CNO Abundances of Bright Giants in the Globular Cluster M5

Astronomy and Astrophysics – Astronomy

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Globular Clusters: Individual: M5, Stars: Abundances

Scientific paper

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 $-2.0 < M_{\rm V} < -1.3$. 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. (SECTION: Stars)

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