Extremely metal-deficient red giants. II - Chemical abundances in 21 halo giants

A model-atmosphere analysis was used to determine chemical abundances in 21 red giants of the galactic halo. Absolute magnitudes calculated from the derived atmospheric parameters, and other spectroscopic and photometric evidence, are used to determine the evolutionary states of the field stars. The lighter alpha-rich metals (Mg, Si, Ca, and Ti) are generally found to be enriched by 0.2-0.5 dex relative to iron in red giants with Fe/H concentration ratio less than -2. It is suggested that massive Population II stars preferentially synthesized these light metals early in the halo-formation era. In addition, a pronounced 'aging effect' is seen in the abundances of s-process elements.