A study of newly discovered close binary open cluster candidates in the Milky Way from Gaia DR3

Institute of Astronomy and Information, Dali University, Dali 671003, China

^★ Corresponding author: zhongmuli@126.com

Received: 1 April 2025
Accepted: 9 July 2025

Abstract

Context. With the release of Gaia data, the number of known Galactic open clusters (OCs) has increased rapidly. This provides an excellent opportunity to confirm more binary OCs in the Milky Way.

Aims. Using a recently released OC catalogue, we employed the photometric and astrometric data of OCs and their member stars to find close binary open clusters (CBOCs).

Methods. Three-dimensional spatial coordinates, proper motions, and colour-magnitude diagrams were used to identify candidate CBOCs. The fundamental parameters of 26 star clusters were determined by fitting colour-magnitude diagrams to stellar population isochrones to confirm the similarity of reddenings, ages, and metallicities of the sub-clusters of candidate CBOCs. The virial equilibrium was then used to exclude false-positive CBOCs, including unbound moving groups. To further confirm the binary nature of the CBOC candidates, we calculated their Roche radii and orbital parameters. The tidal radius and radial velocity difference were then compared to the Roche radius and orbital velocity, respectively, to determine gravitationally bound pairs.

Results. We identified nine new CBOC candidates from bound candidate OCs, seven of which are shown to be candidates for primordial binary OCs. Only the pair CWNU 1024 and OCSN 82 was identified as a gravitationally bound CBOC when the uncertainties were considered, however. The other eight CBOC candidates appear to be gravitationally unbound pairs, but the results depend on the methods with which the tidal radius was determined and with which the gravitational binding was examined.