Discovery of a 21 Myr old stellar population in the Orion complex
  The Orion complex is arguably the most studied star-forming region in the
Galaxy. While stars are still being born in the Orion nebula, the oldest part
was believed to be no more than 13 Myr old. In order to study the full
hierarchy of star formation across the Orion complex, we perform a clustering
analysis of the Ori OB1a region using new stellar surveys and derive robust
ages for each identified stellar aggregate. We use Gaia DR2 parameters
supplemented with radial velocities from the GALAH and APOGEE surveys to
perform clustering of the Ori OB1a association. Five overdensities are resolved
in a six-dimensional parameter space (positions, distance, proper motions, and
radial velocity). Most correspond to previously known structures (ASCC 16, 25
Orionis, ASCC 20, ASCC 21). We use Gaia DR2, Pan-STARRS1 and 2MASS photometry
to fit isochrones to the colour-magnitude diagrams of the identified clusters.
The ages of the clusters can thus be measured with ~10% precision. While four
of the clusters have ages between 11 and 13 Myr, the ASCC 20 cluster stands out
at an age of 21 $\pm$ 3 Myr. This is significantly greater than the age of any
previously known component of the Orion complex. To some degree, all clusters
overlap in at least one of the six phase-space dimensions. We argue that the
formation history of the Orion complex, and its relation to the Gould belt,
must be reconsidered. A significant challenge in reconstructing the history of
the Ori OB1a association is to understand the impact of the newly discovered 21
Myr old population on the younger parts of the complex, including their
formation.
