High-Dispersion Spectroscopy of Giants in Metal-poor Globular Clusters. II. Oxygen and Sodium Abundances

Abstract

Oxygen and sodium abundances or upper limits have been determined for 17 red giant members of the metal-poor Galactic globular clusters M68 (two stars), M30 (two stars), M15 (two stars), M55 (two stars), and NGC 6397 (five stars), as well as for the moderately metal-poor clusters NGC 4833 (one star), NGC 6144 (one star), and NGC 6752 (three stars). The behavior of [O/Fe] with respect to luminosity and metallicity is investigated. As found in previous works, the oxygen-to-iron ratios show an upper envelope, in agreement with the mean level of field halo stars of similar metallicities ([O/Fe]=+0.4{plus_minus}0.15). Moreover, as is true of the more metal-rich giants in globular clusters, about one-third of the stars do not have enhanced oxygen with respect to iron, in contrast to field giants of similar metallicity, which are almost invariably oxygen-rich. Several explanations for this deficiency are examined, including emission possibly associated with mass-loss filling in the oxygen lines and evolutionary mixing effects. In particular, the abundances of oxygen and sodium are seen to be anticorrelated, in agreement with previous results (Kraft {ital et} {ital al}. 1995). The enhancement of oxygen in these extremely metal-poor clusters appears similar to that of less metal-deficient clusters, anmore » important datum for determinations of their relative ages. {copyright} {ital 1996 The American Astronomical Society.}« less