The astrophysical site of the origin of the solar system inferred from extinct radionuclides

Abstract

Ab initio abundances of extinct radio nuclides in the solar abundance distribution (SAD) provide clues sufficient to characterize the molecular cloud environment within which the solar system formed 4566±2 Ma ago. Key observations are: (i) the low abundance of the longer‐lived r‐process radionuclide 129I indicates an ∼102 Ma isolation time from freshly‐synthesized Type II supernova products; (ii) abundances of the shorter‐lived species (26Al, 60Fe, 53Mn) are consistent with late ‘‘injection’’ of freshly‐synthesized Type II supernova produces. This apparent contradiction is resolved in a simple two time scale molecular cloud self‐contamination model consistent with formation of the sun in an evolved star‐forming region in the vicinity of an OB association. Admixture of an ∼10−5 to ∼10−6 mass fraction of Type II supernova ejecta into the cloud dominates the shorter‐lived species and 107Pd, but contributes only ∼1/2 of the 129I budget. 129I consequently preserves the longer time scale information constraining the mean isolation/condensation/accretion model age of the cool molecular mass reservoir with respect to 129I/127I in the global ISM. All extinct radionuclide abundances except 26Al are reproduced in this model. An alternate hypothesis involving late admixture of matter from a mass‐losing low mass AGB star can account for 26Al, 60Fe, and 107Pd, but fails for 53Mn, requires unusual s‐process conditions, and otherwise appears ad hoc and unlikely. Cosmic ray spallation in an OB association environment may contribute significantly to 53Mn in the protosolar reservoir, but is limited as a putative 26Al source by over production of 53Mn, 92gNb, and Li.