Chemical composition of UV-bright star ZNG 4 in the globular cluster M 13

Abstract

We present a detailed model-atmosphere analysis of ZNG 4, a UV-bright star in the globular cluster M 13. From the analysis of a high resolution ( R ≈ 45 000 ) spectrum of the object, we derive the atmospheric parameters to be Teff = 8500± 250 K, log g = 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 0.8 dex. Sodium is enhanced by a factor of 0.2 dex. 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 Teff of 8500 K. Detailed and more accurate abundance analysis of post-HB stars in several globular clusters is needed to further understand their abundance anomalies. Based on observations obtained with the Subaru 8.2 m Telescope, which is operated by the National Astronomical Observatory of Japan.

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