The Timing of Potential Last Nucleosynthetic Injections into the Protosolar Molecular Cloud Inferred from 41Ca–26Al Systematics of Bulk CAIs

Abstract

Short-lived radionuclides (SLRs) provide important information about the chronology of the early solar system. Among them, 41Ca, due to its decay to 41K with a half-life of only 0.1 Ma, is particularly valuable in constraining the timescales and origins of both SLRs and the formation of the oldest solar system materials, the Ca–Al-rich inclusions (CAIs). The initial abundance of 41Ca in the solar system, expressed as the (41Ca/40Ca)I ratio, is the key to unveiling the origin of this nuclide. Here, we report a new solar system (41Ca/40Ca)I ratio of 2.0 × 10−8 derived from the K isotope compositions of two CAIs. This new ratio is about four times higher than the previous value inferred from a mineral isochron. Such a high (41Ca/40Ca)I ratio in the CAIs exceeds that expected for the protosolar molecular cloud by ∼1000×, implying very late injection of the 41Ca (and possibly other SLRs) into the protosolar molecular cloud. The correlated enrichments of 41Ca and 26Al in the bulk CAI samples hint at a common stellar origin of both SLRs. The injection time estimated from our new data depends on the stellar source—it ranges from 0.6 Ma for a Wolf–Rayet wind to 1.0 Ma for a TP-AGB star ejecta.