UOCS. XIV. Uncovering extremely low mass white dwarfs and blue lurkers in NGC 752

TL;DR

This study identifies extremely low mass white dwarfs and blue lurkers in the open cluster NGC 752 using multiwavelength data, revealing a high binary fraction and evidence of past mass transfer events.

Contribution

It provides the first detailed identification of low mass white dwarfs and blue lurkers in NGC 752, highlighting the importance of multiwavelength photometry in binary population studies.

Findings

Detected 8 white dwarfs as companions to cluster members.

Found 4 main sequence stars with extremely low mass white dwarf companions.

Estimated binary fraction of 50-70% in NGC 752.

Abstract

Evolutionary pathways of binary systems are vastly different from single stellar evolution, and thus, there is a need to quantify their frequency and diversity. Open clusters are the best test-bed to unveil the secrets of binary populations due to their coeval nature. And the availability of multi-wavelength data in recent years has been critical in characterising the binary population. NGC 752 is a solar metallicity, intermediate-age open cluster located at 460 pc. In this work, we aim to identify the optically subluminous white dwarfs in NGC 752 and identify the illusive blue lurkers by association. We used multiwavelength photometry from Astrosat/UVIT, swift/UVOT, Gaia DR3 and other archival surveys to analyse the colour-magnitude diagrams and spectral energy distributions of 37 cluster members. We detected eight white dwarfs as companions to cluster members. Four of the systems are main sequence stars with extremely low mass white dwarfs as their companions. Two are these main sequence stars are also fast rotators. The presence of low mass white dwarfs and high rotation signals a past mass transfer, and we classified the four main sequence stars as blue lurkers. The binary fraction in NGC 752 was estimated to be 50--70%, and it shows that the contribution of optically undetected stars is crucial in quantifying the present-day binary fraction.