CNO ABUNDANCES OF BRIGHT GIANTS IN THE GLOBULAR CLUSTER M5

Abstract

ABSTRACT Spectra obtained with the Lick 3-m telescope show that a CN-CH band strength anticorrelation exists among a sample of six red giant members of the globular cluster M5 having absolute magnitudes in the range -2.0 < MV < -1.3. By contrast, a seventh giant (star IV-59 in the color-magnitude survey of Arp [1955]) is found to have both 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 comparison with the [C/Fe] abundances of typical halo subdwarfs, (ii) nitrogen is greatly enhanced (+0.5 < [N/Fe] < +1.2) relative to the [N/Fe] abundances of typical subdwarfs, and (iii) the nitrogen and carbon abundances are anticorrelated and correlated respectively with the [O/Fe] abundances determined by Sneden et~al. (1992). These properties are similar to the pattern of CNO abundances reported in the literature for upper-giant-branch stars in other globular clusters such as M92, M3, and M13. By contrast, star M5 IV-59 has [C/Fe] and [N/Fe] abundance ratios that are near the upper limits for red giants in M5. This star also appears to have an undepleted [O/Fe] abundance, despite a greatly enhanced nitrogen abundance.