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A New Model for the Evolution of Light Elements in an Inhomogeneous Galactic Halo

We present predictions of the evolution of the light elements, Li, Be, and B, in the early epochs of the Galactic halo, using a model of supernova-induced chemical evolution based on contributions from supernovae (SNe) and cosmic rays (CRs), as recently proposed by Tsujimoto et al. and Suzuki et al. This model has the great advantage of treating various elements self-consistently, even under inhomogeneous conditions, as might arise from stochastic star formation processes triggered by SN explosions. The most important prediction from our model is that the abundances of light elements in extremely metal-poor stars might be used as age indicators in the very early stages of an evolving halo population, at times when the abundances of heavy elements ("metallicity") in most stars are dominated by local metal enrichment due to nearby SN events, and is poorly correlated with age. Plots of the expected frequency distribution of stars in the age versus elemental abundance diagram show that the best "cosmic clock" is the 6Li abundance. We have derived relationships among various cosmic-ray parameters such as energy input to CRs by SNe, the spectral shape of the CRs, and the chemical composition in CRs, and find that we can reproduce very well recent observations of 6Li, Be, and B in metal-poor stars. Although our model is successful for certain sets of cosmic-ray parameters, larger energy should be absorbed by energetic particles from each SN than required to the current situation of Galactic disk. We discuss an alternative hypothesis of active galactic nuclei activity in the early Galaxy as another possible accelerator of CRs.