SIMBAD references

We present a study of the three dimensional structure, kinematics, and age distribution of the Orion OB association, based on the second data release of the Gaia satellite (Gaia DR2). Our goal is to obtain a complete picture of the star formation history of the Orion complex and to relate our findings to theories of sequential and triggered star formation. We selected the Orion population with simple photometric criteria, and we constructed a three dimensional map in galactic Cartesian co-ordinates to study the physical arrangement of the stellar clusters in the Orion region. The map shows structures that extend for roughly 150 pc along the line of sight, divided in multiple sub-clusters. We separated different groups by using the density-based clustering algorithm DBSCAN. We studied the kinematic properties of all the groups found by DBSCAN first by inspecting their proper motion distribution, and then by applying a kinematic modelling code based on an iterative maximum likelihood approach, which we used to derive their mean velocity, velocity dispersion, and isotropic expansion. We derived ages and extinction values for all the groups by using an isochrone fitting procedure. We confirm the presence of an old population (∼15Myr) towards the 25 Ori region, and we find that groups with ages of 12-15Myr are present also towards the Belt region. We notice the presence of a population of ∼10Myr also in front of the Orion A molecular cloud. Our findings suggest that star formation in Orion does not follow a simple sequential scenario, but instead consists of multiple events, which caused kinematic and physical sub-structure. To fully explain the detailed sequence of events, specific simulations and further radial velocity data are needed.