The purpose of my research is to understand the properties and formation processes of exoplanets in order to deepen our understanding of the universe, the solar system, Earth, and ourselves.
Many exoplanets have been discovered so far, but it has become clear that their properties differ from those of the planets in our solar system.
What kinds of planets exist?
How common are Earth-like planets?
How were these planets formed?
To answer these questions, I am advancing my research through the observation of exoplanets.
Searching for Exoplanets through Near-Infrared Observations
I am conducting research related to the infrared astrometry satellite JASMINE and the exoplanet survey IRD-SSP using the Subaru Telescope’s near-infrared high-resolution spectrograph IRD, which are projects aimed at detecting planets around M dwarfs (stars with a mass less than half that of the Sun).
JASMINE (Japan Astrometry Satellite Mission for INfrared Exploration)
JASMINE is a Japanese satellite project focused on astrometry of the Galactic center and precise photometric exoplanet exploration. As a member of the JASMINE project, I am involved in data analysis and detector development.
Link to JASMINE website
Assessing the Impact of Earth’s Atmospheric Absorption Lines on Near-Infrared Radial Velocity Measurements
The radial velocity method detects the Doppler shift of absorption lines in the spectrum caused by the wobble of a host star due to the orbiting planet. Observations of cool late M dwarfs (around 3000K), which are the main targets of IRD-SSP (InfraRed Doppler, Subaru Strategic Program), are expected to lead to the discovery of more low-mass planets. However, distinguishing the radial velocity variations caused by such planets from the instabilities that arise during measurements can be challenging. Among the various factors contributing to this instability, the contamination of spectra by Earth’s atmospheric absorption lines is considered to have a significant impact. I estimated the extent of this impact using simulated spectra and explored the potential for achieving higher stability.
Unveiling Properties of Brown Dwarfs Using High-Resolution Spectra
Brown dwarfs are objects with a mass between that of stars and planets, ranging from about 0.013 to 0.08 times the mass of the Sun. Since they do not have enough mass to sustain hydrogen fusion at their cores, they gradually cool down as they age.
I am observing brown dwarfs using IRD and REACH on the Subaru Telescope, working on characterizing their atmospheres and searching for planets orbiting around these brown dwarfs.
(Image credit: NASA, ESA, J. Olmsted (STScI))
Atmospheric Characterizations of Brown Dwarfs
REACH (Rigorous Exoplanetary Atmosphere Characterization with High Dispersion Coronography) is an instrument that combines the near-infrared high-dispersion spectrograph IRD with the extreme adaptive optics system SCExAO. This allows to get high-resolution spectra of faint targets located close to bright stars. By applying spectral models to this observational data, it is possible to estimate the temperature-pressure structure of the atmosphere, as well as the abundances of various molecules, thereby revealing the atmospheric characteristics of brown dwarfs.
Exoplanet Survey around Brown Dwarfs with Radial Velocity Method
Recent observations have discovered brown dwarfs that are fainter than predicted by existing theoretical evolutionary models. These brown dwarfs may host unknown planetary-mass objects that could cause their mass to be overestimated. However, the high-precision radial velocity measurements needed to verify this hypothesis have been challenging due to the faintness of brown dwarfs, resulting in very few such measurements to date. By using high-resolution spectra obtained with IRD and REACH on the Subaru Telescope, I aim to explore planets around brown dwarfs using the radial velocity method, with the goal of validating evolutionary models and uncovering the processes governing their evolution. This approach is expected to enable the estimation of mass and age from the brightness of brown dwarfs and planets, thereby advancing our understanding of the atmospheric structures of planetary-mass objects.
Characterizing Young Stars Showing Irregular Dimming
When and where planets form within protoplanetary disks around young stars remains a critical unsolved question. Observations have revealed gaps and spiral structures within these disks, which are thought to be caused by forming planets. However, the structures and gas flows in the regions close to the star are still not well understood.
One promising clue for probing these regions is the recent discovery of young stars known as “dippers,” which exhibit quasi-periodic dips in their light curves. This dimming is believed to be caused by material near the central star within the protoplanetary disk that obscures the star’s light as it orbits.
(Image credit: ESO/L. Calçada)
Characterization of dippers with spectroscopic observations
Using data from the Transiting Exoplanet Survey Satellite (TESS), which conducts an all-sky photometric survey, we identified four new dippers and performed spectroscopic observations using the High Dispersion Spectrograph (HDS) on the Subaru Telescope, and other instruments, to investigate their characteristics. By analyzing the emission line features in the spectra and their time variations, we determined that the dimming in these dippers is caused by dust within disk winds and accretion flows aligned with tilted magnetic axes. Additionally, one of the four objects was found to be a dipper in a binary system, which is a relatively rare occurrence. Future observations are expected to provide new insights into planet formation around binary systems.