UOCS. V. UV study of the old open cluster NGC 188 using AstroSat
  We present the UV photometry of the old open cluster NGC188 obtained using
images acquired with Ultraviolet Imaging Telescope (UVIT) on board the ASTROSAT
satellite, in two far-UV (FUV) and one near-UV (NUV) filters. UVIT data is
utilised in combination with optical photometric data to construct the optical
and UV colour-magnitude diagrams (CMDs). In the FUV images, we detect only hot
and bright blue straggler stars (BSSs), one hot subdwarf, and one white dwarf
(WD) candidate. In the NUV images, we detect members up to a faintness limit of
~22 mag including 21 BSSs, 2 yellow straggler stars (YSSs), and one WD
candidate. This study presents the first NUV-optical CMDs, and are overlaid
with updated BaSTI-IAC isochrones and WD cooling sequence, which are found to
fit well to the observed CMDs. We use spectral energy distribution (SED)
fitting to estimate the effective temperatures, radii, and luminosities of the
UV-bright stars. We find the cluster to have an HB population with three stars
(Teff = 4750 - 21000 K). We also detect two yellow straggler stars, with one of
them with UV excess connected to its binarity and X-ray emission.
