Production of 26A1 and other extinct radionuclides by low-energy heavy cosmic rays in molecular clouds

Abstract

THE high abundance of radioactive 26A1 in meteoritic materials1 presents a puzzle in attempts to describe the formation of the Solar System. It has been suggested11 that collapse of the solar nebula may have been accompanied by an injection of 26A1 from nucleosynthesis in a neighbouring star. The relatively high level of 26A1 in the interstellar medium2,3 is also unexplained. Here I suggest that this isotope may be formed in nuclear reactions between hydrogen and heavy cosmic rays. This suggestion is prompted by the recent discovery4 of γ-ray line emission from 12C and 16O in the Orion cloud complex, thought to be caused by the interaction of these cosmic-ray ions with interstellar hydrogen. I show that these observations also imply enhanced production of 26A1 in the Orion molecular clouds, and that such nuclear reactions can account for the meteoritic abundances. Reactions involving heavy cosmic rays might also account for the presence of other extinct radioactive nuclides in meteorites.