LITHIUM ABUNDANCES IN THE MOST METAL-DEFICIENT STARS

Five halo dwarfs in the temperature range of the Population II Li plat eau and all having [Fe/H] < - 3.0 were found to have mildly depleted L i abundances, down on average by 0.15 dex, compared to higher metallic ity halo dwarfs of comparable temperature. Evolutionary models tracing Li abundances in halo stars in the presence of several Li depletion m echanisms do not predict this difference. Four comparison stars with - 2.8 < [Fe/H] < - 0.6 were found to be in excellent agreement with pre vious studies. Our sample also includes the very metal poor dwarf, CS 22876-32 ([Fe/H] = - 4.2), for which we report a lithium abundance in the range log (n(Li)/n(H)) + 12.00 = 1.8 - 2.0, based on our observati on of a 15 mA Li line, contrary to a nondetection reported by Molaro, but in agreement with recent measurements reported by Thorburn & Beers . We also trace the dependence of Li abundances on stellar effective t emperatures, amounting to 0.03 dex per 100 K, in agreement with the pr edictions of depletion models. Observed Li abundances are normalized t o an effective temperature near the hot end of the Spite plateau, 6200 K, and must be interpreted with this renormalization in mind. Two gen eral possibilities are considered for the lower Li abundances seen in stars with [Fe/H] < - 3.0: (1) that a small amount of Li production ha s occurred during halo formation, in which case the primordial Li abun dance is near log (n(Li)/n(H)) + 12.00 = 2.05 or lower, or (2) that th e most metal poor halo stars have depleted their surface Li abundances in a fashion not predicted by current models, and the primordial Li v alue lies instead near 2.20. The difference in the implied primordial value alters the constraints on various cosmological models, most nota bly the baryon density resulting from the big bang. Until the differen ce between higher and lower metallicity halo stars is understood, it c annot be stated with certainty whether observing more metal poor stars leads one closer toward or further away from the primordial Li abunda nce.