Peculiarities in the Horizontal Branch Stars of Globular Cluster NGC 1851: Discovery of a Blue Straggler Companion to an EHB Star

We present a study of far-UV (FUV) bright horizontal branch (HB) stars to understand the peculiarities seen in the HB sequence of the globular cluster NGC 1851, using ground- and space-based multiwavelength data. Optical and UV color-magnitude diagrams are used to classify HB stars and their membership from Hubble Space Telescope and Gaia DR2 data. The spectral energy distributions (SEDs) of the hot HB stars located from the core to tidal radii are constructed. The SEDs reveal that the HB stars near the "Grundahl jump" show a decrease in the FUV flux when atmospheric models of cluster metallicity are used for fitting, but a better fit is found with higher-metallicity models, as expected due to atmospheric diffusion. We report on four particularly interesting extreme HB (EHB) stars, two each in the inner and outer regions. We detect a subluminous EHB and "blue-hook" candidates with temperatures Teff ∼ 25,000 K and 31,000 K, respectively. We found an EHB star (Teff ∼ 17,000 K) with a radius that lies between the BHB and normal EHB stars. The most peculiar of our EHB stars (Teff ∼ 28,000 K) is found to be a photometric binary to a blue straggler star (BSS; Teff ∼ 7000 K), which is an important target for spectroscopic study. This discovery of the candidate EHB+BSS binary system could help to explain the mass loss in the red giant branch phase, leading to the formation of EHB stars.