~(60)Fe IN CHONDRITES: DEBRIS FROM A NEARBY SUPERNOVA IN THE EARLY SOLAR SYSTEM?

摘要

~(60)Fe decays to ~(60)Ni with a half-life of 1.49 x 10~6 yr, so all of the original ~(60)Fe atoms incorporated into the solar system have decayed. Because ~(60)Fe is produced only in stars, its initial abundance in the solar system provides a constraint on the stellar contribution of radionuclides to the early solar system and on the nature of the stellar source. Because of its short half-life, ~(60)Fe is also a potential high-resolution chronometer of early-solar-system events. The presence of ~(60)Fe in primitive meteorites has been confirmed in sulfides, but the initial abundance of ~(60)Fe in the solar system has been only loosely constrained because it is uncertain when the sulfides formed. We show that ~(60)Fe was present with abundance ratios of ~(60)Fe/~(56)Fe = (2.2-3.7) x 10~(-4) when ferromagnesianchondrules formed. By applying the time difference of 1.5-2.0 million years between formation of ferromagnesian chondrules and Ca-Al-rich inclusions (CAIs), the oldest known solar system solids, a solar system initial ~(60)Fe/~(56)Fe ratio [(~(60)Fe/~(56)Fe)_0] of (5-10) x 10~(-7) is estimated. This new solidly based (~(60)Fe/~(56)Fe)_0 ratio is consistent with predictions for nucleosynthesis in a supernova or in an intermediate-mass asymptotic giant branch (AGB) star just before the solar system formation, but is too high for the source to have been a low-mass AGB star. Considering the rarity of encounters between a molecular cloud and an AGB star, our results can be considered strong evidence of a contribution of material from a nearby supernova and of a role for a supernova in the origin of the solar system.