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

Abstract

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 T eff ∼ 25,000 K and 31,000 K, respectively. We found an EHB star (T eff ∼ 17,000 K) with a radius that lies between the BHB and normal EHB stars. The most peculiar of our EHB stars (T eff ∼ 28,000 K) is found to be a photometric binary to a blue straggler star (BSS; T eff ∼ 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.

UOCS. IV. Discovery of diverse hot companions to blue stragglers in the old open cluster King 2

“…The environment is quite similar to outskirts of GCs, which are also metal-poor, old and of comparable density. While most of the BSSs in GCs lie in core and are formed via mergers (Chatterjee et al 2013), BSSs in outskirts of GCs can form through mass transfer as seen in EHB-4 of NGC 1851 (Singh et al 2020). Our study suggests that the at least 15% of the BSSs in King 2 are formed via mass transfer pathway of formation.…”

“…Figure 3 shows the density distributions of the best 100 fits for the hotter companions. Figure 3 also shows the location of various companions to BSS in NGC 188 and NGC 2682, and EHB stars in NGC 1851, one of which has a BSS as its companion (Singh et al 2020).…”

“…Old OCs such as NGC 188 and NGC 2682 are rich with BSSs, binary stars and contain many such optically sub-luminous UV-bright companions (Subramaniam et al 2016;Sindhu et al 2019;Jadhav et al 2019). Similar companions have been identified to BSSs in the outskirts of GCs (Sahu et al 2019;Singh et al 2020). Subramaniam et al (2020) provides a summary of the BSSs and post mass transfer systems in star clusters.…”

“…The blue lurkers are identified by their high stellar rotation (Leiner et al 2019) or evidence of extremely low-mass (ELM) white dwarf (WD) companion (Jadhav et al 2019). Depending on the evolutionary status of binary components, the binary system can be observed as main sequence (MS)?MS, contact binaries (Rucinski 1998), common envelop, MS?horizontal branch (HB;Subramaniam et al 2016), MS?extreme HB (EHB; Singh et al 2020), MS? subdwarf-B (sdB; Han et al 2002), MS?WD (Jadhav et al 2019), WD?WD (Marsh et al 1995) and many more combinations.…”

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

“…Thus, in order to identify stars in the outer region (region outside HUGS coverage) and to select the cluster members among them, the UVIT data were combined with other datasets. The membership probabilities of the stars were estimated based on the Gaia DR2 proper-motion data using the technique of Singh et al (2020). The Johnson-Cousins UBVRI photometry of cluster members thus selected, were obtained by a cross-match with the data from Stetson et al (2019).…”