Protostellar cosmic rays and extinct radioactivities in meteorites

Calcium-aluminum-rich inclusions (CAIs) and chondrules of chondritic meteorites may originate with the melting of dustballs launched by a magnetically driven bipolar outflow from the inner edge of the primitive solar nebula. Bombardment by protostellar cosmic rays may make the rock precursors of CAIs and chondrules radioactive, producing radionuclides found in meteorites that are difficult to obtain with other mechanisms. Reasonable scalings from the observed hard X-rays for the cosmic-ray protons released by flares in young stellar objects yield the correct amounts of ⁴¹Ca, ⁵³Mn, and ¹³⁸La inferred for meteorites, but proton- and α-induced transformations underproduce ²⁶Al by a factor of about 20. The missing ²⁶Al may be synthesized by ³He nuclei accelerated in impulsive flares reacting primarily with ²⁴Mg, an abundant isotope in the target precursor rocks. The mechanism allows a simple explanation for the very different ratios of ²⁶Al/ ²⁷Al inferred for normal CAIs, CAIs with fractionated and unidentified nuclear (FUN) anomalies, and chondrules. The overproduction of ^4lCa by analogous ³He reactions and the case of ⁶⁰Fe inferred for eucritic meteorites require special interpretations in this picture.