Early Solar System irradiation quantified by linked vanadium and beryllium isotope variations in meteorites
Résumé
X-ray emission in young stellar objects (YSOs) is orders of magnitude more intense than in main sequence stars(1,2), suggestive of cosmic ray irradiation of surrounding accretion disks. Protoplanetary disk irradiation has been detected around YSOs by the Herschel Space Observatory(3). In our Solar System, short-lived Be-10 (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 tracers(6,7), suggest 10Be likely originates from cosmic ray irradiation caused by solar flares(8-10). 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 Be-10 abundance to quantify irradiation conditions in the early Solar System(11). For the initial Be-10 abundances recorded in most CAIs, V-50 excesses of a few per mil (%) relative to chondrites have been predicted(8,9). We report V-50 excesses in CAIs up to 4.4% that co-vary with Be-10 abundance. Their co-variation dictates that excess V-50 and Be-10 were synthesized through irradiation of refractory dust. Modelling of the production rate of V-50 and Be-10 demonstrates that the dust was exposed to solar cosmic rays produced by gradual flares for less than 300 years at approximate to 0.1 au from the protosun.
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