Evolution of beryllium and boron in the inhomogeneous early galaxy

A model of supernova-driven chemical evolution of the Galactic halo, recent ly proposed by Tsujimoto, Shigeyama, & Yoshii, is extended in order to inve stigate the evolution of light elements such as Be and B (BeB), which are p roduced mainly through spallative reactions with Galactic cosmic rays (GCRs ). In this model, each supernova (SN) sweeps up the surrounding interstella r gas into a dense shell and directly enriches it with ejecta which consist of heavy elements produced in each Type II supernova with different progen itor masses. We propose a two-component source for GCRs such that both inte rstellar gas and fresh SN ejecta engulfed in the shell are accelerated by t he shock wave. The released GCRs travel much faster than the expansion of t he shell and thus produce the BeB elements far outside the shell which will be incorporated in subsequent formation of shells arising from later SNe. As a result, stars formed from coeval shells are predicted to show a large scatter in the abundance of heavy elements while exhibiting BeB abundances similar to that in the gas, with no appreciable scatter. This indicates tha t, contrary to heavy elements, stellar BeB abundances might be used as a go od age indicator in the inhomogeneous Galactic halo. The production of BeB at early epochs is dominated by the primary process through spallation of h eavy GCRs, although it is a minor component in the bulk of the GCR composit ion at present. We have calculated the frequency distribution of long-lived stars in the log (BeB/H)-[Fe/H] plane and find that the contour of constan t frequency covering a range of -3 < [Fe/H] < -1 in this plane is consisten t with the observed linear trend between BeB and Pe. We show from our calcu lations that there is an intriguing possibility of distinguishing between s tandard and nonstandard big bang nucleosynthesis models if BeB abundances i n several hundred halo stars are observed in the future.