Study of UV bright stellar populations in the globular cluster NGC 1261
  using AstroSat
  We present the UV photometry of the globular cluster NGC 1261 using images
acquired with the Ultraviolet Imaging Telescope (UVIT) on-board ASTROSAT. We
performed PSF photometry on four near-UV (NUV) and two far-UV (FUV) images and
constructed UV colour-magnitude diagrams (CMDs), in combination with HST, Gaia,
and ground-based optical photometry for member stars. We detected the full
horizontal branch (HB) in NUV, blue HB in the FUV and identified two extreme HB
(EHB) stars. HB stars have a tight sequence in UV-optical CMDs well-fitted with
isochrones generated (12.6 Gyr age, [Fe/H] = -1.27 metallicity) using updated
BaSTI-IAC models. Effective temperatures (Teff), luminosities and radii of
bright HB stars were estimated using spectral energy distribution. As we detect
the complete sample of UV bright HB stars, the hot end of the HB distribution
is found to terminate at the G-jump (Teff ~ 11500 K). The two EHB stars, fitted
well with single spectra, have Teff= 31,000 K and a mass = 0.495Msun and follow
the same Teff-Radius relation of the blue HB stars. We constrain the formation
pathways of these EHB stars to extreme mass loss in the RGB phase (either due
to rotation or enhanced Helium), OR early hot-flash scenario.
