Protostellar cosmic rays and extinct radioactivities in meteorites

摘要

[[abstract]]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 Ca-41, Mn-53, and La-138 inferred for meteorites, but proton- and alpha-induced transformations underproduce Al-26 by a factor of about 20. The missing Al-26 may be synthesized by He-3 nuclei accelerated in impulsive flares reacting primarily with Mg-24, an abundant isotope in the target precursor rocks. The mechanism allows a simple explanation for the very different ratios of Al-26/Al-27 inferred for normal CAIs, CAIs with fractionated and unidentified nuclear (FUN) anomalies, and chondrules. The overproduction of Ca-41 by analogous He-3 reactions and the case of Fe-60 inferred for eucritic meteorites require special interpretations in this picture.