SIMBAD references

The first Gaia Data Release presents an opportunity to characterize the low-mass population of OB associations, providing larger statistical samples and better constraints on the formation and evolution of clusters and associations. Using previously known low-mass members in a small region of Vela OB2, we have designed selection criteria that combine Gaia and 2MASS photometry, independently of any astrometric information, to identify low-mass pre-main-sequence stars over the wider association area. Our method picks out the known clusters of young stars around γ² Velorum and NGC-2547, but also identifies other overdensities that may represent previously unknown clusters. There are clear differences in the spatial distributions of the low-mass and the high-mass OB populations, suggesting either that the structure and dynamics of these populations has evolved separately or that the initial mass function can vary considerably on small scales within a single association.