Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...466.1026h&link_type=abstract
Astrophysical Journal v.466, p.1026
Astronomy and Astrophysics
Astronomy
23
Meteors, Meteoroids, Nuclear Reactions, Nucleosynthesis, Abundances, Solar System: Formation
Scientific paper
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 129I(T½ = 16 Ma) indicates a long (˜ 102 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 60Fe (T½ = 1.5 Ma), 53Mn (T½ = 3.7 Ma), and 107Pd (T½ = 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 ˜10-5 to ˜10-6 mass fraction of Type II supernova ejecta into the presolar cloud dominates the shorter-lived species and 107Pd, whereas longer- lived 129I 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 26Al (T½ = 0.7 Ma) in the early solar system at ˜102 times the model prediction may point to 26Al/27Al ratio of ˜0.2 in the source, or an ˜102 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 41Ca, 26Al, 60Fe, and 107Pd, but fails for 53Mn, 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.
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