2004A&A...419.1123M

We present model atmosphere analyses of high resolution Keck and VLT optical spectra for three evolved stars in globular clusters, viz. ZNG-1 in M10, ZNG-1 in M15 and ZNG-1 in NGC6712. The derived atmospheric parameters and chemical compositions confirm the programme stars to be in the post-Asymptotic Giant Branch (post-AGB) evolutionary phase. Differential abundance analyses reveal CNO abundance patterns in M10 ZNG-1, and possibly M15 ZNG-1, which suggest that both objects may have evolved off the AGB before the third dredge-up occurred. The abundance pattern of these stars is similar to the third class of optically, bright post-AGB objects discussed by van Winckel (1997A&A...319..561V). Furthermore, M10 ZNG-1 exhibits a large C underabundance (with Δ[C/O]~-1.6dex), typical of other hot post-AGB objects. Differential Δ[α/Fe] abundance ratios in both M10 ZNG-1 and NGC6712 ZNG-1 are found to be approximately 0.0dex, with the Fe abundance of the former being in disagreement with the cluster metallicity of M10. Given that the Fe absorption features in both M10 ZNG-1 and NGC6712 ZNG-1 are well observed and reliably modelled, we believe that these differential Fe abundance estimates to be secure. However, our Fe abundance is difficult to explain in terms of previous evolutionary processes that occur on both the Horizontal Branch and the AGB.