COSMIC LITHIUM: GOING UP OR COMING DOWN ?

Abstract

Observations of interstellar lithium provide a valuable complement to studies of lithium in Pop I and Pop II stars. Large corrections for unseen LiII and for non-gas phase lithium have provided obstacles to using interstellar data for abundance determinations. An approach to surmounting these difficulties is proposed and is applied to the Galaxy and the LMC. The key is that since potassium and lithium behave similarly regarding ionization and depletion, their observed ratio (LiI/KI) can be used to probe the abundance and evolution of lithium. For ten lines-of-sight in the interstellar medium of the Galaxy (ISM) the Li/K ratio observed $(\log (N_{Li}/N_K)_{ISM} = - 1.88 \pm 0.09)$ is entirely consistent with the solar system value $(\log (N_{Li}/N_{K})_{\odot} = - 1.82 \pm 0.05)$. The absence of LiI in front of SN87A in the LMC, coupled with the observed KI, corresponds to an upper bound (at $\rsim 95 \% \ CL$) of $\log (N_{Li}/N_K)_{LMC} < - 0.3 + \log (N_{Li}/N_K)_{ISM}.$ This low upper bound to LMC lithium suggests that cosmic lithium is on its way up from a primordial abundance lower, by at least a factor of two, than the present Pop I value of $[Li]_{PopI} \equiv 12 + \log (Li/H)_{PopI} = 3.2 \pm 0.1$.