# Structure, kinematics, and ages of the young stellar populations in the Orion region

## TL;DR

This study uses Gaia DR2 data to analyze the 3D structure, kinematics, and ages of stellar populations in Orion, revealing complex, multi-episode star formation history with multiple sub-structures and ages.

## Contribution

It provides a detailed 3D map and kinematic analysis of Orion's stellar groups, highlighting multiple star formation events and challenging simple sequential models.

## Key findings

- Presence of an old (~15 Myr) population near 25 Ori
- Detection of 12-15 Myr groups in the Belt region
- Identification of a ~10 Myr population in front of Orion A cloud

## Abstract

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.