Cosmological cosmic rays and the observed⁶Li plateau in metal-poor halo stars

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 over-production 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.