Data from the literature are used to explore the relation between $\lambda$3883 CN band strength and the sodium and oxygen abundances of red giants in the globular cluster Messier 5. Although there is a broad tendency for CN-strong giants in this cluster to have higher sodium abundances and lower oxygen abundances than CN-weak giants of comparable absolute magnitude there are some secondary features in these relations. The oxygen abundance [O/Fe] shows a greater range (0.6-0.7 dex) among the CN-strong giants than the CN-weak giants (approximately 0.3 dex). By contrast [Na/Fe] shows a 0.6-0.7 dex range among the CN-weak giants, but a more limited range of 0.3-0.4 dex among the CN-strong giants. The $\lambda$3883 CN band anticorrelates in strength with [O/Fe] among the CN-strong giants, but there is little, if any, such trend among the CN-weak giants. In contrast, the CN band strength may show a modest correlation with [Na/Fe] among the CN-weak giants, but there is little evidence for such among the CN-strong giants. Neither oxygen or sodium abundance define a continuous relation with CN band strength. Instead, the CN-strong and CN-weak giants overlap in their sodium and possibly their oxygen abundances. At oxygen abundances of [O/Fe] = 0.20 +/- 0.05 it is possible to have both CN-weak and CN-strong giants, although there may be a discontinuity in [O/Fe] between these two groups of stars that has been smeared out by observational errors. Both CN-weak and CN-strong giants populate the sodium abundance range 0.4 <= [Na/Fe] <= 0.6. Messier 5 may be displaying the results of spatially heterogeneous chemical self-enrichment.
Date added: Mon, 21 Oct 13