Abundances In Very Metal‐Poor Dwarf Stars

We discuss the detailed composition of 28 extremely metal-poor dwarfs, 22 ofwhich are from the Hamburg/ESO Survey, based on Keck Echelle spectra. Oursample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhatless metal-poor than was expected from [Fe/H](HK,HES) determined from lowresolution spectra. Our analysis supports the existence of a sharp decline inthe distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So farno additional turnoff stars with [Fe/H]}<-3.5 have been identified in ourfollow up efforts. For the best observed elements between Mg and Ni, we findthat the abundance ratios appear to have reached a plateau, i.e. [X/Fe] isapproximately constant as a function of [Fe/H], except for Cr, Mn and Co, whichshow trends of abundance ratios varying with [Fe/H]. These abundance ratios atlow metallicity correspond approximately to the yield expected from Type II SNwith a narrow range in mass and explosion parameters; high mass Type II SNprogenitors are required. The dispersion of [X/Fe] about this plateau level issurprisingly small, and is still dominated by measurement errors rather thanintrinsic scatter. The dispersion in neutron-capture elements, and theabundance trends for Cr, Mn and Co are consistent with previous studies ofevolved EMP stars. Two dwarfs in the sample are carbon stars, while two othershave significant C enhancements, all with C12/C13 ~ 7 and with C/N between 10and 150. Three of these C-rich stars have large enhancements of the heavyneutron capture elements, including lead, which implies a strong s-processcontribution, presumably from binary mass transfer; the fourth shows no excessof Sr or Ba.Comment: Accepted for publication in the Ap