D. Minniti et al., HIGH-DISPERSION SPECTROSCOPY OF GIANTS IN METAL-POOR GLOBULAR-CLUSTERS .2. OXYGEN AND SODIUM ABUNDANCES, The Astrophysical journal, 470(2), 1996, pp. 953-960
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 NG
C 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 sh
ow an upper envelope, in agreement with the mean level of field halo s
tars of similar metallicities ([O/ Fe] = +0.4 +/- 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, i
n contrast to held giants of similar metallicity, which are almost inv
ariably oxygen-rich. Several explanations for this deficiency are exam
ined, 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 agre
ement with previous results (Kraft et al. 1995). The enhancement of ox
ygen in these extremely metal-poor clusters appears similar to that of
less metal-deficient clusters, an important datum for determinations
of their relative ages.