UOCS –VIII. UV study of the open cluster NGC 2506 using ASTROSAT★

We study an intermediate-age open cluster (OC) NGC 2506 using the ASTROSAT/UVIT data and other archival data. We identified 2175 cluster members using a machine learning-based algorithm, ML–MOC, on Gaia EDR3 data. Among the cluster members detected in UVIT filters, F148W, F154W, and F169M, we detect nine blue straggler stars (BSS), three yellow straggler stars (YSS), and three red clump (RC) stars. We construct multiwavelength spectral energy distributions (SEDs) of these objects to characterize them and to estimate their parameters. We discovered hot companions to three BSS, two YSS, and three RC candidates and estimated their properties. The hot companions with estimated temperatures, Teff ∼ 13250–31 000 K, are WDs of extremely low mass (ELM, ∼ 0.20 M⊙), low mass (LM, ∼ 0.20–0.40 M⊙), normal mass (∼ 0.40–0.60 M⊙), and high mass (∼ 0.8 M⊙). We suggest that systems with ELM and LM WDs as companions are formed via Case-A/Case-B mass transfer mechanism. A BSS is the likely progenitor of the high-mass WD, as a star with more than the turn-off mass of the cluster is needed to form a high-mass WD. Thus, systems with a high-mass WD are likely to be formed through merger in triple systems. We conclude that mass transfer as well as merger pathways of BSS formation are present in this cluster.