PRODUCTION OF AL-26 AND OTHER EXTINCT RADIONUCLIDES BY LOW-ENERGY HEAVY COSMIC-RAYS IN MOLECULAR CLOUDS

THE high abundance of radioactive Al-26 in meteoritic materials(1) presents a puzzle in attempts to describe the formation of the Solar System. It has been suggested(11) that collapse of the solar nebula may have been accompanied by an injection of Al-26 from nucleosynthesis in a neighbouring star. The relatively high level of Al-26 in the interstellar medium (2,3) 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 discovery(4) of gamma-ray line emission from C-12 and O-16 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 Al-26 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.