UOCS-IX. AstroSat/UVIT study of the open cluster NGC 2818: Blue Stragglers, Yellow Stragglers, Planetary Nebula, and their membership

TL;DR

This study uses UVIT and Gaia data to analyze the stellar populations and planetary nebula in the open cluster NGC 2818, revealing details about blue and yellow stragglers, binarity, and the PN's membership and evolution.

Contribution

First UV imaging of NGC 2818 revealing detailed properties of blue/yellow stragglers and the planetary nebula, linking stellar evolution with cluster membership.

Findings

Detected four blue stragglers and two yellow stragglers in UV and optical.

Confirmed binarity of yellow stragglers and their formation via mass transfer.

Identified the planetary nebula as a cluster member with a progenitor mass of ~2.1Msun.

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 the optical and FUV-optical color-magnitude diagrams. Based on the parameters estimated using Spectral Energy Distribution (SED), 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 (RV), Gaia EDR3 proper motion of the PN with the cluster, and reddening towards the PN and the cluster does not rule out the membership of the PN. 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 ~0.66Msun. 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.