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

Abstract

The primordial Lithium Problem is intimately connected to the assumption that ${}^7\mathrm{Li}$ 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 ${}^7\mathrm{Li}$: i) the lack of dispersion in Li abundances measured at low metallicity; and ii) the detection of the more fragile ${}^6\mathrm{Li}$ isotope in at least two halo stars. The purported ${}^6\mathrm{Li}$ detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of ${}^6\mathrm{Li}$. This concordance left little room for depletion of ${}^6\mathrm{Li}$ depletion, and implied that the more robust ${}^7\mathrm{Li}$ largely evaded destruction. Recent (re)-observations of halo stars challenge the evidence against ${}^7\mathrm{Li}$ depletion: i) lithium abundances now show significant dispersion, and ii) sensitive ${}^6\mathrm{Li}$ searches now reveal only firm upper limits to the ${}^6\mathrm{Li}/{}^7\mathrm{Li}$ ratio. The tight new ${}^6\mathrm{Li}$ upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial ${}^6\mathrm{Li}$ depletion has occurred--by factors up to 50. We show that in stars with ${}^6\mathrm{Li}$ limits and thus lower bounds on ${}^6\mathrm{Li}$ depletion, an equal amount of ${}^7\mathrm{Li}$ depletion is more than sufficient to resolve the primordial ${}^7\mathrm{Li}$ Problem. This picture is consistent with stellar models in which ${}^7\mathrm{Li}$ is less depleted than ${}^6\mathrm{Li}$, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas. (abridged)

Comments: 29 pages, 3 figures. Comments welcome