High-resolution spectroscopy of globular cluster post-Asymptotic Giant Branch stars^*

¹ Department of Pure & Applied Physics, The Queen's University of Belfast, Belfast, BT7 1NN, Northern Ireland, UK
² Isaac Newton Group, Apartado de Correos 368, Santa Cruz de La Palma, Tenerife 38780, Canary Islands, Spain
³ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
⁴ Physics and Astronomy Department, University of California, Los Angeles, CA 90095, USA

Corresponding author: F. P. Keenan, f.keenan@qub.ac.uk

Received: 5 April 2002
Accepted: 12 February 2004

Abstract

We present model atmosphere analyses of high resolution Keck and VLT optical spectra for three evolved stars in globular clusters, viz. ZNG-1 in M 10, ZNG-1 in M 15 and ZNG-1 in NGC 6712. 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 M 10 ZNG-1, and possibly M 15 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 ([CITE]). Furthermore, M 10 ZNG-1 exhibits a large C underabundance (with [C/O] ~ -1.6 dex), typical of other hot post-AGB objects. Differential [ α/Fe] abundance ratios in both M 10 ZNG-1 and NGC 6712 ZNG-1 are found to be approximately 0.0 dex, with the Fe abundance of the former being in disagreement with the cluster metallicity of M 10. Given that the Fe absorption features in both M 10 ZNG-1 and NGC 6712 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.