  It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct
radionuclide, was uniformly distributed in the nascent Solar System with the
initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar
origin. However, the stellar source of Al-26 and the manner in which it was
injected into the solar system remain controversial: the Al-26 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 or protoplanetary disk.
Corundum (Al2O3) is thermodynamically predicted to be the first condensate from
a cooling gas of solar composition. Here we show that micron-sized corundum
condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition
recorded heterogeneous distribution of Al-26 at the birth of the solar system:
the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50%
of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich
refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich
calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite
crystals in CM chondrites, and FUN (fractionation and unidentified nuclear
isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the
apparently early and short duration (<0.3 Ma) of condensation of refractory
O-16-rich solids in the solar system, we infer that Al-26 was injected into the
collapsing protosolar molecular cloud and later homogenized in the
protoplanetary disk. The apparent lack of correlation between Al-26 abundance
and O-isotope compositions of corundum grains put important constraints on the
stellar source of Al-26 in the solar system.
