The First Extensive Exploration of UV-bright Stars in the Globular Cluster NGC 2808

Abstract

In this study, we identified and characterized the hot and luminous UV-bright stars in the globular cluster NGC 2808. We combined data from the Ultra Violet Imaging Telescope (UVIT) on board the Indian space satellite AstroSat with the Hubble Space Telescope UV Globular Cluster Survey data for the central region (within ~¢ ´ ¢ 2.7 2.7) and Gaia and ground-based optical photometry for the outer parts of the cluster. We constructed the UV and UV-optical color–magnitude diagrams, compared the horizontal branch (HB) members with the theoretical zero- and terminal-age HB models, and identified 34 UV-bright stars. The spectral energy distributions of the UVbright stars were fitted with theoretical models to estimate their effective temperatures (12,500–100,000 K), radii (0.13–2.2 Re), and luminosities (∼40–3000 Le) for the first time. These stars were then placed on the Hertzsprung–Russell diagram, along with theoretical post-HB evolutionary tracks, to assess their evolutionary status. The models suggest that most of these stars are in the asymptotic giant branch (AGB)-manqué phase, and all except three have evolutionary masses <0.53 Me. We also calculated the theoretically expected number of hot post-(early)-AGB stars in this cluster and found the range to match our observations. Seven UV-bright stars located in the outer region of the cluster, identified from the AstroSat/UVIT images, are ideal candidates for detailed follow-up spectroscopic studies.