In Situ Isotopic Analysis of Presolar Graphite Challenges Stellar Models

We have analyzed zirconium, molybdenum, and barium isotopes for the first time in situ in two presolar graphite grains found in polished thin sections from the CM2 meteorites Maribo and Murchison. Both grains most likely originated in low-mass, low-metallicity AGB stars. The Maribo grain has almost pure s -process molybdenum in a subgrain, challenging AGB star models that assume homogenization of s -process material in the convective stellar envelope prior to grain condensation. Either this grain formed in a chemically and isotopically distinct high-density region or after its parent star had already lost its envelope. Furthermore, neutrons from the 22 Ne( α , n ) 25 Mg reaction were only marginally captured by 95 Zr in both grains’ parent stars, which further challenge stellar models and the theoretically derived neutron-capture cross section of 95 Zr. Finally, both grains showed contamination with molybdenum of solar composition in some regions of the grains, probably from aqueous alteration on their parent asteroids, emphasizing the importance of depth-resolved isotope studies.