The primordial lithium abundance from extreme subdwarfs: New observations

Abstract

High-resolution (R approximately equals 28,000), high signal-to-noise (S/N approximately equals 100) spectra of the Li I lambda-6707 region have been obtained for 90 halo dwarfs and main-sequence turnoff stars with (Fe/H) approximately less than or equal to -2.2. The mean lithium abundance at 6300 K is found to be N(Li) = 12 + log (Li/H) = 2.32 +/- 0.20 (95% confidence interval), where the quoted uncertainty reflects the error in the absolute abundance zero point from all known sources, random and systematic. Contrary to the findings of Spite and Spite (1982), these data show a larger lithium abundance dispersion than can be explained by observational errors alone. The standard deviation of data points about the mean trend, excluding all upper limits, is 0.13 dex, while the typical relative abundance error due to uncertainties in the temperatures and equivalent widths is 0.08-0.09 dex. A formal dispersion analysis in the temperature-equivalent width plane rejects the null hypothesis (i.e., no intrinsic dispersion) at a greater than 6 sigma confidence level (100% - 10-8%). In order for the observed scatter to be consistent with noise, the relative equivalent width and temperature errors must both be increased by approximately 55% from their typical values of 3 mA and 100 K (1 sigma), respectively. A trend of declining N(Li) with decreasing stellar metallicity is identified as evidence of lithium production by Galactic sources. All excess scatter about the N(Li)-(Fe/H) relation is attributed to the combination of lithium production and a approximately 2 Gyr dispersion (1 sigma) in the halo metallicity-to-age relation. This additional source of lithium abundance variations from star to star also accounts for the observed intrinsic dispersion about the Spite plateau. The detection of Li-6 in HD 84937 (Smith, Lambert, and Nissen 1993c) suggests that Galactic cosmic-ray alpha + alpha reactions are the dominant source of lithium production in the early interstellar medium. The rate of Li-6 production inferred from the N(Li)-(Fe/H) trend can account for the current abundance of Li-6 observed in the interstellar medium toward zeta Oph, zeta Per (Meyer, Hawkins, & Wright 1993), and rho Oph (Lemoine et al. 1993). The primordial lithium fraction is estimated from the surface lithium abundances of the hottest, most metal-poor stars in this program: 2.22 +/- 0.20 dex.