Abstract:
We present the first far-UV (FUV) imaging results of the intermediate-age Galactic open cluster NGC 2818 that
has a planetary nebula (PN) within the field using images taken from the Ultra Violet Imaging Telescope (UVIT)
aboard AstroSat. We identify cluster members by combining UVIT-detected sources with Gaia EDR3 data. We
detect four bright and hot blue straggler stars (BSSs) and two yellow straggler stars (YSSs) based on their location
in optical and FUV–optical color–magnitude diagrams. Based on the parameters estimated using spectral energy
distributions, we infer that BSSs are either collisional products or might have undetectable white dwarf (WD)
companions. Our photometric analysis of YSSs confirms their binarity, consistent with the spectroscopic results.
We find YSSs to be formed through a mass-transfer scenario and the hot components are likely to be A-type
subdwarfs. A comparison of the radial velocity, Gaia EDR3 proper motion of the PN with the cluster, and
reddening toward the PN and the cluster does not rule out the membership of the PN. Comparing the central star’s
position with theoretical post‐AGB (pAGB) models suggest that it has already entered the WD cooling phase, and
its mass is deduced to be ∼0.66 Me. The corresponding progenitor mass turns out to be ∼2.1 Me, comparable to
the turn-off mass of the cluster, implying that the progenitor could have formed in the cluster. We suggest that the
NGC 2818 might be one of the few known clusters to host a PN, providing a unique opportunity to test stellar
evolution models.
