RESEARCH

Stars have a wide variety of internal structures depending on their mass and age. These internal structures are reflected in observable properties at the stellar surface. I investigate methods for inferring stellar properties from surface oscillations and abundances.

Surface Oscillation (Asteroseismology)

Just as seismology on Earth probes the Earth's interior, observations of stellar surface oscillations allow us to infer the internal structure of stars. Asteroseismology has therefore become a powerful tool for probing stellar interiors.

• 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, in progress, PI).

Surface Abundance (Dredge-up)

When convection develops, surface abundance has been altered by mixing from the signature of nuclear reactions in the stellar interior through a process known as dredge-up. By comparing the resulting surface abundance changes predicted by stellar evolution models with observations, we can constrain the physical properties of stars.

• I am investigating dredge-up in red supergiants, the progenitors of core-collapse supernovae (in progress, Co-I).

Rotation and magnetic activity are key phenomena in the Sun and FGKM-type stars (Solar-type), and are also important because they can influence planetary environments. By comparing these phenomena between the Sun and solar-type stars, I investigate where the Sun lies in the evolution of stellar rotation and magnetic activity.

Stellar rotational evolution

Solar-type stars are known to spin down over time due to angular momentum loss through magnetic fields, a process known as magnetic braking. Understanding the rotational evolution of cool stars is important because it provides an indirect constraint on the stellar dynamo mechanism.

• I developed a simple model for the rotational evolution of solar-type stars incorporating differential rotation and investigated its properties (Tokuno et al. 2023).

Solar and stellar magnetic activity

Magnetic activity, such as starspots and flares, is a defining phenomenon of cool stars. Some solar-type stars are known to exhibit magnetic activity far more energetic than that observed on the Sun. Whether such extreme activity could also occur on the present-day Sun is therefore an important question.

• By investigating flare occurrence at different stages of spot evolution on the Sun and solar-type stars, I showed that neither exhibits a clear difference in flare frequency between the spot-growth and decay phases (Tokuno et al. 2025).

Some planets outside the Solar System (exoplanets) are known to be giant gaseous planets orbiting very close to their host stars, known as hot Jupiters. I investigate how these planets form and evolve.

Orbital evolution of hot Jupiters

Tidal interactions between stars and planets are thought to affect the long-term evolution of stellar rotation and planetary orbits. However, how efficiently tides operate and how strongly they alter these systems remain uncertain, and both theoretical and observational studies are actively being pursued.

• I proposed a new method to place an upper limit on the efficiency of tidal interactions in systems consisting of a low-mass star and a hot Jupiter (Tokuno et al. 2024).

• I suggested that the unusual rotational structure observed in the red giant Kepler-56 may be a remnant of hot-Jupiter engulfment (Tokuno 2025).

Internal structure of gas giant planets

In the Solar System, planetary interiors can be studied through spacecraft observations. For exoplanets, however, such direct measurements are generally impossible, making their internal structures much more difficult to constrain. Nevertheless, attempts are being made to infer their interiors from observables such as orbital evolution and luminosities measured by direct imaging.

• I am participating in a project that uses long-term transit observations to detect orbital changes and constrain planetary internal structure (in progress, Co-I).

• I am participating in a project investigating how a diluted core affects the thermal evolution of giant planets (in progress, Co-I).

Supernova explosions at the final stages of massive-star evolution produce extremely dense objects known as compact objects, such as neutron stars and black holes. I investigate both the physical properties of compact objects and how they can be detected.

Light curves of failed supernovae

Failed supernovae are associated with black-hole formation and are expected to produce characteristic light curves, including fading at optical wavelengths and brightening in the infrared. Understanding what physical information can be extracted from these light curves is important for identifying and characterizing compact objects.

• I am participating in a project that models dust composition and grain-size distributions for comparison with infrared spectra (in progress, Co-I).

Compact binaries

Binary systems containing compact objects (compact binaries) provide important probes of the origin of compact objects and the evolution of binary systems. Detecting more compact binaries by measuring the motions of stars is therefore important.

• I am participating in a project searching for compact binaries using techniques such as astrometry and radial-velocity measurements (Shiraishi et al. 2026).

Magnetic Fields in Neutron Stars

Neutron stars are extremely dense and strongly magnetized objects. Their properties are of particular interest because they provide a laboratory for physics under extreme conditions, and theoretical studies can help constrain their possible behavior.

• I am participating in a project investigating the stability of internal magnetic fields in neutron stars (in progress, Co-I).

Minerals sometimes preserve signatures of processes that occurred in stars. I also investigate how information recorded in mineral samples can be used to understand the physics of the Sun and other stars.

Presolar grains

Supernovae and asymptotic giant branch (AGB) stars eject large amounts of gas into their surroundings, some of which cools and condenses into solid particles (dust). Some of these particles were incorporated into the Solar System during its formation and are now found in meteorites and other materials as presolar grains with distinctive elemental and isotopic compositions. By combining stellar evolution and dust-formation models, we can investigate the sizes and compositions of these grains.

• I am participating in a project investigating the origin of the sizes of presolar grains formed around supernovae (in progress, Co-I).

Noble-gas implantation

Noble gases are sometimes detected in minerals even when they are unlikely to have been incorporated during mineral formation. Some of these noble gases are thought to have been implanted when high-energy particles emitted by the Sun or other stars collided with the minerals. By physically modeling the implantation process, we can use noble gases preserved in minerals to infer the properties of particles emitted by the Sun and stars.

• I am leading a project interpreting noble gases in asteroid samples (in progress, PI).