Astrophysical site of the origin of the solar system inferred from extinct radionuclide abundances

Extinct radionuclides in the solar abundance distribution (SAD) provide a basis with which to characterize the molecular cloud environment in which the solar system formed 4566 +/- 2 Ma ago. The low abundance of the longer-lived r-process radionuclide I-129 (T-1/2 = 16 Ma) indicates a long (similar to 10(2) Ma) isolation time from energetic interstellar medium (ISM) reservoirs containing most of the Galaxy's budget of freshly-synthesized Type II supernova products. However, the abundances of the shorter-lived species Fe-60 (T-1/2 = 1.5 Ma), Mn-53 (T-1/2 = 3.7 Ma), and Pd-107 (T-1/2 = 6.5 Ma) are consistent with late admixture of freshly synthesized Type II supernova products. The fit for these species is based on an average yield distribution obtained by decomposition of the SAD. The apparent timescale contradiction is resolved in a simple two timescale molecular cloud self-contamination model consistent with formation of the Sun in an old evolved stellar complex at the eroding boundary of a molecular cloud interacting with an adjacent OB association. Admixture of an similar to 10(-5) to similar to 10(-6) mass fraction of Type II supernova ejecta into the presolar cloud dominates the shorter-lived species and Pd-107, whereas longerlived I-129 preserves information on the longer timescale constraining the mean isolation/condensation/accretion age of the molecular material in the protosolar reservoir. The inferred model age of nucleosynthetic isolation in the long timescale is consistent with cyclicity in the nucleosynthesis rate in an orbiting ISM parcel controlled by galactic spiral structure and beads-on-a-string organization of star formation in ''stellar complexes'' in arms. Abundant Al-26 (T-1/2 = 0.7 Ma) in the early solar system at similar to 10(2) times the model prediction may point to Al-26/Al-27 ratio of similar to 0.2 in the source, or an similar to 10(2) times greater mixing fraction for pre-explosion winds over postexplosion ejecta. A mass-losing low-mass asymptotic giant branch (AGB) star model can be tuned to account for Ca-41, Al-26, Fe-60, and Pd-107, but fails for Mn-53, requires unusual s-process conditions, and is a priori improbable. Another alternate hypothesis, cosmic-ray spallation in an OB association, is limited as a radionuclide source by LiBeB overproduction, except for improbably fine-tuned conditions. Supernova self-contamination may be a widespread process in evolved star-forming regions, but mixing dynamics and their relation to star formation are poorly understood.