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