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 ) 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 UV-bright stars were fitted with theoretical models to estimate their effective temperatures (12,500–100,000 K), radii (0.13–2.2 R ⊙), and luminosities (∼40–3000 L ⊙) 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 M ⊙. 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.

UV photometry of spotted stars in the horizontal branch of the globular cluster NGC 2808 using AstroSat

“…We also find that two of the vEHBs have very high luminosities (vEHB-7 and vEHB-10). These stars have been classified as AGBmanqué stars in the works of Schiavon et al (2012) and Prabhu et al (2021). Also, since the amplitude of variability of all the vEHB stars is very small (DU Johnson $ 0:04-0.22 mag), it highly improbable that the results obtained in this study are significantly affected due to the variability.…”

“…One vEHB (vEHB-10) is seen to be located just above the TAHB sequence. This star along with vEHB-7 have been previously identified as pHB candidates by Prabhu et al (2021) with vEHB-7 corresponding to Star 21 and vEHB-10 corresponding to Star 33 in their catalogue. One of these stars (vEHB-10) is also listed as an AGB-manqué star (NGC 2808-594) in the study by Schiavon et al (2012).…”

“…We then executed crowded field photometry on these images using the DAOPHOT software package of IRAF/NOAO (Stetson 1987). The detailed steps of the procedure can be found in Prabhu et al (2021). The final magnitudes (AB system) in all the filters were corrected for extinction by choosing the reddening value of EðB À VÞ ¼ 0:22 mag (Harris 1996(Harris , 2010 and the ratio of total to selective extinction, R V ¼ 3:1.…”

“…For identifying the different classes of stars detected in the UVIT images and to obtain their cluster membership probabilities, the stars within the central $ 2:7 0 Â 2:7 0 region (inner region) were cross-matched with the HUGS catalogue as described in Prabhu et al (2021). The HUGS catalogue consists of data in five filters, namely, the WFC3/UVIS F275W (NUV), F336W (U) and F438W (B) filters, along with the ACS/WFC F606W (V) and F814W (I) filters.…”

“…Jain et al (2019) used the UVIT data for this cluster to study the photometric gaps in the colour-magnitude diagrams (CMDs) and the multiple stellar populations. Prabhu et al (2021) presented a detailed analysis of the stars in the pHB phase in this cluster utilizing the UVIT data in combination with other multiwavelength datasets. In this work, we study the UV properties of the spotted EHB stars in the cluster.…”