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

Abstract

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. Recent (re)-observations of halo stars challenge the evidence against ${}^{7}{\rm Li}$ depletion: i) lithium abundances now show significant dispersion, and ii) sensitive ${}^{6}{\rm Li}$ searches now reveal only firm upper limits to the ${}^{6}{\rm Li}/{}^{7}{\rm Li}$ ratio. The tight new ${}^{6}{\rm Li}$ upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial ${}^{6}{\rm Li}$ depletion has occurred--by factors up to 50. We show that in stars with ${}^{6}{\rm Li}$ limits and thus lower bounds on ${}^{6}{\rm Li}$ depletion, an equal amount of ${}^{7}{\rm Li}$ depletion is more than sufficient to resolve the primordial ${}^{7}{\rm Li}$ Problem. This picture is consistent with stellar models in which ${}^{7}{\rm Li}$ is less depleted than ${}^{6}{\rm Li}$, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas. (abridged)