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In agreement with previous work, we show that the presence of the short-livedradionuclide Al-26 in the early Solar System was unlikely (<2% a prioriprobability) to be the result of direct introduction of supernova ejecta intothe gaseous disk during the Class II stage of protosolar evolution. We alsoshow that any Bondi-Hoyle accretion of contaminated residual gas from the natalstar cluster made a negligible contribution to the primordial Al-26 inventoryof the Solar System. These results are consistent with the absence of theoxygen isotopic signature expected with any late introduction of supernovaejecta into the protoplanetary disk. Instead, the presence of Al-26 in theoldest Solar System solids (calcium-aluminum-rich inclusions or CAIs) and itsapparent uniform distribution with the inferred canonical Al-26/Al-27 ratio of(4.5-5)E-5 support the inheritance of Al-26 from the parent giant molecularcloud. We propose that this radionuclide originated in a prior generation ofmassive stars that formed in the same host molecular cloud as the Sun andcontaminated that cloud by Wolf-Rayet winds. We calculated the Galacticdistribution of Al-26/Al-27 ratios that arise from such contamination using theestablished embedded cluster mass and stellar initial mass functions, publishednucleosynthetic yields from the winds of massive stars, and by assuming rapidand uniform mixing into the cloud. Although our model predicts that themajority of stellar systems contain no Al-26 from massive stars, and that the apriori probability that the Al-26/Al-27 ratio will reach or exceed thecanonical Solar System value is only ~6%, the maximum in the distribution ofnon-zero values is close to the canonical ratio.

Al-26 and the formation of the Solar System from a molecular cloud contaminated by Wolf-Rayet winds