SIRIUS Project. IV. The formation history of the Orion Nebula Cluster
  driven by clump mergers
  The Orion Nebula Cluster (ONC) is an excellent example for understanding the
formation of star clusters. Recent studies have shown that ONC has three
distinct age populations and anisotropy in velocity dispersions, which are key
characteristics for understanding the formation history of the ONC. In this
study, we perform a smoothed-particle hydrodynamics/$N$-body simulation of star
cluster formation from a turbulent molecular cloud. In this simulation, stellar
orbits are integrated using a high-order integrator without gravitational
softening; therefore, we can follow the collisional evolution of star clusters.
We find that hierarchical formation causes episodic star formation that is
observed in the ONC. In our simulation, star clusters evolve due to mergers of
subclumps. The mergers bring cold gas with the clumps into the forming cluster.
This enhances the star formation in the cluster centre. The dense cold gas in
the cluster centre continues to form stars until the latest time. This explains
the compact distribution of the youngest stars observed in the ONC. Subclump
mergers also contribute to the anisotropy in the velocity dispersions and the
formation of runaway stars. However, the anisotropy disappears within 0.5 Myr.
The virial ratio of the cluster also increases after a merger due to the
runaways. These results suggest that the ONC recently experienced a clump
merger. We predict that most runaways originated from the ONC have already been
found, but walkaways have not.
