Origin of Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions in Meteorites

Abstract

Most meteoritic calcium-rich, aluminum-rich inclusions (CAIs) formed from a reservoir with ${}^{26}{\rm Al}/{}^{27}{\rm Al} \approx 5 \times 10^{-5}$, but some record lower $({}^{26}{\rm Al}/{}^{27}{\rm Al})_0$, demanding they sampled a reservoir without live ${}^{26}{\rm Al}$. This has been interpreted as evidence for "late injection" of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them "Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions" (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel or hibonite grains $0.5 - 2 \, \mu{\rm m}$ in size with no live ${}^{26}{\rm Al}$; live ${}^{26}{\rm Al}$ is carried on smaller ($<$50 nm) presolar chromium spinel grains from recent nearby Wolf-Rayet stars or supernovae. In hot ($\approx$ 1350-1425 K) regions of the disk these grains, and perovskite grains, would be the only survivors. These negatively charged grains would grow to sizes $1 - 10^3 \, \mu{\rm m}$, even incorporating positively charged perovskite grains, but not the small, negatively charged ${}^{26}{\rm Al}$-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ${}^{48}{\rm Ca}$ and ${}^{50}{\rm Ti}$ anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ${}^{26}{\rm Al}$ in a solar nebula with uniform, canonical ${}^{26}{\rm Al}/{}^{27}{\rm Al}$. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.