UVIT Open Cluster Study. X. Rich Collection of Post Mass Transfer Systems in NGC 6791

TL;DR

This study uses UVIT imaging to identify and characterize a diverse population of post-mass transfer stellar systems in the old open cluster NGC 6791, revealing complex binary evolution remnants.

Contribution

It provides a detailed UV-based survey of post-mass transfer systems and identifies new candidates, enhancing understanding of binary evolution in open clusters.

Findings

Detected 20 UV-bright members suggestive of binarity or activity.

Characterized 62 cluster members, including hot subdwarfs and white dwarf candidates.

Discovered four blue lurker candidates with mass accretion history.

Abstract

NGC 6791 is one of the richest old open clusters in the Milky Way. Its position above the Galactic plane and the number density makes it an interesting middle ground between Galactic open and globular clusters. We aim to detect the UV bright population of NGC 6791 using \textit{AstroSat}/UVIT images in near-UV and far-UV filters and characterise the known post mass transfer systems such as blue straggler stars (BSSs). We identified 20 members with large UV flux (out of 91 cluster members among 1180 detections), suggestive of binarity, interactions or stellar activity using multi-wavelength spectral energy distribution analysis. We characterised 62 isolated cluster members, including five hot subdwarfs (sdA/sdB). Additionally, we detected ten sdA/sdB/extremely low mass white dwarf (ELM) type candidates hidden alongside other cluster members. Additionally, we report the discovery of four candidate blue lurkers, which are main sequence stars with mass accretion history. We report that this cluster has a variety of stellar (pre-)remnants, such as sdBs, sdAs, and ELM white dwarfs, that are by-products of binary evolution. The above are likely to be post mass transfer binaries found throughout the evolutionary phases from the main sequence to the post horizontal branch. Therefore, this dynamically old open cluster is unique, making it an ideal testbed for dynamical studies.