Ultraviolet Imaging Telescope (UVIT) observation of the Galactic globular cluster NGC 7492

Abstract

We present detailed photometric observations of the Galactic globular cluster NGC 7492 using the data obtained with two far-ultraviolet (FUV: 1300 1800 Å) and three near-ultraviolet (NUV: 2000 3000 Å) filters of Ultraviolet Imaging Telescope (UVIT) on-board the AstroSat satellite. We confirmed the cluster membership of the extracted sources using GAIA data release 2 (Gaia DR2) proper motion data. We have used color-magnitude diagrams (CMDs) using UVIT and GAIA filters to separate out different evolutionary stages of the stars present in the cluster. We have identified a new extreme horizontal branch (EHB) star at the core of the cluster using UV and UV-optical CMDs. The estimated distance-modulus of the cluster is 16.95±0.05 obtained by fitting BaSTI isochrones with cluster parameters, [Fe/H] = −1.8 dex and age = 12.0 Gyr on the V − I vs V CMD. Interestingly, only the EHB star and blue horizontal branch stars (BHBs) among the UV-bright hot sources are detected in FUV filters of UVIT. We have derived the effective temperature of BHBs using color-temperature relation and spectral energy distributions (SEDs) of multi-band filters, which are in the range from 8,000 K to 10,500 K. We find a variation of He abundance of BHBs by fitting the BaSTI ZAHB. The range in the He abundance of the BHBs corresponding to the best fit isochrones is 0.247 to 0.350. We have estimated various physical parameters of the newly identified EHB star in the cluster using SED fit and post-HB evolutionary tracks. We have studied the radial distribution of all the sources of the cluster detected in UVIT. The sources detected in FUV filters extend beyond the half light radius (1.15′) of the cluster, whereas the sources detected in NUV filters extend beyond the tidal radius (9.2′) of the cluster.