A study of newly discovered close binary open clusters in the Milky Way

Abstract

With the release of Gaia data, the number of known Galactic open clusters (OCs) has increased rapidly, providing an excellent opportunity to confirm more binary open clusters in the Milky Way. 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). The three dimensional spatial coordinates, proper motions, and colour-magnitude diagrams (CMDs) are used for identifying candidate CBOCs. The fundamental parameters of 26 star clusters are determined by fitting CMDs to stellar population isochrones, to check the similarity of reddenings, ages and metallicities of the sub-clusters of candidate CBOCs. The virial equilibrium is then used to exclude fake 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 are then compared to the Roche radius and orbital velocity respectively, to find out gravitationally bound pairs. We identified nine new CBOC candidates from bound candidate open clusters, seven of which are shown to be candidates for primordial binary open clusters (PBOCs). However, only the pair CWNU 1024 and OCSN 82 is identified as a gravitationally bound CBOC, when considering the uncertainties. The other eight CBOC candidates appear to be gravitationally unbound pairs, but the results depend on the methods of tidal radius determination and gravitational binding examination.