Do Agb Stars Differ Chemically from Rgb Stars in Globular Clusters?

The recent improvements in globular cluster colour-magnitude diagrams, coupled with an increase in large-sample spectroscopic abundance studies of cluster giants, finally allow some attempts at a statistically meaningful comparison of the chemical compositions of red giant branch and asymptotic branch cluster stars. We review some of the extant data here, concluding that in a few clusters the AGB stars show on average smaller amounts of high-temperature proton-capture synthesis products (low oxygen, high sodium and aluminum) at their surfaces than do the first-ascent RGB stars. This suggests that those RGB stars with envelopes that have been enriched with proton-capture material also have high helium contents. Such stars after the He-flash then take up residence on the bluest parts of the HB (as a consequence of their high envelope helium), probably never to return to the AGB during subsequent evolutionary stages.