X-ray emission in young stellar objects (YSOs) is orders of magnitude more intense than in main sequence stars1,2, suggestive of cosmic ray irradiation of surrounding accretion disks. Protoplanetary disk irradiation has been detected around YSOs by the Herschel Space Observatory3. In our Solar System, short-lived 10Be (with a half-life of 1.39 Myr)4, which cannot be produced by stellar nucleosynthesis, was discovered in the oldest Solar System solids, the calcium–aluminium-rich inclusions (CAIs)5. The high 10Be abundance, as well as the detection of other tracers6,7, suggest 10Be likely originates from cosmic ray irradiation caused by solar flares8–10. Nevertheless, the nature of these flares (gradual or impulsive), the target (gas or dust), and the duration and location of irradiation remain unknown. Here we use the vanadium isotopic composition, together with the initial 10Be abundance to quantify irradiation conditions in the early Solar System11. For the initial 10Be abundances recorded in most CAIs, 50V excesses of a few per mil (‰) relative to chondrites have been predicted8,9. We report 50V excesses in CAIs up to 4.4‰ that co-vary with 10Be abundance. Their co-variation dictates that excess 50V and 10Be were synthesized through irradiation of refractory dust. Modelling of the production rate of 50V and 10Be demonstrates that the dust was exposed to solar cosmic rays produced by gradual flares for less than 300 years at ≈0.1 au from the protosun.
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P.A.S. and F.M. are grateful to the European Research Council under the European Community’s Horizon 2020framework program/ERC grant agreement no. 637503 (Pristine). M.C. and F.M. thank the LabEx UnivEarths (ANR-10-LABX-0023) and ANR Cradle (ANR-15-CE31-0004-01). M.G. and F.M. were supported by the Institut Universitaire de France. We appreciate for the assistance of D. Limmois and L. Faure in the clean lab, and J. Moureau on the Neptune at Institut de Physique du Globe de Paris. We are grateful for the constructive and detailed comments of three anonymous reviewers, which resulted in a more complete framing of the debate and discussion of isotopic fractionation and existing irradiation models.
The authors declare no competing financial interests.
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Sossi, P., Moynier, F., Chaussidon, M. et al. Early Solar System irradiation quantified by linked vanadium and beryllium isotope variations in meteorites. Nat Astron 1, 0055 (2017). https://doi.org/10.1038/s41550-017-0055
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