A UVIT/AstroSat Study of Blue Straggler Stars of NGC 7789

Abstract

We present a study of blue straggler stars (BSS) of open cluster NGC 7789 using the Ultraviolet (UV) data from the UVIT/AstroSat. NGC 7789 is a ∼1.6 Gyr old, populous open cluster located at ∼2000 pc. There are 13 BSS candidates of this cluster which are members according to their radial velocities as well as Gaia DR2 proper motions and parallaxes. We identify 4 additional BSS candidates in this cluster using Gaia DR2 proper motions and parallaxes. We construct spectral energy distributions (SED) of these BSS candidates using multi-wavelength data ranging from UV to IR wavelengths. We discover five of the BSS with a hot companion with T _{eff} ∼11750--15500 K, R ∼0.069--0.242 R _{sun}, and L ∼0.25--1.55 L _{sun}. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however we are unable to characterize them. All the five hot companions to the BSS are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than ∼0.18 M _{sun}, and thereby confirmed post mass transfer systems. We suggest that at least 33% of the BSS candidates studied in this cluster have formed via Case-A/Case-B mass transfer mechanism.