Abstract. High spectral resolution and signal-to-noise observations of the absorption and emission line spectra in two postasymptotic-giant-branch (PAGB) stellar candidates, LSIV −12 111 and HD 314617 are discussed. The absorption line spectra have been analysed using non-LTE model atmosphere techniques to determine stellar atmospheric parameters and chemical compositions, both in absolute terms and relative to a standard star, HD 13841. The atmospheric parameters differ from previous estimates based on LTE model atmospheres, probably due to non-LTE effects. In turn these imply stellar masses that are generally larger than have been previously estimated. Both PAGB candidates have relative uniform underabundances of metals with mean values of −0.35 dex for LSIV −12 111 and −0.50 dex for HD 314617. Furthermore, their abundance patterns are remarkably similar to that observed for optically bright, F-type PAGBs. From the emission spectra, the plasma parameters and angular size of the circumstellar material are constrained, and these are consistent with previous estimates and with a PAGB evolutionary stage.

Absolute and differential abundance analyses have been performed from high‐resolution, high signal‐to‐noise ratio optical (Keck I) spectra for three evolved Galactic halo stars, namely PG 1704 + 222, HD 341617 and LS IV −04 01. Their derived atmospheric parameters indicate that all three objects are undergoing a post‐asymptotic giant branch (post‐AGB) phase of evolution. A differential abundance analysis reveals HD 341617 as having a mild carbon deficiency of 0.74 dex, possibly due to the star having evolved off the AGB before the onset of the third dredge‐up. Although such carbon underabundances are typical of hot post‐AGB objects, the same trend is not observed in PG 1704 + 222, where the carbon abundance is found to be consistent with those derived for nitrogen and oxygen. Hence, a dredge‐up scenario need not be invoked to explain the chemical composition of PG 1704 + 222. For LS IV −04 01 no iron deficiency is apparent relative to magnesium and silicon, and hence a gas–dust separation event in the AGB progenitor need not be invoked for this star.

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 (1997). 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.

We present a model‐atmosphere analysis for the bright (V∼13) star ZNG‐1, in the globular cluster M10. From high‐resolution (R∼40 000) optical spectra we confirm ZNG‐1 to be a post‐asymptotic giant branch (post‐AGB) star. The derived atmospheric parameters are Teff=26 500±1000 K and log g=3.6±0.2 dex. A differential abundance analysis reveals a chemical composition typical of hot post‐AGB objects, with ZNG‐1 being generally metal poor, although helium is approximately solar. The most interesting feature is the large carbon underabundance of more than 1.3 dex. This carbon deficiency, along with an observed nitrogen enhancement relative to other elements, may suggest that ZNG‐1 evolved off the AGB before the third dredge‐up occurred. Also, iron depletions observed in other similar stars suggest that gas–dust fractionation in the AGB progenitor could be responsible for the observed composition of these objects. However, we need not invoke either scenario since the chemical composition of ZNG‐1 is in good agreement with abundances found for a Population II star of the same metallicity.