60 Fe in Chondrites: Debris from a Nearby Supernova in the Early Solar System?

Abstract

60Fe decays to 60Ni with a half-life of 1.49 × 106 yr, so all of the original 60Fe atoms incorporated into the solar system have decayed. Because 60Fe 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, 60Fe is also a potential high-resolution chronometer of early-solar-system events. The presence of 60Fe in primitive meteorites has been confirmed in sulfides, but the initial abundance of 60Fe in the solar system has been only loosely constrained because it is uncertain when the sulfides formed. We show that 60Fe was present with abundance ratios of 60Fe/56Fe = (2.2-3.7) × 10-7 when ferromagnesian chondrules 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 60Fe/56Fe ratio [(60Fe/56Fe)0] of (5-10) × 10-7 is estimated. This new solidly based (60Fe/56Fe)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.