Globular Clusters UVIT Legacy Survey (GlobULeS) I. FUV-optical
  Color-Magnitude Diagrams for Eight Globular Clusters
  We present the first results of eight Globular Clusters (GCs) from the
AstroSat/UVIT Legacy Survey program GlobULeS based on the observations carried
out in two FUV filters (F148W and F169M). The FUV-optical and FUV-FUV
color-magnitude diagrams (CMDs) of GCs with the proper motion membership were
constructed by combining the UVIT data with HST UV Globular Cluster Survey
(HUGS) data for inner regions and Gaia Early Data Release (EDR3) for regions
outside the HST's field. We detect sources as faint as F148W $\sim$ 23.5~mag
which are classified based on their locations in CMDs by overlaying stellar
evolutionary models. The CMDs of 8 GCs are combined with the previous UVIT
studies of 3 GCs to create stacked FUV-optical CMDs to highlight the
features/peculiarities found in the different evolutionary sequences. The FUV
(F148W) detected stellar populations of 11 GCs comprises 2,816 Horizontal
Branch (HB) stars (190 Extreme HB candidates), 46 post-HB (pHB), 221 Blue
Straggler Stars (BSS), and 107 White Dwarf (WD) candidates. We note that the
blue HB color extension obtained from F148W$-$G color and the number of FUV
detected EHB candidates are strongly correlated with the maximum internal
Helium (He) variation within each GC, suggesting that the FUV-optical plane is
the most sensitive to He abundance variations in the HB. We discuss the
potential science cases that will be addressed using these catalogues including
HB morphologies, BSSs, pHB, and, WD stars.
