Using stellar clusters to trace the properties of the Milky Way in the Gaia era

The Gaia DR2 astrometric data allows us to identify over a thousand clusters as compact groups of stars sharing a common motion and parallax, and to study their distribution across the Galactic disk. The data also allows us to discover new objects, and to show that some putative clusters are in fact asterisms. We clearly see that old clusters are distributed at higher Galactic altitudes, and seem to be virtually absent from the inner disk. Young clusters are known to be associated with the spiral pattern of the Milky Way, but we remark a conspicuous interruption of the Perseus spiral arm, which may be an observational bias but could also be a physical feature of a flocculent Milky Way.

In studies focusing on smaller scale structure we investigated the distribution of young stars in the direction of the Vela-Puppis constellations, in the region of the Vela OB2 association. We report that the young stars are organised in seven main groups with distinct kinematics and ages, each one exhibiting significant spatial substructure. One of those groups includes the known open clusters Collinder 135, Collinder 140, NGC 2451B, NGC2547, and UBC 7, showing they share a common origin. The complex and continuous distribution of these stars in 7D (3D positions, 3D velocities, and age) might be explained by an episode of star formation sustained by a mechanism such as supernova-driven turbulence.