Larry R. Nittler, Conel M. O’D. Alexander, Nan Liu¹, and Jianhua Wang
Astrophysical Journal Letters 856, L24 Link to Article [DOI: 10.3847/2041-8213/aab61f]
Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USA
¹Present address: Dept. of Physics, Washington University, 1 Brookings Drive, St. Louis, MO 63130, USA.

We report the identification of 19 presolar oxide grains from the Orgueil CI meteorite with substantial enrichments in ⁵⁴Cr, with ⁵⁴Cr/⁵²Cr ratios ranging from 1.2 to 56 times the solar value. The most enriched grains also exhibit enrichments at mass-50, most likely due in part to ⁵⁰Ti, but close-to-normal or depleted ⁵³Cr/⁵²Cr ratios. There is a strong inverse relationship between ⁵⁴Cr enrichment and grain size; the most extreme grains are all <80 nm in diameter. Comparison of the isotopic data with predictions of nucleosynthesis calculations indicate that these grains most likely originated in either rare, high-density Type Ia supernovae (SN Ia), or in electron-capture supernovae (ECSN), which may occur as the end stage of evolution for stars of mass 8–10 M _⊙. This is the first evidence for preserved presolar grains from either type of supernova. An ECSN origin is attractive, as these likely occur much more frequently than high-density SN Ia, and their evolutionary timescales (~20 Myr) are comparable to those of molecular clouds. Self-pollution of the Sun’s parent cloud from an ECSN may explain the heterogeneous distribution of n-rich isotopic anomalies in planetary materials, including a recently reported dichotomy in Mo isotopes in the solar system. The stellar origins of three grains with solar ⁵⁴Cr/⁵²Cr, but anomalies in ⁵⁰Cr or ⁵³Cr, as well as of a grain enriched in ⁵⁷Fe, are unclear.