A Hot Companion to a Blue Straggler in NGC 188 as Revealed by the Ultra-Violet Imaging Telescope (Uvit) on Astrosat

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 µm to 7.8 µm. 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±150 K, confirming that it is a BSS. Assuming it to be a main-sequence star, we estimate its mass to be ∼ 1.1 -1.2M . The hotter component, with an estimated temperature of 17 000±500 K, has a radius of ∼ 0.6R and L ∼ 30L . 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.

UVIT Open Cluster Study. II. Detection of Extremely Low Mass White Dwarfs and Post–Mass Transfer Binaries in M67

“…The multi-wavelength SEDs were created and compared with model SEDs to determine their characteristics. The analysis presented here is similar to that presented in Subramaniam et al (2016) and Sindhu et al (2019a).…”

The Calibration of the UVIT Detectors for the ASTROSAT Observatory

“…A description of the software package is given in Postma et al (2017). The scientific performance of the UVIT instrument and its space calibration is discussed in Subramaniam et al (2017) and Tandon et al (2017). Some of the first scientific results with UVIT are described in e.g.…”

Ultraviolet Imaging Telescope on AstroSat

“…Lastly we present the first fully analysed result from UVIT which has been published by Subramanian et al 4 . The images of the old open cluster, NGC 188, taken by UVIT revealed a blue straggler star which has a companion from which it has been accreting material.…”

“…Figure 6 shows an analysis of the properties of the stars in this binary system. For more details the reader is referred to Subramaniam et al 4 . These precise measurements from UVIT helped identify and study the companion.…”

“…This pair is a unique example to study details of the formation of blue straggler stars. As the accreted material from the aging star will be enriched with elements processed within it, this blue straggler will throw light on the kind of nuclear processing that happens in aging stars 4 . Many other observations and analyses are under way, and the UVIT payload is proving to be a versatile instrument of wide interest.…”