Blue Straggler Stars of NGC 7789 and NGC 2506 Using AstroSat/UVIT

Blue straggler stars are intriguing objects that seem to defy the standard theory of single-star evolution. They manage to elongate their main-sequence lifetimes by acquiring mass either in a direct stellar collision or through mass transfer in a binary or in mergers. We study the candidate blue straggler stars in two intermediate-age open clusters NGC 7789 and NGC 2506 using the far-UV and near-UV observations from the UV imaging telescope of AstroSat in combination with other multi-wavelength data. Around 45% of blue straggler star candidates show an excess in the UV wavelengths and are fitted with two-component spectral energy distributions. We detect most of these blue straggler stars to contain a low-mass or extremely low-mass white dwarf as a hot companion, whereas some yellow straggler stars and red clump stars with white dwarfs of normal mass or high mass as a companion. Based on our analysis, we infer that around 36% of blue straggler stars have likely formed via mass transfer by the Case-A/Case-B mechanism in these two clusters.