We present a model-atmosphere analysis for the bright (V∼13) star ZNG-1, in the globular cluster M10. From high-resolution (R∼40000) optical spectra we confirm ZNG-1 to be a post-asymptotic giant branch (post-AGB) star. The derived atmospheric parameters are Teff=26500±1000K and logg=3.6±0.2dex. 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 1.3dex. 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.
The Royal Astronomical Society
stars: abundances - stars: AGB and post-AGB - stars: early-type - stars: individual: ZNG-1 - stars: Population II - globular clusters: individual: M10