RESEARCH

Solar/Stellar physics : Surface Phenomena

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. However, it is known that there are still many properties of these phenomena that remain to be elucidated. I am conducting research into these issues by analyzing observational data and theoretically interpreting the observational results.

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. However, there are many phenomena that cannot be interpreted by the recent stellar physics.

We have theoretically investigated the newly discovered phenomenon, resonance between the inner core and the outer layer in the oscillations of γ Doradus stars, a fast rotating 1.2-1.8 solar mass star (Tokuno & Takata 2022).

Spin Evolution of Low-mass Stars

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, Suzuki & Shoda 2023).

Magnetic Activity of the Sun / Solar-type Stars

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, Namekata, Maehara, Toriumi 2025).

Planetary science : 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 these hot Jupiters are still largely unknown. I am conducting research into these by analyzing observational data and theoretically interpreting the observational results.

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 orbiting gas planets by applying the mathematical field of dynamical systems (Tokuno, Fukui & Suzuki 2024).
I have investigated a formation scenario of the spin rotational structure of the red giant Kepler-56 (a rapidly rotating envelope and a internal spin misalignment) as a consequence of tides with its planets. I show that the engulfment of a hot Jupiter is plausible to accelerate Kepler-56 (Tokuno 2025).

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