Can Galactic Cosmic Rays Account for Solar 6Li without Overproducing Gamma Rays?

Abstract

Cosmic-ray interactions with interstellar gas produce both Li-6, which accumulates in the interstellar medium (ISM), and mesons, which decay to gamma rays that propagate throughout the cosmos. Local Li-6 abundances and extragalactic gamma rays thus have a common origin that tightly links them. We exploit this connection to use gamma-ray observations to infer the contribution to Li-6 nucleosynthesis by standard Galactic cosmic-ray (GCR) interactions with the ISM. Our calculation uses a carefully propagated cosmic-ray spectrum and accounts for Li-6 production from both fusion reactions (alpha alpha -> Li-6) as well as spallation channels (p, alpha + CNO -> Li-6). We find that, although extreme assumptions yield a consistent picture, more realistic ones indicate that solar Li-6 cannot be produced by standard GCRs alone without overproducing the hadronic gamma rays. Implications for the primordial Li-6 production by decaying dark matter and cosmic rays from cosmological structure formation are discussed. Upcoming gamma-ray observations by the Gamma-Ray Large Area Space Telescope will be crucial for determining the resolution of this problem.