RESEARCH
RESEARCH
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The Sun and Low-mass Stars
The Sun and Low-mass Stars
The Sun and stars are the most fundamental object in astronomy. By analyzing the rotation, convection, vibrations, and magnetic activity that occur on the surface, we can characterize what kind of star they are. I am participating in research projects exploring these phenomena (as PI/co-I).
Asteroseismology
Asteroseismology is the field to study stellar internal structures by using surface oscillations of stars. Thanks to space telescopes missions (e.g. Kepler), our understanding about star interiors is proceeded.
We have theoretically investigated the newly discovered phenomenon, resonance between the inner core and the outer layer in the oscillations of γ Dor stars (Tokuno & Takata 2022).
Spin Evolution
The angular momentum transport of magnetized stellar winds spins down cool stars, including the Sun (magnetic braking). The spin-down of cool stars exhibits the complicated behavior and its physical interpretation is actively discussed.
We have proposed a simple model to track the spin-down of solar-type stars that takes into account differential rotation and discussed its properties (Tokuno et al. 2023).
Magnetic Activity
Surface magnetic activity such as spots and flares is key to characterizing the Sun and understanding its impact on planetary environments. Some solar-type stars show much stronger activity, and comparing them with the Sun helps build a unified picture of stellar magnetism.
We have investigated the temporal evolution of flare frequencies via spot evolution in the Sun and solar-type stars. We show the similar patterns regardless of spot size or flare energy, which suggests common underlying processes (Tokuno et al. 2025).
Hot Jupiter
Hot Jupiter
Among the planets outside our solar system (exoplanets), there are giant planets that orbit near stars, called "hot Jupiters". The formation and evolution processes of hot Jupiters are still largely unknown. I am conducting research into these by analyzing observational data and theoretically interpreting the observational results (as PI/co-I).
Tidai Evolution of Exoplanetary System
Tidal interactions are thought to play an important role in the orbital evolution of planets. However, the specific mechanisms and efficiency of tidal interactions are still unclear.
We have proposed a novel method for determining the upper limit of the efficiency of tides in a system of low-mass stars and hot Jupiter (Tokuno et al. 2024).
We have pointed out that the engulfment of a hot Jupiter could be a process responsible for the peculiar rotation structure of the red giant Kepler-56 (Tokuno 2025).
Super Nova / Compact Object
Super Nova / Compact Object
The phenomenon known as gravitational collapse supernovae, which occur during the final evolutionary stages of massive stars, creates compact objects such as neutron stars and black holes. I am also participating in a project exploring the process of these supernovae and the formation and evolution of compact objects (as co-I).
Compact Binaries
Binaries containing compact objects (compact binaries) are considered key to understanding the origin of compact objects and verifying binary evolution. I participate in projects searching for compact binaries using astrometry, radial velocity methods, etc.
We have discovered white dwarf + low-mass star binary candidates using the TESS and Gaia satellites (Shiraishi et al. 2026).
Red Supergiant Evolution
Red supergiants are important precursors to gravitational collapse supernovae. I participate in a project investigating red supergiant evolution and comparing it with observations to explore the stage a star is in.
In progress !
Dust Formation from Supernova Explosions
Supernova explosions scatter gas into their surroundings; as this gas cools, it forms solid particles (dust). I am participating in a project to calculate the composition and size distribution of this dust, with the goal of comparing it to presolar particles and infrared spectra.
In progress !
Internal Magnetic Fields in Neutron Stars
Neutron stars are extremely dense, strong magnetic field objects, and their formation and evolution are attracting attention as a laboratory for physics. I am participating in a project to calculate phenomena caused by these high densities and strong magnetic fields.
In progress !
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