Meteoritic evidence of a late superflare as source of 7Be in the early Solar System

Abstract

Fossil meteoritic records of short-lived, now-extinct radionuclides provide crucial high-resolution temporal information about the events, processes and activity of the Sun during the early phases of Solar System formation1. The proposed genesis of one such radionuclide,10Be, by spallation reactions of carbon and oxygen2–5 led to the hypothesis of enhanced irradiation in the early Solar System6–8. An alternative scenario of production of 10Be (half-life t1/2 = 1.386 ± 0.016 million years9) by a neutrino process in a supernova arising from the core collapse of a low-mass star (11.8 solar masses, M⊙) has recently been suggested10 and can explain the observed abundance of 10Be in the early Solar System. Here, we report well-resolved excesses in 7Li/6Li of up to ~21.5% in a calcium- and aluminium-rich inclusion (CAI) from the Efremovka meteorite that correlate with 9Be/6Li, suggestive of in situ decay of 7Be. The in situ decay of 7Be to 7Li, with a characteristic half-life of 53.12 ± 0.07 days11, entails multiple episodes of enhanced irradiation in the early Solar System, which have been observed recently in other Sun-like stars12,13. The short half-life of 7Be limits its production by interaction of solar energetic particles (SEPs) with the nebular gas and solids, and provides constraints on the genealogy and chronology of CAIs. Irradiation of the solid and gaseous precursors of CAIs of solar composition by a superflare (X-ray luminosity approximately 1032 erg s−1) during the terminal phase of class I or II of the pre-main-sequence stages of the Sun explains the isotopic properties, distinctive petrographic features and diffusivity constraints in the CAI. Li and Be isotopic measurements of calcium–aluminium inclusions (CAIs) in the Efremovka meteorite suggest that the solid and gaseous precursors of CAIs were irradiated by a superflare during the last stages of the pre-main-sequence evolution of the Sun.