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 Ultraviolet (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 multi-wavelength data. Optical and UV color magnitude diagrams are used to classify HB stars and their membership from HST 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 G-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 inner and outer regions. We detect a sub-luminous EHB and a "blue-Hook" candidates with temperatures T ef f ∼ 25,000 K and 31,000 K, respectively. We found an EHB star (T ef f ∼ 17,000 K) with the radius lies between the BHB and normal EHB stars. The most peculiar of our EHB stars (T ef f ∼ 28,000 K) is found to be a photometric binary to a Blue Straggler star (BSS) (T ef f ∼ 7,000 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 RGB phase, leading to the formation of EHB stars.

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

“…Among the UVIT detected stars in the outer regions, some bright stars were found to be non-members as per the list of possible members from Gaia Collaboration et al (2018). We re-assessed the membership of these stars in the outer region, using methods presented in Singh et al (2020) which also uses the Gaia DR2 data. Two stars were found to have a membership probability of ∼ 90%.…”

UVIT study of UV bright stars in the globular cluster NGC 4147

“…In particular, UVIT observations of star clusters are able to identify the hot binary companions (WD, HB, EHB, etc.) of BS stars (Subramaniam et al, 2016;Sindhu et al, 2019;Sahu et al, 2019b;Jadhav et al, 2019;Singh et al, 2020). Present observational data-set allows us to identify one FUV bright BSS in the outer region of the cluster, at a distance of 2.15 from the center.…”

“…The glimpse of ultraviolet (UV) light in the old stellar population of the Galactic globular clusters (GGCs) is dominated by hot luminous UV-bright stars which are mostly the stars of blue horizontal branch (BHB), bluestragglers (BS), post-asymptotic giant branch (pAGB), and extreme horizontal branch (EHB) phases having temperature more than 7,000 K. Various physical properties of these UV-bright populations have been explored using large sample of GGCs observed by Galaxy Evolution Explorer (GALEX, Schiavon et al, 2012) and Hubble space telescope (HST, Nardiello et al, 2018). Ultraviolet Imaging Telescope (UVIT) on-board the AstroSat satellite (Kumar et al, 2012) has also performed the imaging observations of several GGCs in UV with a better spatial resolution than GALEX enabling in resolving the core of the clusters and also in distinguishing UV bright stars of different evolutionary stages to study their physical parameters in a great detail (Subramaniam et al, 2017;Sahu et al, 2019a;Jain et al, 2019;Kumar et al, 2020a,b;Rani et al, 2020;Singh et al, 2020). Using the spectroscopic data of a large number of stars in many GGCs along with the UV photometric observations with HST, the evidence for the presence of multiple stellar populations in GGCs is now well established (see reviews Bastian & Lardo, 2018;Gratton et al, 2019;Cassisi & Salaris, 2020, and references therein).…”

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 U BVRI photometry of cluster members thus selected, were obtained by a cross-match with the data from Stetson et al (2019).…”