The Horizontal Branch population of NGC 1851 as revealed by the
  Ultra-violet Imaging Telescope (UVIT)
  We present the UV photometry of the globular cluster NGC 1851 using images
acquired with the Ultra-violet Imaging Telescope (UVIT) onboard the ASTROSAT
satellite. PSF-fitting photometric data derived from images in two far-UV (FUV)
filters and one near-UV (NUV) filter are used to construct color-magnitude
diagrams (CMD), in combination with HST and ground-based optical photometry. In
the FUV, we detect only the bluest part of the cluster horizontal branch (HB);
in the NUV, we detect the full extent of the HB, including the red HB, blue HB
and a small number of RR Lyrae stars. UV variability was detected in 18 RR
Lyrae stars, and 3 new variables were also detected in the central region. The
UV/optical CMDs are then compared with isochrones of different age and
metallicity (generated using Padova and BaSTI models) and synthetic HB (using
helium enhanced $Y^2$ models). We are able to identify two populations among
the HB stars, which are found to have either an age range of 10-12~Gyr, or a
range in Y$_{ini}$ of 0.23 - 0.28, for a metallicity of [Fe/H] =$-$1.2 to
$-$1.3. These estimations from the UV CMDs are consistent with those from
optical studies. The almost complete sample of the HB stars tend to show a
marginal difference in spatial/azimuthal distribution among the blue and red HB
stars. This study thus show cases the capability of UVIT, with its excellent
resolution and large field of view, to study the hot stellar population in
Galactic globular clusters.
