The Rigakubu News

Disclaimer: machine translated by DeepL which may contain errors.

~ Message from a graduate student~.
Learn physics with an easygoing approach


Ayano Komaki
Department of Physics, 1st Year Doctoral Student
Tokyo, Japan
High School
Rikkyo Jogakuin High School
Department of Physics, Faculty of Science, The University of Tokyo


I entered the Department of Physics at the Faculty of Science with questions about how the universe was formed and how star and planetary systems evolved. Every year at the May Festival, the Department of Physics holds an academic exhibition called the Physics Lab. I joined the Space Group in my fourth year. That year, the first black hole observation by the Event Horizon Telescope (EHT) was just around the corner, so the theme of the Space Group's exhibit was decided to be physical phenomena around black holes. Around a heavy object such as a black hole, the direction of light traveling from a distant object is bent by the distorted gravitational field. As a result, a gravitational lensing effect occurs, in which distant objects are observed in a distorted form. In order to explain this effect in an easy-to-understand manner, we decided to develop a gravitational lensing simulation. The Department of Physics required a computer class, so the students learned the basics of programming, but they had no experience with simulations. They investigated the gravitational lensing effect on their own and completed the simulation through a process of trial and error, including repeated test calculations. Through the experience of reproducing actual observed phenomena using calculations they had made on their own, the students learned the importance of understanding through actual hands-on experience. They were also impressed by the convenience of being able to easily solve the equations on their own computers as long as they knew the equations they wanted to solve. From this experience, I became interested in reproducing various phenomena through simulations.

Currently, I am studying the formation of stars and planetary systems. Planetary systems are formed in protoplanetary disks called protoplanetary disks. More than 5,000 exoplanets have been discovered so far, and it is clear that they have a wide variety of masses and periods. Since planets are formed inside disks from the materials (gas and dust) that make up the disk, understanding disk evolution provides clues to the planet formation process. The lifetime of protoplanetary disks is estimated to be several million years based on observations, but it is suggested to be shorter around more massive stars. Such diverse disk evolution is thought to lead to the diversity of planetary systems, but the details of the disk dissipation process that is responsible for such diversity have not been clarified. We focused on photoevaporation, which is considered to be a promising disk dissipation process, and performed the first simulation of photoevaporation using radiation hydrodynamics with the central star mass as a parameter. Photoevaporation is a phenomenon in which disk gas heated by high-energy radiation such as UV and X-ray from the central star overcomes the gravitational potential of the central star and flows out from the disk. It is found that as the central star mass increases, the stellar luminosity also increases, and the mass-loss rate increases due to the efficient heating of the gas. We also show that the dependence of the mass-loss rate due to photoevaporation on the central star mass explains the observation that the disk lifetime is shorter around massive stars. Recently, I have created my own disk evolution simulations to investigate when and how disks dissipate under various environmental conditions.  Understanding how the solar system was formed is an important theme in science. I would like to continue to work with my hands throughout my doctoral program to understand the formation of planetary systems, including the Earth. I hope that everyone will feel free to enjoy the physical phenomena that can be reproduced computationally, because they are all around us.

Snapshot of the disk photoevaporation simulation. The color map represents the gas density and the arrows represent the gas velocity. The disk cross-section shows the central star at the lower left. The disk gas is heated by the high-energy radiation from the central star, and gas is flowing out from the disk surface.



The Rigaku-bu News, March 2023

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