J. Astrophys. Astr. (2020) 41:35 Ó Indian Academy of Sciences https://doi.org/10.1007/s12036-020-09664-5Sadhana(0123456789().,-volV)FT3](0123456789().,-volV) The horizontal branch morphology of the globular cluster NGC 1261 using AstroSat SHARMILA RANI1,2,*, GAJENDRA PANDEY1, ANNAPURNI SUBRAMANIAM1, SNEHALALA SAHU1 and N. KAMESWARA RAO1 1Indian Institute of Astrophysics, Koramangala II Block, Bengaluru 560 034, India. 2Pondicherry University, R.V. Nagar, Kalapet, Puducherry 605 014, India. *Corresponding author. E-mail: [email protected] MS received 6 September 2020; accepted 24 September 2020 Abstract. We present the results obtained from the UV photometry of the globular cluster NGC 1261 using far-UV (FUV) and near-UV (NUV) images acquired with the Ultraviolet Imaging Telescope (UVIT) on- board the AstroSat satellite. We utilized the UVIT data combined with HST, GAIA, and ground-based optical photometric data to construct the different UV colour-magnitude diagrams (CMDs). We detected blue HB (BHB), and two extreme HB (EHB) stars in FUV, whereas full HB, i.e., red HB (RHB), BHB as well as EHB is detected in NUV CMDs. The 2 EHB stars, identified in both NUV and FUV, are confirmed members of the cluster. The HB stars form a tight sequence in UV-optical CMDs, which is almost aligned with Padova isochrones. This study sheds light on the significance of UV imaging to probe the HB mor- phology in GCs. Keywords. Galaxy: globular clusters: individual: NGC 1261—stars: horizontal-branch—stars: blue stragglers—stars: Hertzsprung–Russell and colour-magnitude diagrams. 1. Introduction studied the wide-field multi-colour photometry of this cluster using data from 1.3-m Warsaw telescope at Las Globular clusters (GCs) are the ideal laboratories to Campanas Observatory. They mainly focused on the study the formation and evolution of different stellar brighter sequences of the cluster’s CMD. Identification populations as they are the oldest objects known to of hot exotic populations in optical images is difficult as exist in our galaxy. They provide the best platform to optical images are more crowded than UV images, study exotic populations such as blue straggler stars dominated by main-sequence (MS) and red giants (BSSs), cataclysmic variables, low mass X-ray bina- (RGBs), and the hot stars are optically faint. UV study ries (LMXB), etc. NGC 1261 is an old, metal-inter- of evolved populations in GCs is very important mediate ([Fe/H] = - 1.42 dex) GC located in the because a few critical evolutionary phases, found in the constellation Horologium at a distance of 17.2 kpc GCs, are brighter in UV than in optical wavelength. The (Arellano Ferro et al. 2019). main contributors to the luminosity of GCs in the UV Optical imaging using the Hubble Space Telescope are horizontal branch (HB) stars, BSSs, post-asymptotic has been carried out by Piotto et al. (2002), along with giant branch (PAGB) stars, and white dwarfs (WDs) other 74 Galactic GCs. They constructed the optical (Zinn et al. 1972; Harris et al. 1983). UV colour- colour-magnitude diagrams (CMDs) using HST magnitude diagrams (CMDs) play a vital role to identify F439W and F555W bandpasses. Kravtsov et al. (2010) and study the properties of UV bright stellar populations in GCs (Ferraro et al. 2003; Haurberg et al. 2010; This article is part of the Topical Collection: Chemical Dieball et al. 2010). The HB stars are core helium Elements in the Universe: Origin and Evolution. burning low mass stars with a core mass of 35 Page 2 of 6 J. Astrophys. Astr. (2020) 41:35 approximately 0.5M (Iben & Rood 1970). The performed on 26 August 2017 by UVIT in two far-UV different parts of the HB are red HB (RHB), blue HB (FUV) and four near-UV (NUV) filters. The spatial (BHB), and extreme HB (EHB). RR Lyrae instability resolution of UVIT is better than 1:800 in both FUV strip separates the RHB stars from the BHB stars. EHB and NUV bands. We obtained the magnitudes in all stars are core helium-burning stars with an envelope too filters by performing a PSF photometry on the sci- thin to sustain hydrogen burning. These stars are ence-ready UV images using the DAOphot package in expected to lie at the blue/hot end of the blue tail of the IRAF. The aperture and saturation corrections were HB in the optical CMDs, and their effective temperature also done to obtain the final magnitudes in each filter is found to be more than 20,000 K. The UV wavelength (Tandon et al. 2017b). The instrumental magnitudes acts as an excellent probe to study the HB stars and their were calibrated to the AB magnitude system using characteristics, as these star appear brighter in the UV. zero-points reported in Tandon et al. (2017b). The Brown et al. (2016) studied the HB morphology in stars as faint as 21 magnitude in NUV and 22 mag- 53 GCs, including NGC 1261. They found less than nitude in FUV with typical errors 0.2 and 0.3 mag are four HB stars blue-ward of the G-jump in NGC 1261 detected. The details of the basic parameters and (see Figure 5 in Brown et al. (2016)). Schiavon et al. UVIT observations of the cluster are tabulated in 2012 constructed UV CMDs (FUV-NUV vs. FUV) Table 1. The FUV image of the cluster in the F172M for 44 galactic globular clusters using GALEX data. filter of the UVIT is shown in Figure 1 In this image, They found that the HB stars follow a diagonal we show the area of the UVIT image used in this sequence, unlike the horizontal distribution in optical study, and the region covered with HST with black and its slope mainly depends on the bolometric cor- square box. rection effects. The BSSs are defined as the stars brighter and bluer than the main-sequence (MS) turn-off in the colour- magnitude diagram of the star clusters. The standard 2.1 UV and optical colour-magnitude diagrams theory of stellar evolution does not explain the origin of BSSs. The main leading scenario suggested for the To identify different stellar populations detected with BSS formation in star clusters are (1) mass transfer in UVIT in the inner region (within 3.40 diameter), we close binary systems (McCrea 1964) and (2) merger of cross-matched UVIT detected stars with HST UV binaries induced by collisions (Hills & Day 1976). In legacy survey catalog of GCs (Nardiello et al. 2018). star clusters, the formation scenario due to stellar The proper motion (PM) membership probability for collisions dominates in high-density environments, the HST detected stars is provided by Nardiello et al. whereas other formation scenarios dominate in low- (2018), and they mentioned that the most likely mem- density environments. UV photometry helps to sepa- bers of the cluster have PM membership probability rate BSSs from the hot HB stars, as the hot HB stars greater than 90%. The common stars detected with both are very much luminous in the UV, when compared to HST and UVIT are identified, and the proper motion the relatively cooler BSSs. (PM) members of the cluster are selected for further In this study, we present the results of a UV imaging study. To adopt the same magnitude system, we study of the GC NGC 1261 in six filters (2 FUV and 4 transformed the VEGA magnitude system (of the HST) NUV), using the Ultra-Violet Imaging Telescope into the AB magnitude system. We cross-matched the (UVIT) on AstroSat. We used the proper motion esti- variable star catalog provided by Bustos Fierro and mated using HST and GAIA data to select cluster Caldero´n(2019) with UVIT data to identify the vari- members in the inner and outer regions of the cluster, able stars including RR Lyrae and SX Phe. Our sample respectively. We detect the HB morphology, including of HB stars is not complete in the inner region as we are a few UV bright stars among the HB population. not able to resolve the inner 10 region due to crowding and outside the 10 region, we detected about 90% stars compared to the HST. We constructed the PM cleaned optical and UV-optical CMDs for the members in the 2. Observations and data analysis inner region as shown in the left panel of Figure 2 and Figure 3 respectively. The over-plotted solid grey lines The data analyzed in this study are taken from UVIT correspond to a Padova model isochrone for the on-board AstroSat, the first Indian Space Observatory. metallicity [Fe/H] = –1.42 dex and age 12.6 Gyr gen- Observations of the cluster, NGC 1261, were erated using flexible stellar population synthesis J. Astrophys. Astr. (2020) 41:35 Page 3 of 6 35 Table 1. Details of basic parameters and observations of NGC 1261. R.A. (J2000) Decl. (J2000) lbFilter kmean Exposure Cluster ID (deg) (deg) (deg) (deg) (A˚ ) Time (s) NGC 1261 48.0675 -55.2162 270.5387 52.1243 F169M 1608 1746 F172M 1717 6662 N219M 2196 2847 N245M 2447 740 N263M 2632 1022 N279N 2792 3831 horizontal sequence as found in optical CMDs (Subramaniam et al. 2017; Sahu et al. 2019). We detected two stars at the blue end of the HB in both FUV and NUV CMDs (see Figure 2). From the UV-optical colour of these two stars, we suggest that these stars are likely to be the hotter counter-parts of the HB. Only BHB stars are detected in FUV as these stars are hot enough to emit at this wavelength. We detected full stretch of the HB including RHB, RR Lyrae and BHB in NUV-optical CMDs shown in the lower four panels in Figure 1.