Results
Dynamical ejection of stars during star-cluster formation
Star clusters form dense molecular clouds. We performed simulations of star-cluster formation starting from turbulent molecular clouds. Once star clusters form, binaries in the cluster cores dynamically eject stars formed in them. If massive stars are ejected from the star clusters, they form HII regions outskirts of the clusters. This process can be investigated using simulations without gravitational softening for stars.
Figure: Gas surface density (left) and gas temperature (right) of the simulated cluster-forming region. Red dots indicate massive stars, and the others are less massive stars. Yellow regions in the right panel correspond to HII regions. (Fujii et al. 2021, SIRIUS III., PASJ, 3, 4, pp.1074-1099, Fig. 13)
Formation of runaway and walkaway stars
As a result of the ejection of stars from forming star clusters, a fraction of stars become runaway and walkway stars. Runaway (walkaway) stars are stars that have a peculiar velocity of more than 30 km/s (5 km/s). The number of runaway stars formed in the Orion Nebula Cluster model is consistent with that found around the Orion Nebula Cluster. The simulation suggests that more walkway stars have not been identified.
Figure: Cumulative mass function of runaway (green-dotted curve) and walkway (blue-dashed curve) stars in the simulation. Red and black curves show the mass function of the cluster and all stars. Blue and green shaded regions indicate the number of walkaway and runaway stars observed around the Orion Nebula Cluster. (Fujii et al. 2022, SIRIUS IV., MNRAS, 514, 2, pp.2513-2526, Fig. 12)
Age distribution of stars in the Orion Nebula Cluster
Star clusters are formed via repeating mergers of small sub-clusters. This process results in multiple ages of stars in star clusters. In the Orion Nebula Cluster, three population has been observed. In the simulation, when sub-clusters merge with the main cluster, they bring cold, dense gas with the clumps. As a result, star formation simultaneously increases just after clump mergers. This causes multiple peaks of the stellar ages in a star cluster, as seen in the ONC. Star formation is the most active in the central region of the cluster, where the gravitational potential is the deepest. This causes the age gradient from the cluster center to the outer region. This is also observed in the ONC.
Figure: Age distribution (left) and the averaged age of the simulated cluster. The black lines in the left panel indicate the clump merger time. The blue, green, and red bars indicate the observationally identified three age populations of the ONC (Beccari et al. 2017). (Fujii et al. 2022, SIRIUS IV., MNRAS, 514, 2, pp.2513-2526, Fig. 5)