Bimodal production of Be and B in the early Galaxy

Abstract

Recently models based on the acceleration of metal-rich material inside superbubbles have been proposed to account for the observed abundances of Be and B in metal-poor halo stars. We analyse some of the implications of these models for the distribution of the Be/O and B/O abundance ratios. In particular, we discuss the possible scatter in the data and argue that LiBeB production in the very early Galaxy was probably bimodal, with isolated supernovae giving rise to a low-efficiency mechanism, and collective supernovae exploding in an OB association inducing a high-efficiency mechanism. This should produce two populations of halo stars, one with high L/M ratios (light elements/metals), and the other with L/M ratios about ten times lower. The relative weight of these two populations depends on the fraction of supernovae exploding inside superbubbles. In this context, we discuss the recent observation of the B-depleted, Li-normal star HD 160617 (Primas, et al., 1998), as well as the reported spread in the Be data at [Fe/H] ~ -2.2 (Boesgaard, et al., 1999). Finally, we predict that Be will be found to be even more deficient than B in HD 160617.