UVIT-HST-Gaia-VISTA study of KRON 3 in the Small Magellanic Cloud: A cluster with an extended red clump in UV

Abstract: We have demonstrated the advantage of combining multi-wavelength observations, from the ultraviolet (UV) to near-infrared, to study Kron 3, a massive star cluster in the Small Magellanic Cloud. We have estimated the radius of the cluster Kron 3 to be 2.'0 and for the first time, we report the identification of NUV-bright red clump (RC) stars and the extension of the RCin colour and magnitude in the NUV vs (NUV-optical) colour-magnitude diagram (CMD). We found that extension of the RC is an intrinsic property of the cluster and it is not due to contamination of field stars or differential reddening across the field. We studied the spectral energy distribution of the RC stars and estimated a small range in temperature ~5000 - 5500K, luminosity ~60 - 90 Land radius ~8.0 - 11.0 Supporting their RC nature. The range of UV magnitudes amongst the RC stars (~23.3 to 24.8 mag) is likely caused by the combined effect of variable mass loss, variation in initial helium abundance (Y_ini=0.23 to 0.28), and a small variation in age (6.5-7.5 Gyr) and metallicity ([Fe/H]=-1.5 to -1.3). Spectroscopic follow-up observations of RC stars in Kron 3 are necessary to confirm the cause of the extended RC.