ASTROPHYSICAL SITE OF THE ORIGIN OF THE SOLAR-SYSTEM INFERRED FROM EXTINCT RADIONUCLIDE ABUNDANCES

Extinct radionuclides in the solar abundance distribution (SAD) provid e a basis with which to characterize the molecular cloud environment i n which the solar system formed 4566 +/- 2 Ma ago. The low abundance o f the longer-lived r-process radionuclide I-129 (T-1/2 = 16 Ma) indica tes a long (similar to 10(2) Ma) isolation time from energetic interst ellar medium (ISM) reservoirs containing most of the Galaxy's budget o f freshly-synthesized Type II supernova products. However, the abundan ces 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 admixt ure of freshly synthesized Type II supernova products. The fit for the se species is based on an average yield distribution obtained by decom position of the SAD. The apparent timescale contradiction is resolved in a simple two timescale molecular cloud self-contamination model con sistent with formation of the Sun in an old evolved stellar complex at the eroding boundary of a molecular cloud interacting with an adjacen t 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 d ominates the shorter-lived species and Pd-107, whereas longerlived I-1 29 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 isola tion in the long timescale is consistent with cyclicity in the nucleos ynthesis rate in an orbiting ISM parcel controlled by galactic spiral structure and beads-on-a-string organization of star formation in ''st ellar complexes'' in arms. Abundant Al-26 (T-1/2 = 0.7 Ma) in the earl y solar system at similar to 10(2) times the model prediction may poin t 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 po stexplosion ejecta. A mass-losing low-mass asymptotic giant branch (AG B) 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 i s a priori improbable. Another alternate hypothesis, cosmic-ray spalla tion in an OB association, is limited as a radionuclide source by LiBe B overproduction, except for improbably fine-tuned conditions. Superno va self-contamination may be a widespread process in evolved star-form ing regions, but mixing dynamics and their relation to star formation are poorly understood.