Implications of the Non-Observation of ${}^{6}{\rm Li}$ in Halo Stars for the Primordial ${}^{7}{\rm Li}$ Problem

Understand

The primordial Lithium Problem is intimately connected to the assumption that ${}^{7}{\rm Li}$ observed in metal-poor halo stars retains its primordial abundance, which lies significantly below the predictions of standard big-bang nucleosynthesis.

• Two key lines of evidence have argued that these stars have not significantly depleted their initial ${}^{7}{\rm Li}$: i) the lack of dispersion in Li abundances measured at low metallicity; and ii) the detection of the more fragile ${}^{6}{\rm Li}$ isotope in at least two halo stars.
• The purported ${}^{6}{\rm Li}$ detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of ${}^{6}{\rm Li}$.
• This concordance left little room for depletion of ${}^{6}{\rm Li}$ depletion, and implied that the more robust ${}^{7}{\rm Li}$ largely evaded destruction.