A Hot Companion to a Blue Straggler in NGC188 as Revealed by the Ultra-violet Imaging Telescope (UVIT) on ASTROSAT

TL;DR

This study uses UVIT on ASTROSAT to discover a hot companion to a blue straggler in NGC188, revealing a binary system with a post-AGB/HB star and providing insights into BSS formation through mass transfer.

Contribution

First identification of a blue straggler with a post-AGB/HB companion in an open cluster using UVIT data, advancing understanding of BSS formation mechanisms.

Findings

Detected a binary system with a BSS and a hot companion.

The hot companion is likely a post-AGB/HB star, not a white dwarf.

This is the first such system identified in an open cluster.

Abstract

We present early results from the Ultra-Violet Imaging Telescope (UVIT) onboard the ASTROSAT observatory. We report the discovery of a hot companion associated with one of the blue straggler stars (BSSs) in the old open cluster, NGC188. Using fluxes measured in four filters in UVIT's Far-UV (FUV) channel, and two filters in the near-UV (NUV) channel, we have constructed the spectral energy distribution (SED) of the star WOCS-5885, after combining with flux measurements from GALEX, UIT, UVOT, SPITZER, WISE and several ground-based facilities. The resulting SED spans a wavelength range of 0.15~${\mu}$m to 7.8~${\mu}$m. This object is found to be one of the brightest FUV sources in the cluster. An analysis of the SED reveals the presence of two components. The cooler component is found to have a temperature of 6,000$\pm$150~K, confirming that it is a BSS. Assuming it to be a main-sequence star, we estimate its mass to be $\sim$ 1.1 - 1.2M$_\odot$. The hotter component, with an estimated temperature of 17,000$\pm$500~K, has a radius of $\sim$ 0.6R$_\odot$ and L $\sim$ 30L$_\odot$. Bigger and more luminous than a white dwarf, yet cooler than a sub-dwarf, we speculate that it is a post-AGB/HB star that has recently transferred its mass to the BSS, which is known to be a rapid rotator. This binary system, which is the first BSS with a post-AGB/HB companion identified in an open cluster, is an ideal laboratory to study the process of BSS formation via mass transfer.