Oxygen abundances in halo giants. III - Giants in the mildly metal-poor globular cluster M5

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Abstract

High-resolution echelle spectra of 13 giants in the globular cluster M5 are analyzed. Abundances of several elements in each star are determined, with the most attention given to oxygen and sodium. It is found that (Fe/H) = -1.17 +/- 0.01 for M5. If there are (Fe/H) variations from star to star, they are less than 0.1 dex. (O/Fe) varies from about -0.25 to about +0.35 in the present sample of 13 M5 giants. No 'super O-poor' stars were found in M5. Stars with low oxygen typically have enhanced CN bonds and probably have enhanced nitrogen abundances. It is inferred that CNO nuclei have been reshuffled in the material that now makes up the outer layers of the stars with low oxygen abundances. (O/Fe) and (Na/Fe) are anticorrelated in M5 giants. This strongly suggests that the reshuffling of CNO that appears to have occurred in the low-O high-Na stars occurred before the stars were born, unless the creation of new Na nuclei accompanies O yields N nucleosynthesis and mixing in evolving low-mass stars.

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