AstroSat/UVIT Study of the open cluster NGC 2818: Membership, Blue Stragglers, Yellow Stragglers, and Planetary Nebula

We present the first far-UV (FUV) imaging results of the intermediate-age Galactic open cluster (OC)
NGC 2818 that has a Planetary nebula (PN) within the field. We explore whether the PN is a member
of the cluster using images taken from the Ultra-violet Imaging Telescope (UVIT) aboard AstroSat.
We identify the cluster members by combining the 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 the optical and FUV-optical color-magnitude diagrams (CMDs). The theoretical
isochrones more or less fit the observed distribution of detected stars in all the CMDs. Based on the
parameters estimated using Spectral Energy Distribution (SED), we infer that the 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 the 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 (RV), Gaia EDR3 proper-motion (PM) of the PN with the cluster
members, and reddening towards the PN and the cluster does not rule out the membership of the PN.
Using SED, the estimated stellar parameters of the PN’s central star match well with the previous
estimations. Comparing the central star’s position with theoretical pAGB models suggest that it has
already entered the WD cooling phase, and its mass is deduced to be ∼ 0.6Msun. The corresponding
progenitor mass turns out to be ∼ 2.1Msun, 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.