Structure, kinematics, and ages of the young stellar populations in the
  Orion region
  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 select the Orion
population with simple photometric criteria, and we construct a three
dimensional map in galactic Cartesian coordinates to study the physical
arrangement of the stellar clusters in the Orion region. The map shows
structures that extend for roughly $150 \, \mathrm{pc}$ along the line of
sight, divided in multiple sub-clusters. We separate different groups by using
the density based clustering algorithm DBSCAN. We study 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 use to derive their mean
velocity, velocity dispersion and isotropic expansion. By using an isochrone
fitting procedure we provide ages and extinction values for all the groups. We
confirm the presence of an old population ($\sim 15$ Myr) towards the 25 Ori
region, and we find that groups with ages of $12-15 \, \mathrm{Myr}$ are
present also towards the Belt region. We notice the presence of a population of
$\sim 10$ Myr 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.
