Heterogeneous distribution of 26Al at the birth of the solar system

Abstract

It is believed that 26Al, a short-lived (t 1/2 = 0.73Ma) and now extinct radionuclide, was uniformly distributed in the nascent solar system (SS) with the initial 26Al/27Al ratio of 5.2 × 10-5, suggesting an external, stellar origin rather than local, solar source. However, the stellar source of 26Al and the manner in which it was injected into the SS remain controversial: the 26Al could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud, protosolar cloud core, or protoplanetary disk. Corundum (Al2O3) is predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from 16O-rich (Δ17O -25‰) gas of solar composition recorded heterogeneous distribution of 26Al at the birth of the SS: the inferred initial 26Al/27Al ratio ranges from 6.5×10-5 to <2×10-6; 52% of corundum grains measured are 26Al-poor. Abundant 26Al- poor, 16O-rich refractory objects include grossite- and hibonite-rich calcium-aluminum-rich inclusions (CAIs) in CH (high metal abundance and high iron concentration) chondrites, platy hibonite crystals in CM (Mighei-like) chondrites, and CAIs with fractionation and unidentified nuclear effects CAIs chondrites. Considering the apparently early and short duration (<0.3Ma) of condensation of refractory 16O-rich solids in the SS, we infer that 26Al was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between 26Al abundance and O-isotope composition of corundum grains constrains the stellar source of 26Al in the SS. © 2011. The American Astronomical Society. All rights reserved.