LI-7 ABUNDANCES IN HALO STARS - TESTING STELLAR EVOLUTION MODELS AND THE PRIMORDIAL LI-7 ABUNDANCE

A large number of stellar evolution models with [Fe/H] = -2.3 and -3.3 have been calculated in order to determine the primordial Li-7 abunda nce and to test current stellar evolution models by a comparison to th e extensive database of accurate Li abundances in extremely metal-poor halo stars observed by Thorburn (1994). Standard models with gray atm ospheres do a very good job of fitting the observed Li abundances in s tars hotter than approximately 5600 K. They predict a primordial. Li-7 abundance of log N(Li) = 2.24 +/- 0.03. Models which include microsco pic diffusion predict a downward curvature in the Li-7 destruction iso chrones at hot temperatures which is not present in the observations. Thus, the observations clearly rule out models which include uninhibit ed microscopic diffusion of Li-7 from the surface of the star. Rotatio nal mixing inhibits the microscopic diffusion and the [Fe/H] = - 2.28 stellar models which include both diffusion and rotational mixing prov ide an excellent match to the mean trend in T(eff) which is present in the observations. Both the plateau stars and the heavily depleted coo l stars are well fit by these models. The rotational mixing leads to c onsiderable Li-7 depletion in these models and the primordial Li-7 abu ndance inferred from these models is log N(Li) = 3.08 +/- 0.1. However , the [Fe/H] = -3.28 isochrones reveal problems with the combined mode ls. These isochrones predict a trend of decreasing log N(Li) with incr easing T(eff) which is not present in the observations. Possible cause s for this discrepancy are discussed.