SIRIUS Project. V. Formation of off-center ionized bubbles associated with Orion Nebula Cluster

TL;DR

This study uses Gaia data and simulations to show that ejected massive stars from the Orion Nebula Cluster create off-center ionized bubbles, revealing complex stellar dynamics in star cluster formation.

Contribution

It demonstrates that massive star ejections significantly influence the formation of off-center ionized bubbles in star clusters, a novel insight into cluster dynamics.

Findings

Ejected massive stars can form off-center ionized bubbles.

Gaia data supports the ejection scenario of massive stars.

Orbit analysis indicates returning stars to cluster center.

Abstract

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 size H{\sc ii} region (ionized bubble) opening toward 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 this data with star cluster formation simulation results, we demonstrate that massive stars born in the ONC center were ejected via three-body encounters. Further, orbit analysis indicates that $\theta^2$ Ori A, the second massive star in this region, was ejected from the ONC center toward the observer and is now returning to the cluster center. 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{\sc ii} regions that are not always centered by massive stars.