The Primordial Abundance of $^6$Li and $^9$be

Abstract

Light element ($^6$Li, $^7$Li and $^9$Be) depletion isochrones for halo stars have been calculated with standard stellar evolution models. These models include the latest available opacities and are computed through the sub-giant branch. If $^6$Li is not produced in appreciable amounts by stellar flares, then the detection of $^6$Li in HD 84937 by Smith, Lambert \& Nissen (1993) is compatible with standard stellar evolution and standard big bang nucleosynthesis only if HD 84937 is a sub-giant. The present parallax is inconsistent with HD 84937 being a sub-giant star at the $2.5\, \sigma$ level. The most metal poor star with a measured $^9$Be abundance is HD 140283, which is a relatively cool sub-giant. Standard stellar evolution predict that $^9$Be will have been depleted in this star by $\sim 0.3$ dex (for ${\rm T_{eff}} = 5640$ K). Revising the abundance upward changes the oxygen-beryllium relation, suggesting incompatible with standard comic ray production models, and hence, standard big bang nucleosynthesis. However, an increase in the derived temperature of HD 140283 to 5740 K would result in no depletion of $^9$Be and agreement with standard big bang nucleosynthesis.