The chemical composition of the extreme halo stars. II - Green spectra of 20 dwarfs

The abundances of nine elements in the atmospheres of 20 extreme metal-poor stars are derived from green spectra obtained with the ESO Cassegrain Echelle Spectrograph (CASPEC). Sodium is found to be overabundant with respect to aluminum by as much as one dex. There is some scatter in Ca/Fe which cannot be accounted for by random errors in equivalent widths or atmospheric parameters. Scandium is overabundant relative to iron. Some barium lines may be affected by strong departures from local thermodynamic equilibrium. The nitrogen-rich metal-poor star HD 74000, which was found in previous work to be also Al-rich, also presents higher than usual abundances of Na and of s-process elements. It can thus be considered as a mild barium dwarf of Population II.