Cosmological Cosmic Rays and the Observed 6 Li Plateau in Metal‐poor Halo Stars

Abstract

Very recent observations of the 6Li isotope in halo stars reveal a 6Li plateau about 1000 times above the predicted big bang nucleosynthesis abundance. We calculate the evolution of 6Li versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that the pre-Galactic production of the 6Li isotope can account for the 6Li plateau observed in metal-poor halo stars without additional overproduction of 7Li. The derived relation between the amplitude of the CCR energy spectra and the redshift of the initial CCR production puts constraints on the physics and history of the objects, such as Population III stars, responsible for these early cosmic rays. Consequently, we consider the evolution of 6Li in the Galaxy. Since 6Li is also produced in Galactic cosmic-ray nucleosynthesis, we argue that halo stars with metallicities between [Fe/H] = -2 and -1 must be somewhat depleted in 6Li.