Massive stars moving around in star clusters - School of Science, the University of Tokyo
Jun 8, 2022

Massive stars moving around in star clusters


Overview of the press release

A research group at the University of Tokyo has performed a simulation of star cluster formation using a newly developed simulation code. The simulation revealed that some massive stars formed in the star cluster center were ejected to the outskirts of the cluster and ionized the molecular cloud there. These processes caused the formation of off-centered ionized bubbles seen in the Orion Nebula.
In the simulation, the motions of stars in molecular clouds were accurately solved. The importance of the motions of massive stars in the formation phase of star clusters was for the first time shown using the new simulation. The simulation will be upgraded to simulations for up to 100 times more massive star clusters to understand the formation process of massive stars and star clusters.

Figure: The structure of the Orion Nebula.

Credit: Original Image by NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team), the names of stars and explanations are added by Michiko Fujii.


Stars are born in molecular clouds, which are dense and cold molecular hydrogen gas. When many stars are formed in a small region, the stars are gravitationally bound to each other and develop a star cluster. Star clusters usually contain massive stars, which are more than eight times more massive than the sun and rarely form. Massive stars finally ionize the molecular gas and terminate the star cluster formation. The Orion Nebula Cluster, a massive star cluster located in the Orion Nebula, is a star cluster that is currently in the formation stage.
The Orion Nebula is associated with an ionized bubble, which is a high-temperature region filled with hydrogen ionized by massive stars. The most massive star in this region, theta 1 Orionis C, is located in the Orion Nebula Cluster, but this star does not center the ionized bubble in the Orion Nebula. In order to form such an off-centered bubble, ionizing photons emitted from the massive star in the cluster center must reach the outskirt of the cluster overwhelming the dense molecular gas in the cluster center. The simulations showed that 1. massive stars were formed in the central region of star clusters, 2. massive stars were scattered by their gravitational interactions, and 3. the scattered massive stars made a hole in the dense molecular gas in the central region and helped the formation of  off-centered ionized bubbles. The kinematic data of stars around the Orion Nebula Cluster suggests that some massive stars are ejected from the cluster center and that theta 2 Orionis A, which is the second massive star in this region, is now going back to the cluster center.
In this study, the research group performed a state-of-the-art simulation of star cluster formation, in which the motions of individual stars were accurately reproduced. With this code, the scattering process of massive stars during the formation phase of relatively massive star clusters was, for the first time, simulated. The simulations were performed using the supercomputer ATERUI II at the National Astronomical Observatory of Japan.

“Most previous studies have focused on the hydrodynamical aspects of the formation of star clusters. However, we are more interested in the dynamics of star clusters, such as the scattering of stars and the initial density of star clusters. The initial density of star clusters is an important factor in determining the following evolution of star clusters. A high density of star clusters results in the runaway merger of stars and the formation of massive black holes, which we have not fully understood the formation process. Therefore we aim to reproduce individual stars and their accurate motions in our simulations.” Michiko Fujii explained.

“ Our team took more than two years to develop the new code. Four main contributors worked together to combine their codes. We got very excited when we found a simulation that looked similar to the Orion Nebula, and we got excited again when we confirmed that the velocity distribution of stars in our simulations matches the observation.” she said.
“The simulation is part of the SImulations Resolving IndividUal Stars (SIRIUS) project. This project aims to perform novel simulations of star clusters and galaxies, resolving the dynamics of individual stars. This simulation is not the limit of our simulation code. If we use a larger number of CPUs, it can treat even more massive star clusters. We will challenge the first star-by-star star-cluster formation simulation of globular clusters, which are 100 times more massive than the star cluster we simulated in this study.”  Fujii continued.

【Movie】 Massive Stars Moving around in Star Clusters
(Credit: Michiko Fujii, Takaaki Takeda, The 4D2U project, National Astronomical Observatory of Japan))


Research Team

- Michiko Fujii (Associate Professor, Department of Astronomy, Graduate School of Science, The University of Tokyo)

- Kohei Hattori (Assistant Professor, Research Enhancement Strategy Office, National Astronomical Observatory of Japan / School of Statistical Thinking, The Institute of Statistical Mathematics)

- Long Wang (Associate Professor, School of Physics and Astronomy, Sun Yat-sen University / Former: JSPS International Research Fellow, Graduate School of Science, The University of Tokyo)

- Yutaka Hirai (JSPS Research Fellow,Department of Physics and Astronomy, University of Notre Dame/Astronomical Institute, Tohoku University)

   - Jun Kumamoto (Lecturer, Faculty of Business Administration, Chukyo Gakuin University / Former: Research fellow, Department of Astronomy, Graduate School of Science, The University of Tokyo )


 - Yoshito Shimajiri (Professor, Center for General Education, Kyushu Kyoritsu University / Former: Project Associate Professor, ALMA, National Astronomical Observatory of Japan)

- Takayuki Saitoh (Associate Professor, Department of Planetrogy, Graduate School of Science, Kobe University)


Publication details

Monthly Notices of the Royal Astronomical Society
SIRIUS Project. V. Formation of off-center ionized bubbles associated with Orion Nebula Cluster
Michiko S. Fujii*, Kohei Hattori, Long Wang, Yutaka Hirai, Jun Kumamoto, Yoshito Shimajiri, Takayuki R. Saitoh


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