UOCS VII. Blue Straggler Populations of Open Cluster NGC 7789 with UVIT/AstroSat

TL;DR

This study uses UVIT/AstroSat data to analyze blue straggler stars in open cluster NGC 7789, revealing hot companions likely to be low-mass white dwarfs, and suggesting mass transfer as a formation mechanism for a third of these stars.

Contribution

First detailed UV spectral energy distribution analysis of BSS in NGC 7789, identifying hot companions and providing evidence for mass transfer formation mechanisms.

Findings

Hot companions are likely low-mass white dwarfs.

Approximately 33% of BSS formed via mass transfer.

Detected UV excess in some BSS candidates.

Abstract

NGC 7789 is a $\sim$1.6 Gyr old, populous open cluster located at $\sim$2000 pc. We characterize the blue straggler stars (BSS) of this cluster using the Ultraviolet (UV) data from the UVIT/AstroSat. We present spectral energy distributions (SED) of 15 BSS candidates constructed using multi-wavelength data ranging from UV to IR wavelengths. In 8 BSS candidates, a single temperature SED is found to be satisfactory. We discover hot companions in 5 BSS candidates. The hot companions with Teff $\sim$11750-15500 K, R $\sim$0.069-0.242 R$_{\odot}$, and L $\sim$0.25-1.55 L$_{\odot}$, are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than $\sim$0.18 M$_{\odot}$, and thereby confirmed post mass transfer systems. We discuss the implication of this finding in the context of BSS formation mechanisms. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however, we are unable to characterize them. We suggest that at least 5 of the 15 BSS candidates (33%) studied in this cluster have formed via the mass transfer mechanism.