2004A&A...417..293A

We present a detailed model-atmosphere analysis of ZNG 4, a UV-bright star in the globular cluster M13. From the analysis of a high resolution (R ≃45000) spectrum of the object, we derive the atmospheric parameters to be T_eff=8500±250K, logg=2.5±0.5 and [Fe/H]=-1.5. Except for magnesium, chromium and strontium, all other even Z elements are enhanced with titanium and calcium being overabundant by a factor of 0.8dex. Sodium is enhanced by a factor of 0.2dex. The luminosity of ZNG 4 and its position in the color-magnitude diagram of the cluster indicate that it is a Supra Horizontal Branch (SHB) (post-HB) star. The underabundance of He and overabundances of Ca, Ti, Sc and Ba in the photosphere of ZNG 4 indicate that diffusion and radiative levitation of elements may be in operation in M 13 post-HB stars even at T_eff of 8500K. Detailed and more accurate abundance analysis of post-HB stars in several globular clusters is needed to further understand their abundance anomalies.