Lithium-6 and gamma-rays: Constraints on primordial lithium, cosmic rays and cosmic star formation

Abstract

Cosmic-ray (CR) reactions with the interstellar gas produce both Li (predominantly via α<inf>CR</inf> + α →^6,7 Li) and γ-rays (via p<inf>CR</inf> + p → π⁰ → γγ). We establish the link between them analytically, then apply it to constrain both galactic CR nucleosynthesis as well as extragalactic γ-ray production. We exploit the extragalactic γ-ray spectrum to place constraints on the hadronic production by cosmic rays, and then use the γ - π⁰ - Li connection to place upper limits on the production of non-primordial Li via cosmic rays accelerated in the shocks arising from cosmological structure formation. Turning the problem around, we use observations of the Galactic Li both in the Solar System and in the ancient halo stars, to constraint the contribution of the galactic cosmic rays to the diffuse extragalactic γ-ray background. © 2005 Elsevier B.V. All rights reserved.