An Open Star Cluster consists of hundreds to thousands of stars, which are loosely bound. They are formed most likely from a single gas cloud, and are therefore roughly of same age. Open clusters and particularly old open clusters therefore are ideal sites to study the stellar evolution for both single and binary stars. Most stars evolve away from the main sequence once their hydrogen burning phase is over. The turn over point of the Hertzsprung – Russell (HR) diagram of an open cluster is indicative of its age.

Blue Straggler Stars (BSS) are members of old clusters that are brighter and bluer than stars on the upper main sequence. They appear to 'extend' the main sequence in a HR diagram of the cluster and appear as if they are 'younger' stars. They are termed stragglers because they do not move away from the main sequence like the other stars in the same cluster.

NGC-188 is a well-studied old open cluster with an estimated age of 7 Gyr (Billion year, astronomically known as Giga Year- Gyr) and exhibits high metallicity. It is located about 5000 light years away and has about 1050 stars as its members with 20 BSSs confirmed. WOCS-5885, most likely a member of NGC-188 (with a high probability of 53 to 80% quoted in literature), was one of the 3 objects identified with exceptionally blue color. Various classifications -a BSS or a sub-dwarf or a binary with a red giant and a pre-white dwarf to name a few- were attributed to this object, because its spectrum did not match with any single identification. This could only be resolved if the hot (UV, blue) and the cool (red, IR) part of the spectrum of this object could be fitted together with spectral models of stars. This had been done with observations from space (GALEX, UIT, UVOT, SPITZER, WISE) and several ground based observatories, spanning the IR, optical and UV bands.

The UV band observations from the Ultraviolet Imaging Telescope (UVIT) on ASTROSAT have provided additional points in the Spectral Energy Distribution (SED) thus resulting in a much better spectral fit over the wavelength range of 0.15 µm to 7.8 µm. With this data set, WOCS-5885 has been classified as a binary consisting of a BSS and a hot star which is either a post Asymptotic Giant Branch or Horizontal branch (post-AGB/HB) star.

The UVIT contains two 38-cm telescopes; one for the far-ultraviolet (FUV) region, the other for the near-ultraviolet (NUV) and visible (VIS) regions. These are divided using a dichroic mirror for beam splitting. UVIT is primarily an imaging instrument, simultaneously generating images in the FUV, NUV and VIS channels over a 28 arcmin diameter circular field. Each channel can be divided into smaller pass bands using a selectable set of filters.

UVIT observed NGC-188 both as a first light object and for regular calibration. The observations have been done in both NUV and FUV filters in the wavelength band of 0.3 to 0.15 µm. With these observations, it is found that the SED can only be fit with spectra consisting of 2 stars. The cooler star is found to be a BSS with a temperature of 6,000 + 150 K, and the temperature of the hotter star is 17,000 + 500 K. The estimated size and luminosity of the hotter star rule out a white-dwarf or a sub-dwarf classification and hence it is proposed that it could be a post AGB/HB star. If the membership of WOCS-5885 to NGC-188 is confirmed, it could be a rare BSS + post AGB/HB binary, the first of its kind to be identified (for which probability is high) in an open cluster. This system therefore provides a great opportunity to constrain theories of BSS formation via mass transfer.

Thus, observations from the UVIT were used to solve the puzzle of a star WOCS-5885 which appeared as a single star but whose spectra did not match with this identity.

For details:

Subramaniam Annapurni et al., A Hot Companion to a Blue Straggler in NGC-188 as Revealed by the Ultra-Violet Imaging Telescope (UVIT) on ASTROSAT-The Astrophysical Journal Letters, Volume 833, No. 2, 19 December 2016.

FUV (left) and NUV (right) images of NGC-188 obtained on 18 February 2016.

WOCS-5885 is marked as red square

Spectral Energy Distribution (SED) for WOCS-5885. Points in light green are from UVIT