Cosmic Rays and Light Elements

Light Elements, Li Be B, with proton number of 3-5 are heavier than Helium and lighter than Carbon. Although they are very rare on the earth, they are intriguing elements, since their origins in our cosmos are quite different from those of the heavier elements, C N O....
While the heavy elements are produced by processes involving stellar activities, such as supernovae, novae, and AGB stars, the light elements are mainly synthesized by inelastic reactions of cosmic rays, non-thermal particles flying with high speed in the universe. Therefore, we can investigate physics of cosmic rays (acceleration, propagation, evolution, ...) by examining evolutionary histories of the light elements. Firstly, by collaborating with Profs. Yoshii & Kajino, we investigated the evolutions of LiBeB in an inhomogeneous (not well-mixed) early Galaxy (Suzuki, Yoshii, & Kajino 1999, Suzuki, Yoshii & Beers 2000, Suzuki & Yoshii 2001). In these papers, we have pointed out that the light elements can be used as better time indicators than the heavy elements.

Li-6 Galactic Archaeology

Next, we studied evolution of Li-6 which is synthesized via the fusion reaction of two alpha particles, besides spallations of CNO. The fusion reaction is unique in Li, which cannot produce Be & B. At early epoch of our Galaxy, observed Li-6 abundance in metal-poor stars exceed the theoretical prediction from the 'standard' model by 1 dex, which remains as a severe discrepancy. We have proposed a process of Li-6 production by cosmic rays accelerated by virialization shocks through the structure formation of our Galaxy. (Suzuki & Inoue 2002) Namely, the primordial gas consisting of H & He (alpha = He++) can selectively produce the Li isotopes, and this effect is only 'visible' in Li-6 because an enormous amount of Li-7 has already synthesized during big bang. This process, which has completely failed to be noticed before our study, naturally solves the discrepancy. This scenario further predicts that future observation of Li-6 can grab the evidence of the formation of our Milky Way. It is worth establishing Li-6 Archaeology.

Evolutions of Li-6(red) and Be-9(blue). Solid and dashed lines are the results with and without cosmic rays from structure formation shocks.