Constraints for nuclear astrophysics from an unusual presolar silicate-oxide aggregate grain found in primitive ordinary chondrite Meteorite Hills 00526
Constraints for nuclear astrophysics from an unusual presolar silicate-oxide aggregate grain found in primitive ordinary chondrite Meteorite Hills 00526
Larry R. Nittler, Jens Barosch, Conel M. O’D. Alexander & Jianhua Wang
The European Physical Journal A
Volume 62, article number 110, Published: 02 June 2026
“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 ≈1 μm aggregate consists of a Mg- and Ca-rich silicate, an Al-rich oxide, and a tiny TiO2 grain. A large 18O depletion and high inferred 26Al/27Al 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 26Al/27Al ratios. The lack of a large 30Si 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 26Mg, 46Ti, and 44Ca 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.”
