UVIT–HST–GAIAview of NGC 288: a census of the hot stellar population and its properties from UV

Abstract

A complete census of Blue Horizontal Branch (BHB) and Blue Straggler Star (BSS) population within the 10 radius from the center of the Globular Cluster, NGC 288 is presented, based on the images from the Ultraviolet Imaging Telescope (UVIT). The UV and UV−optical Colour-Magnitude Diagrams (CMDs) are constructed by combining the UVIT, HST-ACS and ground data and compared with the BaSTI isochrones generated for UVIT filters. We used stellar proper motions data from GAIA DR2 to select the cluster members. Our estimations of the temperature distribution of 110 BHB stars reveal two peaks with the main peak at T ef f ∼ 10,300 K with the distribution extending up to T ef f ∼ 18,000 K. We identify the well known photometric gaps including the Gjump in the BHB distribution which are located between the peaks. We detect a plateau in the FUV magnitude for stars hotter than T ef f ∼ 11,500 K (G-jump), which could be due to the effect of atomic diffusion. We detect 2 Extreme HB (EHB) candidates with temperatures ranging from 29,000 to 32,000 K. The radial distribution of 68 BSSs suggests that the bright BSSs are more centrally concentrated than the faint BSS and the BHB distribution. We find that the BSSs have a mass range of 0.86 -1.25 M and an age range of 2 -10 Gyr with a peak at 1 M and 4 Gyr respectively. This study showcases the importance of combining UVIT with HST, ground, and GAIA data in deriving HB and BSS properties.

Scientific potential of the Indo-Belgian 3.6-m DOT in the field of Galactic Astronomy

“…Also, it has number of FUV and NUV filters in comparison to other recent similar space missions. UVIT images, in combination with other space and ground based data, are a very powerful tool to understand the formation of UV and blue stellar population in star clusters (see Sahu et al 2019and Subramaniam et al 2016, 2017. However, for proper understanding of their progenitors, extensive photometric and spectroscopic studies at optical and NIR wavelengths are essential.…”

“…The good image quality combined with large field of view make the UVIT extremely valuable for studies of hot stellar populations in GCs since it is able to detect the UV and blue stars beyond the central region of the cluster. Based on FUV and NUV images taken with the UVIT, Sahu et al (2019) have presented a census of the blue horizontal branch (BHB) and Blue Straggler stars (BSSs) population in the GC NGC 288 while Subramaniam et al (2017) have studied it for the NGC 1851. These scientific results prove beyond doubt that the UVIT images, in combination with the Hubble Space Telescope (HST), ground and GAIA DR2 data, are a very powerful tool to understand the formation of UV and blue stellar population in the core of galactic GCs.…”

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

“…The UV photometry is a great tool to identify the BS population in a cluster. The data from UV telescopes (HST, GALEX and UVIT) have been excellent tool to explore BS population and their formation scenario (Ferraro et al 1997(Ferraro et al , 2003Dieball et al 2010;Schiavon et al 2012;Gosnell et al 2015;Subramaniam et al 2017;Sahu et al 2019a;Kumar et al 2020Kumar et al , 2021Rani et al 2020). In particular, UVIT observations of star clusters are able to identify the hot binary companions (WD, HB, EHB, etc.)…”

“…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), blue-stragglers (BS), post-asymptotic giant branch (pAGB), and extreme horizontal branch (EHB) phases having temperature more than 7000 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 2020Kumar et al , 2021Rani 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).…”

AstroSat study of the globular cluster NGC 2298: probable evolutionary scenarios of hot HB stars

“…Crowding is generally not a problem in the UV images as normal cluster stars (MS and RGB) are cooler than late A type stars and considerably fainter at wavelengths less than 2000 Å. Thus, a combination of optical and far-UV (FUV) magnitudes is the most powerful tool to analyze the hot stars in GCs (Ferraro et al 1998;Dieball et al 2010;Dalessandro et al 2011Dalessandro et al , 2013Subramaniam et al 2017;Sahu et al 2019;Singh et al 2020;Rani et al 2020Rani et al , 2021Prabhu et al 2021).…”