UOCS-VIII. UV Study of the open cluster NGC 2506 using ASTROSAT

TL;DR

This study uses UV and archival data to identify and characterize stellar populations in the open cluster NGC 2506, revealing hot companions and multiple formation pathways for blue straggler stars.

Contribution

It presents the first UV study of NGC 2506, identifying hot companions to BSS, YSS, and RC stars, and discusses their formation mechanisms involving mass transfer and mergers.

Findings

Detected 9 BSS, 3 YSS, and 3 RC stars in UV

Discovered hot companions with low to high mass WDs

Indicated multiple formation pathways for BSS

Abstract

We study an intermediate-age open cluster NGC 2506 using the \textit{ASTROSAT}/UVIT data and other archival data. We identified 2175 cluster members using a machine learning-based algorithm, ML--MOC, on Gaia EDR3 data. Among the cluster members detected in UVIT filters, F148W, F154W, and F169M, we detect 9 blue straggler stars (BSS), 3 yellow straggler stars (YSS) and 3 red clump (RC) stars. We construct multi-wavelength spectral energy distributions (SEDs) of these objects to characterize them and to estimate their parameters. We discovered hot companions to 3 BSS, 2 YSS and 3 RC candidates and estimated their properties. The hot companions with estimated temperatures, T$\mathrm{_{eff}}$ $\sim$ 13250--31000 K, are WDs of extremely low-mass ($\sim$ 0.20 M$_\odot$), low-mass ($\sim$ 0.20--0.40 M$_\odot$), normal mass ($\sim$ 0.40--0.60 M$_\odot$), and high-mass ($\sim$ 0.8 M$_\odot$). We suggest that systems with extremely low mass and low mass WDs as companions are formed via Case-A/Case-B mass transfer mechanism. A BSS is the likely progenitor of the high mass WD, as a star with more than the turn-off mass of the cluster is needed to form a high mass WD. Thus, systems with high mass WD are likely to be formed through merger in triple systems. We conclude that mass transfer as well as merger pathways of BSS formation are present in this cluster.