Open superclusters. I. The most populated primordial groups of open clusters in the third quadrant ofthe Galactic disk

Abstract

We define an open supercluster (OSC) as a cluster of at least six open clusters (OCs) born from the same giant molecular cloud (GMC). We surveyed the recent catalogs of OCs based on Gaia data and relevant literature to find 17 OSCs of the third Galactic quadrant, along with 190 likely members of them. OSCs are frequent enough to be considered an extra class of objects in the hierarchy of star formation. Some of these supersystems are new and most of them contain more members than previously thought. The detailed study of some OSCs lead to the discovery of four new young OCs that are members of them, named Casado-Hendy 2 to 5. In certain instances, subgroups with distinct proper motions (PMs) or 3D positions have been found within an OSC, suggesting the presence of multiple generations of stars formed from several bursts of star formation within the same GMC. OSCs are typically unbound and tend to disintegrate on timescales of sim 0.1 Gyr. The present results confirm that young OCs tend to form primordial groups and suggest that globular clusters (GCs) are not formed from the accretion of OSCs, that is, at least in the local Universe at late times.