CNO ABUNDANCES OF BRIGHT GIANTS IN THE GLOBULAR-CLUSTER M5

Spectra obtained with the Lick 3-m telescope show that a CN-CH band st rength anticorrelation exists among a sample of six red giant members of the globular cluster M5 having absolute magnitudes in the range -2. 0<M(V)<-1.3. By contrast, a seventh giant, star IV-59 in the color-mag nitude survey of Arp (1955, AJ, 60, 317), is found to have bath strong CN and strong CH bands. Carbon and nitrogen abundances determined for five of the observed stars reveal that for the giants exhibiting the CN-CH anticorrelation, (i) carbon is depleted ([C/Fe]<-0.5) by compari son with the [C/Fe] abundances of typical halo subdwarfs, (ii) nitroge n is greatly enhanced (+0.5<[N/Fe]<+1.2) relative to the [N/Fe] abunda nces of typical subdwarfs, and (iii) the nitrogen and carbon abundance s are anticorrelated and correlated, respectively, with the [O/Fe] abu ndances determined by Sneden et al. (1992, AJ, 104, 2121). These prope rties are similar to the pattern of CNO abundances reported in the lit erature for upper-giant-branch stars in other globular clusters such a s M92, M3, and M13. By contrast, star M5 IV-59 has [C/Fe] and [N/Fe] a bundance ratios that are near the upper Limits for red giants in M5. T his star also appears to have an undepleted [O/Fe] abundance, despite a greatly enhanced nitrogen abundance.