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-inducedchemical evolution based on contributions from supernovae (SNe) and cosmic rays(CRs). The most important prediction from our model is that the abundances oflight elements in extremely metal-poor stars might be used as age indicators inthe very early stages of an evolving halo population, at times when metallicityin most stars are dominated by local metal enrichment due to nearby SN events,and is poorly correlated with age. Our results show that the best ``cosmicclock'' is the $^6$Li abundance. We have derived relationships among variouscosmic-ray parameters. Although our model is successful for certain sets ofcosmic-ray parameters, larger energy should be absorbed by energetic particlesfrom each SN than required to the current situation of Galactic disk. Wediscuss an alternative hypothesis of AGN activity in the early Galaxy asanother possible accelerator of CRs.Comment: ApJ in press, 32 pages including 8 figures (3 fiures, fig.3,5,& 7 in color