  In agreement with previous work, we show that the presence of the short-lived
radionuclide Al-26 in the early Solar System was unlikely (<2% a priori
probability) to be the result of direct introduction of supernova ejecta into
the gaseous disk during the Class II stage of protosolar evolution. We also
show that any Bondi-Hoyle accretion of contaminated residual gas from the natal
star cluster made a negligible contribution to the primordial Al-26 inventory
of the Solar System. These results are consistent with the absence of the
oxygen isotopic signature expected with any late introduction of supernova
ejecta into the protoplanetary disk. Instead, the presence of Al-26 in the
oldest Solar System solids (calcium-aluminum-rich inclusions or CAIs) and its
apparent 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 molecular
cloud. We propose that this radionuclide originated in a prior generation of
massive stars that formed in the same host molecular cloud as the Sun and
contaminated that cloud by Wolf-Rayet winds. We calculated the Galactic
distribution of Al-26/Al-27 ratios that arise from such contamination using the
established embedded cluster mass and stellar initial mass functions, published
nucleosynthetic yields from the winds of massive stars, and by assuming rapid
and uniform mixing into the cloud. Although our model predicts that the
majority of stellar systems contain no Al-26 from massive stars, and that the a
priori probability that the Al-26/Al-27 ratio will reach or exceed the
canonical Solar System value is only ~6%, the maximum in the distribution of
non-zero values is close to the canonical ratio.
