2022MNRAS.514...43F

Massive stars born in star clusters terminate star cluster formation by ionizing the surrounding gas. This process is considered to be prevalent in young star clusters containing massive stars. The Orion Nebula is an excellent example associated with a forming star cluster including several massive stars (the Orion Nebula Cluster, ONC) and a 2-pc-sized H II region (ionized bubble) opening towards the observer; however, the other side is still covered with dense molecular gas. Recent astrometric data acquired by the Gaia satellite revealed the stellar kinematics in this region. By comparing these data with star cluster formation simulation results, we demonstrate that massive stars born in the ONC centre were ejected via three-body encounters. Further, orbit analysis indicates that θ² Ori A, the second massive star in this region, was ejected from the ONC centre towards the observer and is now returning to the cluster centre. Such ejected massive stars can form a hole in the dense molecular cloud and contribute to the formation of the 2-pc bubble. Our results demonstrate that the dynamics of massive stars are essential for the formation of star clusters and H II regions that are not always centred by massive stars.