UOCS XX: Ultraviolet Signatures of Binary Evolution in the Open Cluster Berkeley 21

Abstract We present a multi-wavelength study of the intermediate-age (~2.1 Gyr) open cluster Berkeley 21 using ultraviolet observations from AstroSat/UVIT together with archival optical and infrared data. A machine learning–based membership analysis (ML-MOC) applied to Gaia DR3 identifies 325 cluster members, of which ten are detected in the UVIT/F172M filter, including four blue straggler stars (BSSs), five main-sequence (MS) stars, and one red clump (RC) star. We construct ultraviolet-to-infrared spectral energy distributions (SEDs) to derive stellar parameters and search for unresolved companions. The BSSs are well reproduced by single-component SEDs, suggesting either merger origins or companions too cool to be detected in the UV. In contrast, four MS stars and the RC star exhibit significant UV excess (>50 %), requiring binary-component SEDs. The inferred hot companions have effective temperatures of ~26,000–49,000 K and properties consistent with low-mass (≤0.4M⊙) white dwarfs. Such white dwarfs cannot form through single-star evolution within a Hubble time, indicating a binary mass-transfer origin. The detection of hot companions in the MS stars identifies them as blue lurker candidates, while the RC star also shows evidence for past binary interaction. Our results demonstrate that binary evolution, most likely through Case-B mass transfer, plays a major role in shaping the hot stellar population of Berkeley 21, with at least five of the ten UV-detected sources formed through mass transfer.