Implications of a Subthreshold Resonance for Stellar Beryllium Depletion

Abstract

Abundance measurements of the light elements lithium, beryllium, and boron are playing an increasingly important role in the study of stellar physics. Because these elements are easily destroyed in stars at temperatures (2 - 4) × 106 K, the abundances in the surface convective zone are diagnostics of the star's internal workings. Standard stellar models cannot explain depletion patterns observed in low-mass stars, and so do not account for all the relevant physical processes. These processes have important implications for stellar evolution and primordial lithium production in big bang nucleosynthesis. Because beryllium is destroyed at slightly higher temperatures than lithium, observations of both light elements can differentiate between the various proposed depletion mechanisms. Unfortunately, the reaction rate for the main destruction channel 9Be(p, α)6Li is uncertain. A level in the compound nucleus 10B is only 25.7 keV below the reaction's energetic threshold. The angular momentum and parity of this level are not well known; current estimates indicate that the resonance entrance channel is either s- or d-wave. We show that an s-wave resonance can easily increase the reaction rate by an order of magnitude at temperatures T ≈ 4 × 106 K. Observations of M < M☉ stars can constrain the strength of the resonance, as can experimental measurements at laboratory energies lower than 30 keV.