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-ind uced chemical evolution based on contributions from supernovae (SNe) and co smic rays (CRs), as recently proposed by Tsujimoto et al. and Suzuki et al. This model has the great advantage of treating various elements self-consi stently, even under inhomogeneous conditions, as might arise from stochasti c star formation processes triggered by SN explosions. The most important p rediction from our model is that the abundances of light elements in extrem ely metal-poor stars might be used as age indicators in the very early stag es of an evolving halo population, at times when the abundances of heavy el ements ("metallicity") in most stars are dominated by local metal enrichmen t due to nearby SN events, and is poorly correlated with age. Plots of the expected frequency distribution of stars in the age versus elemental abunda nce diagram show that the best "cosmic clock" is the Li-6 abundance. We hav e derived relationships among various cosmic-ray parameters such as energy input to CRs by SNe, the spectral shape of the CRs, and the chemical compos ition in CRs, and find that we can reproduce very well recent observations of Li-6, Be, and B in metal-poor stars. Although our model is successful fo r certain sets of cosmic-ray parameters, larger energy should be absorbed b y energetic particles from each SN than required to the current situation o f Galactic disk. We discuss an alternative hypothesis of active galactic nu clei activity in the early Galaxy as another possible accelerator of CRs.