@abia01 samples. The @tomkin99 and @gonzalez98 samples would increase by roughly 0.15 dex. NGC 6752 would therefore have unusually low ratios \[Rb/Zr\] compared to field stars at the same metallicity. Similarly, the $\omega$ Cen compositions would be unusually low though comparable to NGC 6752. The NGC 3201 giants would then have very high \[Rb/Zr\] ratios compared to other globular clusters and field stars at the same metallicity. NGC 3201 is peculiar since it has a retrograde orbit and may have been a captured cluster [@vandenbergh93]. The capture hypothesis could not be demonstrated by @gonzalez98 who found no unusual abundance ratios.

@smith00 compare \[Rb/Zr\] in $\omega$ Cen with predictions from AGB models with various initial masses and initial metallicities (their Figure 14). Their Figure clearly shows how the ratio \[Rb/Zr\] can vary by nearly a factor of 10 depending on whether a low-mass (1.5M$_\odot$) or high-mass (5M$_\odot$) AGB model is synthesizing the $s$-process elements. As anticipated from the arguments given in Section \[sec:intro\], high-mass AGB models produce high \[Rb/Zr\] while low-mass AGB models produce low \[Rb/Zr\]. The magnitude of the difference in the predicted \[Rb/Zr\] between low- and high-mass AGB models does not significantly change as the metallicity decreases from \[Fe/H\] 