Evidence of Third Dredge-Up in Post-AGB Stars in Galactic Globular Clusters

Abstract: To better understand the mixing and mass loss experienced by low-mass stars as they ascend the asymptotic giant branch (AGB), I have gathered from the literature the abundances of CNO and s-process elements in post-AGB stars in Galactic globular clusters. These species are mixed to the surface during third dredge-up (3DU) events, so their abundance should increase as the star ascends the AGB. Of the 17 stars in this sample, CNO abundances are available for 11. Of these, four are enhanced in CNO relative to the RGB stars from which they descended, which I take as evidence of 3DU on the AGB. The enhancement is mainly in the form of carbon. Of the six stars for which only heavy-element abundances are available, one shows s-process enhancements that previous authors have interpreted as evidence of 3DU. Combining these 17 stars with other recent samples reveals that most globular-cluster post-AGB stars have luminosities log (L/L_sun) ~ 3.25. They are the progeny of blue horizontal-branch (HB) stars in clusters with intermediate metallicity ([Fe/H] ~ -1.5). A second group consists of sub-luminous stars associated with high-metallicity clusters ([Fe/H] ~ -1.0) with red HBs. They may be burning helium, rather than hydrogen. A third group of hot, super-luminous stars is evolving quickly across the Hertzsprung-Russell diagram. Some of them may be merger remnants.