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

Abstract

Context. Evolutionary pathways of binary systems are vastly different from single stellar evolution, and thus there is a need to quantify their frequency and diversity. Thanks to their to their coeval nature, open clusters are the best test beds for unveiling the secrets of binary populations. The availability of multi-wavelength data in recent years has been critical in characterising the binary population.

Aims. NGC 752 is a solar-metallicity, intermediate-age open cluster located at 460 pc. In this work, we aim to identify the optically sub-luminous white dwarfs in NGC 752 and identify the elusive blue lurkers by association.

Methods. We used multi-wavelength 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.

Results. We detected eight white dwarf companions of cluster members. Four of the systems are main sequence stars with extremely low-mass white dwarfs as their companions. Two of these main sequence stars are also fast rotators.

Conclusions. The presence of low-mass white dwarfs and fast rotation signals a past mass transfer, and we classify the four main sequence stars as blue lurkers. The binary fraction in NGC 752 is estimated to be 50–70%, and it shows that the contribution of optically undetected stars is crucial in quantifying the present-day binary fraction.