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

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 ${\mathrm{T}}_{\mathrm{eff}}=8500\pm 250$ K, log g $=$ 2.5 $\pm$ 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 ${\mathrm{T}}_{\mathrm{eff}}$ of 8500K. Detailed and more accurate abundance analysis of post-HB stars in several globular clusters is needed to further understand their abundance anomalies.

arXiv comment17 pages, 5 figures, Accepted for publication in A&A

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• A Re-Evaluation of the Evolved Stars in the Globular Cluster M13
• The Na-O anticorrelation in horizontal branch stars. IV. M22
• Ultraviolet Imaging Telescope (UVIT) observation of the Galactic Globular Cluster NGC 7492
• Astrometric Membership Tests for the Zinn-Newell-Gibson "UV-Bright" Stars in Galactic Globular Clusters