Sodium Abundances in Field Metal-Poor Stars

Abstract

We have determined sodium abundances in 60 metal-poor halo subgiants, giants, and horizontal branch stars using high dispersion spectra obtained with the coudé feed telescope at the Kitt Peak National Observatory. The [Na/Fe] ratios show a wide scatter (ranging from -0.6 to nearly +0.3) with a small mean deficiency: <[Na/Fe]>=-0.17±0.04 (σ=0.19). There is a slight tendency for <[Na/Fe]> to increase with advancing evolutionary state. Comparison of our field star sample with giants of similar metallicity (but higher luminosity) in globular clusters suggests there is an intrinsic difference between halo field giants and globular cluster giants. The bright giants among the field population do not appear to have an excess of sodium as found in their cluster counterparts.