Publications (73)
Unexpected Near-Resonant and Metastable States of Young Multi-Planet Systems
Zhecheng Hu, Fei Dai, Wei Zhu +4
Recent observations suggest that the incidence of near-resonant planets declines as planetary systems age, making young planetary systems key signposts of early dynamical evolution…
Possible Outcomes of Coplanar High-eccentricity Migration: Hot Jupiters, Close-in Super-Earths, and Counter-orbiting Planets
Yuxin Xue, Kento Masuda, Yasushi Suto
We investigate the formation of close-in planets in near-coplanar eccentric hierarchical triple systems via the secular interaction between an inner planet and an outer perturber (…
Dippers from TESS Full-frame Images. II. Spectroscopic Characterization of Four Young Dippers
Yui Kasagi, Takayuki Kotani, Hajime Kawahara +9
Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasi-periodic dim…
A Decade of Transit-Timing Measurements Confirm Resonance in the K2-19 System
Paige M. Entrican, Erik A. Petigura, Antoine C. Petit +2
K2-19 is a star, slightly smaller than the Sun, that hosts three transiting planets. Two of these, K2-19 b and c, are between the size of Neptune and Saturn and have orbital period…
The misaligned orbit of the Earth-sized planet Kepler-408b
Shoya Kamiaka, Othman Benomar, Yasushi Suto +3
Kepler-408 is one of the 33 planet-hosting {\it Kepler} stars for which asteroseismology has been used to investigate the orientation of the stellar rotation axis relative to the p…
Transiting Planet Candidates Beyond the Snow Line Detected by Visual Inspection of 7557 Kepler Objects of Interest
Sho Uehara, Hajime Kawahara, Kento Masuda +2
We visually inspected the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) possibly due to long-period giant planets. We identified…
Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the Formation of Warm Jupiters
Kento Masuda
I report the discovery of non-transiting close companions to two transiting warm Jupiters (WJs), Kepler-448/KOI-12b (orbital period , radius $R_{\rm p}=1.23^…
On the Inference of a Star's Inclination Angle from its Rotation Velocity and Projected Rotation Velocity
Kento Masuda, Joshua N. Winn
It is possible to learn about the orientation of a star's rotation axis by combining measurements of the star's rotation velocity () and its projection onto our line of sight ($…
Measurement of Methane Line Broadening in Hot Hydrogen/Helium Atmospheres at = 1.60-1.63 μm for Substellar Object Spectroscopy
Ko Hosokawa, Takayuki Kotani, Hajime Kawahara +4
Recent high-dispersion spectroscopy from ground-based telescopes and high-precision spectroscopy from space observatories have enabled atmospheric observations of substellar object…
Auto-Differentiable Spectrum Model for High-Dispersion Characterization of Exoplanets and Brown Dwarfs
Hajime Kawahara, Yui Kawashima, Kento Masuda +3
We present an auto-differentiable spectral modeling of exoplanets and brown dwarfs. This model enables a fully Bayesian inference of the high--dispersion data to fit the ab initio…
Global Analysis of KOI-977: Spectroscopy, Asteroseismology, and Phase-curve Analysis
Teruyuki Hirano, Kento Masuda, Bun'ei Sato +5
We present a global analysis of KOI-977, one of the planet host candidates detected by {\it Kepler}. Kepler Input Catalog (KIC) reports that KOI-977 is a red giant, for which few c…
On the Reparameterization Between Cartesian Position-Velocity Vectors and Orbital Elements in the Kepler Problem
Kento Masuda, Kansuke Nunota
Reparameterization from the standard set of orbital elements to Cartesian position-velocity vectors can be computationally advantageous for orbit inference problems, particularly w…
On the Evolution of Rotational Modulation Amplitude in Solar-mass Main-sequence Stars
Kento Masuda
We investigate the relation between rotation periods and photometric modulation amplitudes for Sun-like main-sequence stars observ…
Dippers from the TESS Full-Frame Images I: The Results of the first 1 year data and Discovery of A Runaway dipper
Tomoyuki Tajiri, Hajime Kawahara, Masataka Aizawa +9
We present a comprehensive catalog of the dippers---young stellar objects that exhibit episodic dimming---derived from the one year's worth of data of Transiting Exoplanet Survey S…
A Hubble constant measurement from superluminal motion of the jet in GW170817
Kenta Hotokezaka, Ehud Nakar, Ore Gottlieb +5
The Hubble constant () measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decad…
PyIRD: A Python-Based Data Reduction Pipeline for Subaru/IRD and REACH
Yui Kasagi, Hajime Kawahara, Ziying Gu +4
PyIRD is a Python-based pipeline for reducing spectroscopic data obtained with IRD (InfraRed Doppler; Kotani et al. (2018)) and REACH (Rigorous Exoplanetary Atmosphere Characteriza…
Tidal evolution of the spin-orbit angle in exoplanetary systems
Yuxin Xue, Yasushi Suto, Atsushi Taruya +3
The angle between the stellar spin and the planetary orbit axes (spin-orbit angle) is supposed to carry valuable information on the initial condition of the planet formation and th…
Transiting Planets near the Snow Line from Kepler. I. Catalog
Hajime Kawahara, Kento Masuda
We present a comprehensive catalog of cool (period ) transiting planet candidates in the four-year light curves from the prime \kepler\ mission. Most of th…
Very Low-Density Planets around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event
Kento Masuda
We present an analysis of the transit timing variations (TTVs) in the multi-transiting planetary system around Kepler-51 (KOI-620). This system consists of two confirmed transiting…
Inferring the Rotation Period Distribution of Stars from their Projected Rotation Velocities and Radii: Application to late-F/early-G Kepler Stars
Kento Masuda, Erik A. Petigura, Oliver J. Hall
While stellar rotation periods may be measured from broadband photometry, the photometric modulation becomes harder to detect for slower rotators, which could bias…
Bayesian Dynamic Mapping of an Exo-Earth from Photometric Variability
Hajime Kawahara, Kento Masuda
Photometric variability of a directly imaged exo-Earth conveys spatial information on its surface and can be used to retrieve a two-dimensional geography and axial tilt of the plan…
A spectroscopic thermometer: individual vibrational band spectroscopy with the example of OH in the atmosphere of WASP-33b
Sam O. M. Wright, Stevanus K. Nugroho, Matteo Brogi +26
Individual vibrational band spectroscopy presents an opportunity to examine exoplanet atmospheres in detail by distinguishing where the vibrational state populations of molecules d…
Detecting Planetary Oblateness in the Era of JWST: A Case Study of Kepler-167e
Quanyi Liu, Wei Zhu, Yifan Zhou +5
Planets may be rotationally flattened, and their oblateness thus provide useful information on their formation and evolution. Here we develop a new algorithm that can compute the t…
JASMINE: Near-Infrared Astrometry and Time Series Photometry Science
Daisuke Kawata, Hajime Kawahara, Naoteru Gouda +86
Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aero…
Ages of Stars and Planets in the Kepler Field Younger Than Four Billion Years
Luke G. Bouma, Lynne A. Hillenbrand, Andrew W. Howard +3
Recent analyses of FGK stars in open clusters have helped clarify the precision with which a star's rotation rate and lithium content can be used as empirical indicators for its ag…
Bayesian Doppler Imaging: Simultaneous Inference of Surface Maps and Geometric Parameters
Yamato Ureshino, Hajime Kawahara, Hibiki Yama +1
We present a fully Bayesian, pixel-based Doppler imaging framework that enables the simultaneous inference of surface brightness maps and geometric parameters, including the inclin…
Precise Radial Velocity Measurements for Kepler Giants Hosting Planetary Candidates: Kepler-91 and KOI-1894
Bun'ei Sato, Teruyuki Hirano, Masashi Omiya +6
We present results of radial-velocity follow-up observations for the two Kepler evolved stars Kepler-91 (KOI-2133) and KOI-1894, which had been announced as candidates to host tran…
Hidden worlds: a non-transiting candidate planet in the Neptunian desert around the solar-type pulsator KIC 9139163
Sylvain N. Breton, Achrène Dyrek, Carlos Allende Prieto +16
Close-in substellar companions experience strong tidal and magnetic interactions with their host stars and are therefore subject to fast orbital evolution. The solar-type pulsator…
Detectability of Rotational Modulation in Kepler Sun-like Stars as a Function of Age
Kento Masuda
We examine how the fraction of stars for which rotational modulation has been detected in Kepler light curves depends on the stellar mass and age . Our sampl…
ExoJAX Retrievals of VLT/CRIRES Spectra of Luhman 16AB: C/O Ratios and Systematic Uncertainties
Hibiki Yama, Kento Masuda, Yui Kawashima +1
We present atmospheric retrievals of the benchmark brown dwarf binary Luhman 16AB using high-resolution VLT/CRIRES spectra and the differentiable framework ExoJAX. We derive elemen…
Probing 2D Asymmetries of an Exoplanet Atmosphere from Chromatic Transit Variation
Shotaro Tada, Hajime Kawahara, Yui Kawashima +2
We propose a new method for investigating atmospheric inhomogeneities in exoplanets through transmission spectroscopy. Our approach links chromatic variations in conventional trans…
Photochemical CS Gas Detected on a 20-Myr-old Exoplanet
Fei Dai, Erik Petigura, John Livingston +6
Probing the atmospheres of young exoplanets offers a powerful window into how planetary systems evolve and the physical and chemical processes that drive those early evolutions. We…
Absolute Dimensions of a Flat Hierarchical Triple System KIC 6543674 from the Kepler Photometry
Kento Masuda, Sho Uehara, Hajime Kawahara
Many of the Kepler close binaries are suggested to constitute hierarchical triple systems through their eclipse timing variations (ETVs). Eclipses by the third body in those system…
Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects
Ataru Tanikawa, Akito Tajitsu, Satoshi Honda +5
We have discovered a compact hierarchical triple main-sequence star system, which is cataloged as Gaia DR3 1010268155897156864 or TIC 21502513. Hereafter, we call it ``G1010''. G10…
Differentiable Modeling of Planet and Substellar Atmosphere: High-Resolution Emission, Transmission, and Reflection Spectroscopy with ExoJAX2
Hajime Kawahara, Yui Kawashima, Shotaro Tada +11
Modeling based on differentiable programming holds great promise for astronomy, enabling advanced techniques such as gradient-based posterior sampling and optimization. This paradi…
Larger mutual inclinations for the shortest-period planets
Fei Dai, Kento Masuda, Joshua N. Winn
The {\it Kepler} mission revealed a population of compact multiple-planet systems with orbital periods shorter than a year, and occasionally even shorter than a day. By analyzing a…
The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-Puff, Kepler-51d
Jessica E. Libby-Roberts, Aaron Bello-Arufe, Zachory K. Berta-Thompson +14
Kepler-51 is a 500 Myr G dwarf hosting three "super-puffs" and one low-mass non-transiting planet. Kepler-51d, the coolest (T_eq ~ 350 K) transiting planet in this system, is also…
Two unseen massive white dwarf candidates in close binaries
Yuta Shiraishi, Kenta Hotokezaka, Kento Masuda +10
We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TES…
Self-lensing Discovery of a White Dwarf in an Unusually Wide Orbit Around a Sun-like Star
Kento Masuda, Hajime Kawahara, David W. Latham +4
We report the discovery of the fifth self-lensing binary in which a low-mass white dwarf (WD) gravitationally magnifies its 15th magnitude G-star companion, KIC 8145411, during ecl…
Evidence that the Occurrence Rate of Hot Jupiters around Sun-like Stars Decreases with Stellar Age
Shota Miyazaki, Kento Masuda
We investigate how the occurrence rate of giant planets (minimum mass ) around Sun-like stars depends on the age, mass, and metallicity of their host stars.…
The Featureless Transmission Spectra of Two Super-Puff Planets
Jessica E. Libby-Roberts, Zachory K. Berta-Thompson, Jean-Michel Desert +9
The Kepler mission revealed a class of planets known as ''super-puffs,'' with masses only a few times larger than Earth's but radii larger than Neptune, giving them very low mean d…
Prospects of Finding Detached Black Hole-Star Binaries with TESS
Kento Masuda, Kenta Hotokezaka
We discuss prospects of identifying and characterizing black hole (BH) companions to normal stars on tight but detached orbits, using photometric data from the Transiting Exoplanet…
Revisiting a gravity-darkened and precessing planetary system PTFO 8-8695: spin-orbit non-synchronous case
Shoya Kamiaka, Kento Masuda, Yuxin Xue +7
We reanalyse the time-variable lightcurves of the transiting planetary system PTFO 8-8695, in which a planet of 3 to 4 Jupiter mass orbits around a rapidly rotating pre-main-sequen…
It takes two planets in resonance to tango around K2-146
Kristine W. F. Lam, Judith Korth, Kento Masuda +51
K2-146 is a cool, 0.358 M_sun dwarf that was found to host a mini-Neptune with a 2.67-days period. The planet exhibited strong transit timing variations (TTVs) of greater than 30 m…
No longer impossible: the self-lensing binary KIC 8145411 is a triple
Natsuko Yamaguchi, Kareem El-Badry, David R. Ciardi +5
Five self-lensing binaries (SLBs) have been discovered with data from the \textit{Kepler} mission. One of these systems is KIC 8145411, which was reported to host an extremely low…
TOI-2015b: A Warm Neptune with Transit Timing Variations Orbiting an Active mid M Dwarf
Sinclaire E. Jones, Gudmundur Stefansson, Kento Masuda +29
We report the discovery of a close-in () warm Neptune with clear transit timing variations (TTVs) orbiting the nearby ($d=47.3\:\mathrm{pc}…
Atmospheric retrieval of Subaru/IRD high-resolution spectrum of the archetype T-type brown dwarf Gl 229 B
Yui Kawashima, Hajime Kawahara, Yui Kasagi +14
Brown dwarfs provide a unique opportunity to study atmospheres and their physical and chemical processes with high precision, especially in temperature ranges relevant to exoplanet…
A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations
Kento Masuda, Jessica E. Libby-Roberts, John H. Livingston +49
Kepler-51 is a -old Sun-like star hosting three transiting planets with radii - and orbital periods -$130\,\mathrm{day…
Mutual Orbital Inclinations Between Cold Jupiters and Inner Super-Earths
Kento Masuda, Joshua N. Winn, Hajime Kawahara
Previous analyses of Doppler and Kepler data have found that Sun-like stars hosting "cold Jupiters" (giant planets with ) almost always host "inner super-E…
TOI-1136 is a Young, Coplanar, Aligned Planetary System in a Pristine Resonant Chain
Fei Dai, Kento Masuda, Corey Beard +60
Convergent disk migration has long been suspected to be responsible for forming planetary systems with a chain of mean-motion resonances (MMR). Dynamical evolution over time could…
Discovery of three self-lensing binaries from Kepler
Hajime Kawahara, Kento Masuda, Morgan MacLeod +3
We report the discovery of three edge-on binaries with white dwarf companions that gravitationally magnify (instead of eclipsing) the light of their stellar primaries, as revealed…
Towards Detection of Exoplanetary Rings Via Transit Photometry: Methodology and a Possible Candidate
Masataka Aizawa, Sho Uehara, Kento Masuda +2
Detection of a planetary ring of exoplanets remains as one of the most attractive but challenging goals in the field. We present a methodology of a systematic search for exoplaneta…
Weighing Single-lined Spectroscopic Binaries Using Tidal Effects on Radial Velocities: The Case of V723 Monocerotis
Mio Tomoyoshi, Kento Masuda, Teruyuki Hirano +6
In single-lined spectroscopic binaries (SB1s) where flux variations due to tidal deformation of the primary star (ellipsoidal variations, EVs) are detected, the binary mass can be…
An extremely low-density exoplanet spins slow
Quanyi Liu, Wei Zhu, Kento Masuda +3
We present constraints on the shape of Kepler-51d, which is a super-puff with a mass and a radius , based on detailed modeling of the transit lig…
Biases in Planet Occurrence Caused by Unresolved Binaries in Transit Surveys
L. G. Bouma, Kento Masuda, J. N. Winn
Wide-field surveys for transiting planets, such as the NASA Kepler and TESS missions, are usually conducted without knowing which stars have binary companions. Unresolved and unrec…
Spin-Orbit Angles of Kepler-13Ab and HAT-P-7b from Gravity-Darkened Transit Light Curves
Kento Masuda
Analysis of the transit light curve deformed by the stellar gravity darkening allows us to photometrically measure both components of the spin-orbit angle , its sky projection…
Revisiting the Architecture of the KOI-89 System
Kento Masuda, Daniel Tamayo
While high stellar obliquities observed in exoplanetary systems may be attributed to processes that tilt the planetary orbits, it is also possible that they reflect misalignments b…
Reassessment of the Null Result of the HST Search for Planets in 47 Tucanae
Kento Masuda, Joshua N. Winn
We revisit the null result of the Hubble Space Telescope search for transiting planets in the globular cluster 47 Tucanae, in the light of improved knowledge of planet occurrence f…
Tidal Effects on the Radial Velocities of V723 Mon: Additional Evidence for a Dark Companion
Kento Masuda, Teruyuki Hirano
Jayasinghe et al. (2021) identified a dark companion on a nearly edge-on orbit around the red giant star V723 Monoceros as a black h…
First Detection of Hydroxyl Radical Emission from an Exoplanet Atmosphere: High-dispersion Characterization of WASP-33b using Subaru/IRD
Stevanus K. Nugroho, Hajime Kawahara, Neale P. Gibson +23
We report the first detection of a hydroxyl radical (OH) emission signature in the planetary atmosphere outside the solar system, in this case, in the day-side of WASP-33b. We anal…
TOI-4495: A Pair of Aligned, Near-Resonant Sub-Neptunes that Likely Experienced Overstable Migration
Mu-Tian Wang, Fei Dai, Hui-Gen Liu +24
We report the discovery of a sub-Neptune and a Neptune-like planet (, ) orbiting the F-type star TOI-449…
Unveiling the Atmosphere of HR 7672 B from the Near-Infrared High-Resolution Spectrum Using REACH/Subaru
Yui Kasagi, Yui Kawashima, Hajime Kawahara +12
Characterizing the atmospheres of exoplanets and brown dwarfs is crucial for understanding their atmospheric physics and chemistry, searching for biosignatures, and investigating t…
An Earth-sized Planet on the Verge of Tidal Disruption
Fei Dai, Andrew W. Howard, Samuel Halverson +95
TOI-6255~b (GJ 4256) is an Earth-sized planet (1.079 ) with an orbital period of only 5.7 hours. With the newly commissioned Keck Planet Finder (KPF) and CARMEN…
Planet-Planet Eclipse and the Rossiter-McLaughlin Effect of a Multiple Transiting System: Joint Analysis of the Subaru Spectroscopy and the Kepler Photometry
Teruyuki Hirano, Norio Narita, Bun'ei Sato +6
We report a joint analysis of the Rossiter-McLaughlin (RM) effect with Subaru and the Kepler photometry for Kepler Object of Interest (KOI) 94 system. The system comprises four tra…
A young progenitor for the most common planetary systems in the Galaxy
John H. Livingston, Erik A. Petigura, Trevor J. David +39
The Galaxy's most common known planetary systems have several Earth-to-Neptune-size planets in compact orbits. At small orbital separations, larger planets are less common than the…
High-Resolution Spectroscopic Detection of TiO and Stratosphere in the Day-side of WASP-33b
Stevanus K. Nugroho, Hajime Kawahara, Kento Masuda +3
We report high-resolution spectroscopic detection of TiO molecular signature in the day-side spectra of WASP-33 b, the second hottest known hot Jupiter. We used High-Dispersion Spe…
Systematic Search for Rings around Kepler Planet Candidates: Constraints on Ring Size and Occurrence Rate
Masataka Aizawa, Kento Masuda, Hajime Kawahara +1
We perform a systematic search for rings around 168 Kepler planet candidates with sufficient signal-to-noise ratios that are selected from all the short-cadence data. We fit ringed…
Hot Stars With Kepler Planets Have High Obliquities
Emma M. Louden, Joshua N. Winn, Erik A. Petigura +5
It has been known for a decade that hot stars with hot Jupiters tend to have high obliquities. Less is known about the degree of spin-orbit alignment for hot stars with other kinds…
Characterization of the KOI-94 System with Transit Timing Variation Analysis: Implication for the Planet-Planet Eclipse
Kento Masuda, Teruyuki Hirano, Atsushi Taruya +2
The KOI-94 system is a closely-packed, multi-transiting planetary system discovered by the Kepler space telescope. It is known as the first system that exhibited a rare event calle…
Four New Self-lensing Binaries from Kepler: Radial Velocity Characterization and Astrophysical Implications
Kento Masuda, Hajime Kawahara, David W. Latham +5
In Kawahara et al. (2018) and Masuda et al. (2019), we reported the discovery of four self-lensing binaries consisting of F/G-type stars and (most likely) white dwarfs whose masses…
Homogeneous Analysis of Hot Earths: Masses, Sizes, and Compositions
Fei Dai, Kento Masuda, Joshua N. Winn +1
Terrestrial planets have been found orbiting Sun-like stars with extremely short periods --- some as short as 4 hours. These "ultra-short-period planets" or "hot Earths" are so str…
Transiting planets as a precision clock to constrain the time variation of the gravitational constant
Kento Masuda, Yasushi Suto
Analysis of transit times in exoplanetary systems accurately provides an instantaneous orbital period, , of their member planets. A long-term monitoring of those transiting p…
Kepler-730b is probably a hot Jupiter with a small companion
Wei Zhu, Fei Dai, Kento Masuda
Here we draw attention to a candidate system with one hot Jupiter and one small, nearby companion, revealed by the recent Kepler data release (DR25). The hot Jupiter, Kepler-730b,…