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

¹ Indian Institute of Astrophysics, Koramangala, Bangalore, 560034, India
² National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan
³ Faculty of Science, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan
⁴ Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo, 181-0015, Japan
⁵ Kiso Observatory, Institute of Astronomy, University of Tokyo, Mitake, Kiso, Nagano 397-0101, Japan
⁶ Okayama Astrophysical Observatory, National Astronomical Observatory, Kamogata, Okayama 719-0232, Japan

Corresponding author: S. Ambika, ambika@iiap.ernet.in

Received: 20 May 2003
Accepted: 18 November 2003

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 () spectrum of the object, we derive the atmospheric parameters to be  K, log  and . 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 T_eff 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.