.

Contribution

It presents detailed isotopic analysis of a presolar grain, supporting an origin from low-mass AGB stars and highlighting the importance of multi-element data for astrophysical models.

Findings

Supports LM-AGB origin for the grain based on Si isotopes.

Most isotopic ratios suggest a parent star with sub-solar metallicity.

Heterogeneous GCE processes indicated by Mg, Ti, and Ca isotope anomalies.

Abstract

We report O, Mg-Al, Si, Ca, and Ti isotopic data for an unusual presolar oxide/silicate aggregate grain, M526-69, previously reported in the primitive ordinary chondrite Meteorite Hills 00526. The $\approx 1\mu$m aggregate consists of a Mg- and Ca-rich silicate, a Al-rich oxide, and a tiny TiO$_2$ grain. A large $^{18}$O depletion and high inferred $^{26}$Al/$^{27}$Al classifies M526-69 as a Group 2 grain. Both low-mass (LM) and intermediate-mass (IM) asymptotic giant branch (AGB) stars are considered viable candidate parent stars of Group 2 grains based on their O isotopes and inferred $^{26}$Al/$^{27}$Al ratios. The lack of a large $^{30}$Si excess in M526-69 strongly supports an LM-AGB origin for it and other Group 2 grains. The stable Mg, Ca, and Ti isotopes all reflect the initial composition of the parent star, set by galactic chemical evolution (GCE) processes. Presolar O-rich grains provide a better measure of the GCE trends for Ti isotopes than presolar SiC grains as the latter are also affected by neutron capture reactions in the parent stars. Most of the Mg, Ca, and Ti isotopic ratios in M526-69 are consistent with its parent star having metallicity lower than solar. However, small excesses in stable non-radiogenic $^{26}$Mg, $^{46}$Ti, and $^{44}$Ca do not fit this pattern and instead point to heterogeneous GCE processes, though quantitative modeling is needed to test this hypothesis. Multi-phase presolar grains are extremely valuable for nuclear astrophysics as they can both provide isotopic compositions for multiple elements that must be matched at a single time and place in a single star.