Abundances in the red giants of M13 and M22

Abundances of Ca, Na, and Fe are derived for 10 red giant branch stars in the globular clusters M13 and M22, by means of model atmosphere analyses of high-dispersion spectra. It was found that M13 and M22 represent two different types of globular clusters. M13 is similar to M92 and many other globular clusters in that it displays variations in CH and CN band strengths, while showing no variation in either Ca or Fe abundance. The Na abundance was found to vary and its variation correlates with CN band strengths. This suggests that the source of the observed CN band strength variation is the same as that of the Na variation. M22, on the other hand, is similar to ω Cen, in that it displays variations in Ca, Na, and Fe abundances (Δ[Fe/H] ~ Δ[Ca/H] ~ 0.4) and these variations correlate with variations in CH and CN band strengths. These correlations suggest that the cause of the variations in M22 is largely "primordial" and mixing may only make a relatively small contribution to the C and N variations observed.