Study of Blue Straggler Stars in Open Cluster NGC 2506 using UVIT/AstroSat

Abstract

Blue straggler stars (BSS) are one of the exotic stellar populations that prolong their main-sequence lifetimes by acquiring extra mass through either mass transfer or complete merger in a binary or multiple stellar system, or through direct stellar collisions in dense stellar environments. UV-based studies enable detection of hot companions of BSS from the excess flux in the UV wavelengths hence shedding light on the formation mechanisms of BSS. NGC 2506 is an intermediate age (∼ 2.2 Gyr) open cluster located at a distance of ∼ 3110 pc. According to our membership analysis of the cluster based on the Gaia EDR3 data, this cluster has 9 BSS candidates. We use the UVIT/AstroSat data in combination with other multi-wavelength data to characterize these BSS candidates. The spectral energy distributions (SEDs) of these BSS showed excess UV flux in 3 BSS candidates. Based on the properties of the hot companions derived from the SEDs, these hot companions are low-mass (LM) or extremely low-mass (ELM) white dwarfs (WD). The detection of LM/ELM WDs in a binary system strongly indicates the formation of BSS via Case-A/Case-B mass transfer mechanism. We, therefore suggest that at least 3 out of 7 (42%) BSS studied have formed via the Case-A/Case-B mass-transfer mechanism.