Lithium abundances in the most metal-deficient stars

Five halo dwarfs in the temperature range of the Population II Li plateau and all having (Fe/H) is less than -3.0 were found to have mildly depleted Li abundances, down on average by 0.15 dex, compared to higher metallicity halo dwarfs of comparable temperature. Evolutionary models tracing Li abundances in halo stars in the presence of several Li depletion mechanisms do not predict this difference. Four comparison stars with -2.8 is less than (Fe/H) is less than -0.6 were found to be in excellent agreement with previous 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 observation 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 temperatures, amounting to 0.03 dex per 100 K, in agreement with the predictions of depletion models. Observed Li abundances are normalized to an effective temperature near the hot end of the Spite plateau, 6200 K, and must be interpreted with this renormalization in mind. Two general possibilities are considered for the lower Li abundances seen in stars with (Fe/H) is less than -3.0: (1) that a small amount of Li production has occurred during halo formation, in which case the primordial Li abundance is near log (n(Li)/n(H)) + 12.00 = 2.05 or lower, or (2) that the most metal poor halo stars have depleted their surface Li abundances in a fashion not predicted by current models, and the primordial Li value lies instead near 2.20. The difference in the implied primordial value alters the constraints on various cosmological models, most notably the baryon density resulting from the big bang. Until the difference between higher and lower metallicity halo stars is understood, it cannot be stated with certainty whether observing more metal poor stars leads one closer toward or further away from the primordial Li abundance.