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Chemical composition of UV-bright star ZNG 4 in the globular cluster M13

S. Ambika·M. Parthasarathy·W. Aoki·T. Fujii·Y. Nakada·Y. Ita·H. Izumiura
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Abstract

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 \approx 45,000 $) spectrum of the object, we derive the atmospheric parameters to be $\rm T_{eff} = 8500\pm 250$ K, log g $=$ 2.5 $\pm$ 0.5 and $\rm [Fe/H]= -1.5$. 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 $\rm T_{eff}$ of 8500K. Detailed and more accurate abundance analysis of post-HB stars in several globular clusters is needed to further understand their abundance anomalies.

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S. Ambika, M. Parthasarathy, W. Aoki, T. Fujii, Y. Nakada, Y. Ita, H. Izumiura. 2004-02-12. Chemical composition of UV-bright star ZNG 4 in the globular cluster M13. https://doi.org/10.1051/0004-6361%3A20031759

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