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Cluster Formation Revealed by Latest Simulations

Michiko Fujii , Associate Professor, Department of Astronomy

In astronomy, the formation and growth of celestial objects such as galaxies and stars over time is called "evolution. Since the time scale of astronomical evolution is much longer than the history of mankind, the images we see of celestial objects are merely still images of a single moment in time. Therefore, we observe celestial objects at various different evolutionary stages to understand their formation and evolution processes.

Numerical simulation is a method to reproduce celestial objects in a computer and directly "observe" their evolution. Simulations have developed along with the development of computers, and their accuracy has improved year by year. In the case of a star cluster (a collection of tens to millions of stars gravitationally bound to each other)
The simulation of a star cluster (a collection of tens to millions of stars gravitationally bound to each other) can now reproduce each individual star.

Star clusters are born in low-temperature interstellar gas (mainly composed of hydrogen) called molecular clouds. In particularly dense regions of the molecular cloud, the interstellar gas contracts by self-gravity to form stars. When stars are born in dense clusters, they are bound to each other gravitationally and form clusters. The motion of the stars in a cluster is governed by gravity, and it is known that the strong gravitational interactions that occur when stars come close to each other can eject them out of the cluster at high speed.

Once we are able to reproduce the motion of individual stars, the next question is how accurately we can reproduce such motion of stars. The closer the stars are to each other, the faster their motion becomes and the more difficult it becomes to simulate. In previous studies, simulations have been made easier by deliberately weakening the gravity. However, recently developed algorithms have made it possible to perform calculations without the gravity weakening approximation.

Figure: Top: A star cluster and the interstellar gas around it formed by numerical simulation. The blue-white dots represent stars and the red to green regions represent the interstellar gas. The low-temperature gas (molecular clouds) is colored red and the high-temperature gas (the edge of the ionized region) is colored green (image credit: Michiko Fujii, Takaaki Takeda, 4D Digital Universe Project, National Astronomical Observatory of Japan).

Bottom: The Orion Nebula taken by the Hubble Space Telescope (Image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)

Simulations that solve for the motion of the stars without any approximations reveal how new stars born in the molecular cloud near the center of the cluster are ejected out of the cluster by the gravitational interaction with the surrounding stars during the formation of the cluster. In the actual Orion Nebula, unlike in the simulation, we cannot see the movement of the stars, but we can measure their velocities. By comparing the latest stellar velocity data with the simulation results, it was found that gravitational scattering of stars also occurs in the Orion Nebula.

How does the motion of stars, which is more dynamic than previously thought, affect the formation process of star clusters? Simulations show that stars ejected from the cluster center ionize the surrounding gas, creating an ionized region like that seen in the Orion Nebula (see figure). In the future, it is necessary to confirm whether the same thing is happening in other clusters in the process of formation. In addition, we hope that the formation process of massive star clusters, which remain a mystery, can be elucidated by simulating the formation process of clusters of many more stars in the same way.

This study was supported by M. S. Fujii et al . , published in Monthly Notices of the Royal Astronomical Society 43, 514 (2022).

(Press release on June 8, 2022)

Published in the September 2022 issue of Faculty of Science News

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