Paper

• Search for exoplanetary ring systems with TESS
  Tsubasa Umetani; Masataka Aizawa; Yuichiro Ezoe; Yoshitaka Ishisaki, Photometric surveys for exoplanetary ring systems have not yet confirmed any object with Saturn-sized rings. We systematically analyse 308 TESS planet candidates, mainly comprising giant short-period planets orbiting nearby bright stars. These targets are selected based on the optimistic detectability of rings, assuming a favourable ring orientation. We develop a pipeline with a two-step noise reduction and compare the fitting results of both ringless and ringed transit models to the resulting phase-folded light curves. Although we identify six systems where ringed models are statistically favoured, visual inspection of the signals suggests that none of them is conclusively attributed to the presence of rings. Assuming the ring orientation favourable for detection, we determine the $3\sigma$ upper limits on ring sizes for 125 objects. Using these ring size limits, we derive upper limits on the ring occurrence rate, such as rings with an outer radius larger than 1.8 times the planetary radii occurring at rates lower than 2%. However, these limits can be relaxed if tidal alignment between the spin and orbital axes holds. We explore an alternative detection method using transit depth variations by ring precession and estimate that 10 and 13 systems are likely detectable in TESS and Kepler data, respectively., OXFORD UNIV PRESS
  PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN, 13 Dec. 2024
• JASMINE image simulator for high-precision astrometry and photometry
  Takafumi Kamizuka; Hajime Kawahara; Ryou Ohsawa; Hirokazu Kataza; Daisuke Kawata; Yoshiyuki Yamada; Teruyuki Hirano; Kohei Miyakawa; Masataka Aizawa; Masashi Omiya; Taihei Yano; Ryouhei Kano; Takehiko Wada; Wolfgang Loeffler; Michael Biermann; Pau Ramos; Naoki Isobe; Fumihiko Usui; Kohei Hattori; Satoshi Yoshikawa; Takayuki Tatekawa; Hideyuki Izumiura; Akihiko Fukui; Makoto Miyoshi; Daisuke Tatsumi; Naoteru Gouda, SPIE
  Proceedings of SPIE, 23 Aug. 2024
• Revealing asymmetry on mid-plane of protoplanetary disc through modelling of axisymmetric emission: methodology
  Masataka Aizawa; Takayuki Muto; Munetake Momose, ABSTRACT
  
  This study proposes an analytical framework for deriving the surface brightness profile and geometry of a geometrically thin axisymmetric disc from interferometric observation of continuum emission. Such precise modelling facilitates the exploration of faint non-axisymmetric structures, such as spirals and circumplanetary discs. As a demonstration, we simulate interferometric observations of geometrically thin axisymmetric discs. The proposed method can reasonably recover the injected axisymmetric structures, whereas Gaussian fitting of the same data yielded larger errors in disc orientation estimation. To further test the applicability of the method, it was applied to the mock data for $m=1,2$ spirals and a point source, which are embedded in a bright axisymmetric structure. The injected non-axisymmetric structures were reasonably recovered except for the innermost parts, and the disc geometric parameter estimations were better than Gasussian fitting. The method was then applied to the real data of Elias 20 and AS 209, and it adequately subtracted the axisymmetric component, notably in Elias 20, where substantial residuals remained without our method. We also applied our method to continuum data of PDS 70 to demonstrate the effectiveness of the method. We successfully recovered emission from PDS 70 c consistently with previous studies, and also tentatively discovered new substructures. The current formulation can be applied to any data for disc continuum emission, and aids in the search of spirals and circumplanetary discs, whose detection is still limited., Oxford University Press (OUP)
  Monthly Notices of the Royal Astronomical Society, 25 Jun. 2024
• Frequencies of Warm Debris Disks Based on Point Source Catalogs of Spitzer, WISE, and Gaia
  Toshiyuki Mizuki; Munetake Momose; Masataka Aizawa; Hiroshi Kobayashi, Abstract
  
  More than a thousand warm debris disks have been detected as infrared excess at mid-infrared wavelengths, and their frequencies have been obtained for various spectral types of stars. However, the dependence of the frequencies on spectral type is still debated because the number of stars with significant and detectable infrared excess is limited. Herein, we present the largest systematic search for infrared excess using data from Gaia, WISE, and Spitzer. We identified 373, 485, and 255-reliable infrared excesses in the mid-infrared archival data at wavelengths of 12, 22, and 24 μm for WISE/W3, W4, and Spitzer/MIPS ch1, respectively. Although we confirmed that more massive stars tend to show higher frequencies of debris disks, these disk frequencies are relatively flat for both low- and intermediate-mass stars, with a jump at 7000 K for all three wavelengths. Assuming that bright, warm debris disks have lifetimes of a few to several hundred million years, the disk frequency can be understood as the ratio between the timescale and the upper limits of the sample ages. We also found that intermediate-mass stars with infrared excess tend to be bluer and fainter along the evolutionary track than those without, implying that massive stars hosting debris disks are relatively young, with an isochronal age of approximately 500 Myr. These tendencies are reasonably explained by a standard scenario in which debris disks are likely to be produced by collisions of planetesimals in the early stages of stellar evolution, such as the Late Heavy Bombardment., American Astronomical Society
  The Astronomical Journal, 20 May 2024
• JASMINE: Near-infrared astrometry and time-series photometry science
  Daisuke Kawata; Hajime Kawahara; Naoteru Gouda; Nathan J Secrest; Ryouhei Kano; Hirokazu Kataza; Naoki Isobe; Ryou Ohsawa; Fumihiko Usui; Yoshiyuki Yamada; Alister W Graham; Alex R Pettitt; Hideki Asada; Junichi Baba; Kenji Bekki; Bryan N Dorland; Michiko Fujii; Akihiko Fukui; Kohei Hattori; Teruyuki Hirano; Takafumi Kamizuka; Shingo Kashima; Norita Kawanaka; Yui Kawashima; Sergei A Klioner; Takanori Kodama; Naoki Koshimoto; Takayuki Kotani; Masayuki Kuzuhara; Stephen E Levine; Steven R Majewski; Kento Masuda; Noriyuki Matsunaga; Kohei Miyakawa; Makoko Miyoshi; Kumiko Morihana; Ryoichi Nishi; Yuta Notsu; Masashi Omiya; Jason Sanders; Ataru Tanikawa; Masahiro Tsujimoto; Taihei Yano; Masataka Aizawa; Ko Arimatsu; Michael Biermann; Celine Boehm; Masashi Chiba; Victor P Debattista; Ortwin Gerhard; Masayuki Hirabayashi; David Hobbs; Bungo Ikenoue; Hideyuki Izumiura; Carme Jordi; Naoki Kohara; Wolfgang Löffler; Xavier Luri; Ichiro Mase; Andrea Miglio; Kazuhisa Mitsuda; Trent Newswander; Shogo Nishiyama; Yoshiyuki Obuchi; Takafumi Ootsubo; Masami Ouchi; Masanobu Ozaki; Michael Perryman; Timo Prusti; Pau Ramos; Justin I Read; R Michael Rich; Ralph Schönrich; Minori Shikauchi; Risa Shimizu; Yoshinori Suematsu; Shotaro Tada; Aoi Takahashi; Takayuki Tatekawa; Daisuke Tatsumi; Takuji Tsujimoto; Toshihiro Tsuzuki; Seitaro Urakawa; Fumihiro Uraguchi; Shin Utsunomiya; Vincent Van Eylen; Floor van Leeuwen; Takehiko Wada; Nicholas A Walton, Abstract
  
  The 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 Aerospace Exploration Agency. JASMINE has two main science goals. One is Galactic archaeology with a Galactic Center survey, which aims to reveal the Milky Way’s central core structure and formation history from Gaia-level (∼25 ${\mu} $as) astrometry in the near-infrared (NIR) Hw band (1.0–1.6 ${\mu} $m). The other is an exoplanet survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic Center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information on the stars in the Galactic Center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic Center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars, and microlensing studies, including discovery of (intermediate-mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions., Oxford University Press (OUP)
  Publications of the Astronomical Society of Japan, 10 Apr. 2024
• Modeling photometric variations due to a global inhomogeneity on an obliquely rotating star: Application to light curves of white dwarfs
  Yasushi Suto; Shin Sasaki; Masataka Aizawa; Kotaro Fujisawa; Kazumi Kashiyama, Abstract
  
  We develop a general framework to compute photometric variations induced by the oblique rotation of a star with an axisymmetric inhomogeneous surface. We apply the framework to compute light curves of white dwarfs adopting two simple models of their surface inhomogeneity. Depending on the surface model and the location of the observer, the resulting light curve exhibits a departure from a purely sinusoidal curve that is observed for a fraction of white dwarfs. As a specific example, we fit our model to the observed phase-folded light curve of a fast-spinning white dwarf ZTF J190132.9+145808.7 (with a rotation period of 419 s). We find that the size and obliquity angle of the spot responsible for the photometric variation are Δθs ≈ 60° and θ⋆ ≈ 60° or 90°, respectively, implying an interesting constraint on the surface distribution of the magnetic field on white dwarfs., Oxford University Press (OUP)
  Publications of the Astronomical Society of Japan, 09 Dec. 2022
• 3D Selection of 167 Substellar Companions to Nearby Stars
  Fabo Feng; R. Paul Butler; Steven S. Vogt; Matthew S. Clement; C. G. Tinney; Kaiming Cui; Masataka Aizawa; Hugh R. A. Jones; J. Bailey; Jennifer Burt; B. D. Carter; Jeffrey D. Crane; Francesco Flammini Dotti; Bradford Holden; Bo Ma; Masahiro Ogihara; Rebecca Oppenheimer; S. J. O’Toole; Stephen A. Shectman; Robert A. Wittenmyer; Sharon X. Wang; D. J. Wright; Yifan Xuan, Abstract
  
  We analyze 5108 AFGKM stars with at least five high-precision radial velocity points, as well as Gaia and Hipparcos astrometric data, utilizing a novel pipeline developed in previous work. We find 914 radial velocity signals with periods longer than 1000 days. Around these signals, 167 cold giants and 68 other types of companions are identified, through combined analyses of radial velocity, astrometry, and imaging data. Without correcting for detection bias, we estimate the minimum occurrence rate of the wide-orbit brown dwarfs to be 1.3%, and find a significant brown-dwarf valley around 40 M _Jup. We also find a power-law distribution in the host binary fraction beyond 3 au, similar to that found for single stars, indicating no preference of multiplicity for brown dwarfs. Our work also reveals nine substellar systems (GJ 234 B, GJ 494 B, HD 13724 b, HD 182488 b, HD 39060 b and c, HD 4113 C, HD 42581 d, HD 7449 B, and HD 984 b) that have previously been directly imaged, and many others that are observable at existing facilities. Depending on their ages, we estimate that an additional 10–57 substellar objects within our sample can be detected with current imaging facilities, extending the imaged cold (or old) giants by an order of magnitude., American Astronomical Society
  The Astrophysical Journal Supplement Series, 26 Aug. 2022
• Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen
  Masataka Aizawa; Kojiro Kawana; Kazumi Kashiyama; Ryou Ohsawa; Hajime Kawahara; Fumihiro Naokawa; Tomoyuki Tajiri; Noriaki Arima; Hanchun Jiang; Tilman Hartwig; Kotaro Fujisawa; Toshikazu Shigeyama; Ko Arimatsu; Mamoru Doi; Toshihiro Kasuga; Naoto Kobayashi; Sohei Kondo; Yuki Mori; Shin-ichiro Okumura; Satoshi Takita; Shigeyuki Sako, Abstract
  
  We report on a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3–M5 dwarfs with a rise time of 5 s ≲ trise ≲ 100 s and an amplitude of 0.5 ≲ ΔF/F⋆ ≲ 20. The flare light-curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with a temperature of 9000–15000 K, the peak luminosities and energies are estimated to be 1029 erg s−1 ≲ Lpeak ≲ 1031 erg s−1 and 1031 erg ≲ Eflare ≲ 1034 erg, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than $90\%$ of the host stars of the detected flares are magnetically active based on their Hα-emission-line intensities obtained by LAMOST. An estimated occurrence rate of detected flares is ∼0.7 per day per active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light-curves can be explained by the chromospheric compression model: the rise time is broadly consistent with the Alfvén transit time of a magnetic loop with a length scale of lloop ∼ 104 km and a field strength of 1000 gauss, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down near to the photosphere with a temperature of ≳ 10000 K. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types., Oxford University Press (OUP)
  Publications of the Astronomical Society of Japan, 08 Aug. 2022
• ET White Paper: To Find the First Earth 2.0
  Jian Ge; Hui Zhang; Weicheng Zang; Hongping Deng; Shude Mao; Ji-Wei Xie; Hui-Gen Liu; Ji-Lin Zhou; Kevin Willis; Chelsea Huang; Steve B. Howell; Fabo Feng; Jiapeng Zhu; Xinyu Yao; Beibei Liu; Masataka Aizawa; Wei Zhu; Ya-Ping Li; Bo Ma; Quanzhi Ye; Jie Yu; Maosheng Xiang; Cong Yu; Shangfei Liu; Ming Yang; Mu-Tian Wang; Xian Shi; Tong Fang; Weikai Zong; Jinzhong Liu; Yu Zhang; Liyun Zhang; Kareem El-Badry; Rongfeng Shen; Pak-Hin Thomas Tam; Zhecheng Hu; Yanlv Yang; Yuan-Chuan Zou; Jia-Li Wu; Wei-Hua Lei; Jun-Jie Wei; Xue-Feng Wu; Tian-Rui Sun; Fa-Yin Wang; Bin-Bin Zhang; Dong Xu; Yuan-Pei Yang; Wen-Xiong Li; Dan-Feng Xiang; Xiaofeng Wang; Tinggui Wang; Bing Zhang; Peng Jia; Haibo Yuan; Jinghua Zhang; Sharon Xuesong Wang; Tianjun Gan; Wei Wang; Yinan Zhao; Yujuan Liu; Chuanxin Wei; Yanwu Kang; Baoyu Yang; Chao Qi; Xiaohua Liu; Quan Zhang; Yuji Zhu; Dan Zhou; Congcong Zhang; Yong Yu; Yongshuai Zhang; Yan Li; Zhenghong Tang; Chaoyan Wang; Fengtao Wang; Wei Li; Pengfei Cheng; Chao Shen; Baopeng Li; Yue Pan; Sen Yang; Wei Gao; Zongxi Song; Jian Wang; Hongfei Zhang; Cheng Chen; Hui Wang; Jun Zhang; Zhiyue Wang; Feng Zeng; Zhenhao Zheng; Jie Zhu; Yingfan Guo; Yihao Zhang; Yudong Li; Lin Wen; Jie Feng; Wen Chen; Kun Chen; Xingbo Han; Yingquan Yang; Haoyu Wang; Xuliang Duan; Jiangjiang Huang; Hong Liang; Shaolan Bi; Ning Gai; Zhishuai Ge; Zhao Guo; Yang Huang; Gang Li; Haining Li; Tanda Li; Yuxi; Lu; Hans-Walter Rix; Jianrong Shi; Fen Song; Yanke Tang; Yuan-Sen Ting; Tao Wu; Yaqian Wu; Taozhi Yang; Qing-Zhu Yin; Andrew Gould; Chung-Uk Lee; Subo Dong; Jennifer C. Yee; Yossi Shvartzvald; Hongjing Yang; Renkun Kuang; Jiyuan Zhang; Shilong Liao; Zhaoxiang Qi; Jun Yang; Ruisheng Zhang; Chen Jiang; Jian-Wen Ou; Yaguang Li; Paul Beck; Timothy R. Bedding; Tiago L. Campante; William J. Chaplin; Jørgen Christensen-Dalsgaard; Rafael A. García; Patrick Gaulme; Laurent Gizon; Saskia Hekker; Daniel Huber; Shourya Khanna; Yan Li; Savita Mathur; Andrea Miglio; Benoît Mosser; J. M. Joel Ong; Ângela R. G. Santos; Dennis Stello; Dominic M. Bowman; Mariel Lares-Martiz; Simon Murphy; Jia-Shu Niu; Xiao-Yu Ma; László Molnár; Jian-Ning Fu; Peter De Cat; Jie Su; the ET consortium, We propose to develop a wide-field and ultra-high-precision photometric
  survey mission, temporarily named "Earth 2.0 (ET)". This mission is designed to
  measure, for the first time, the occurrence rate and the orbital distributions
  of Earth-sized planets. ET consists of seven 30cm telescopes, to be launched to
  the Earth-Sun's L2 point. Six of these are transit telescopes with a field of
  view of 500 square degrees. Staring in the direction that encompasses the
  original Kepler field for four continuous years, this monitoring will return
  tens of thousands of transiting planets, including the elusive Earth twins
  orbiting solar-type stars. The seventh telescope is a 30cm microlensing
  telescope that will monitor an area of 4 square degrees toward the galactic
  bulge. This, combined with simultaneous ground-based KMTNet observations, will
  measure masses for hundreds of long-period and free-floating planets. Together,
  the transit and the microlensing telescopes will revolutionize our
  understandings of terrestrial planets across a large swath of orbital distances
  and free space. In addition, the survey data will also facilitate studies in
  the fields of asteroseismology, Galactic archeology, time-domain sciences, and
  black holes in binaries.
  14 Jun. 2022
• Global Mapping of Surface Composition on an Exo-Earth Using Sparse Modeling
  Atsuki Kuwata; Hajime Kawahara; Masataka Aizawa; Takayuki Kotani; Motohide Tamura, Abstract
  
  The time series of light reflected from exoplanets by future direct imaging can provide spatial information with respect to the planetary surface. We apply sparse modeling to the retrieval method that disentangles the spatial and spectral information from multiband reflected light curves termed as spin–orbit unmixing. We use the ℓ ₁-norm and the total squared variation norm as regularization terms for the surface distribution. Applying our technique to a toy model of cloudless Earth, we show that our method can infer sparse and continuous surface distributions and also unmixed spectra without prior knowledge of the planet surface. We also apply the technique to the real Earth data as observed by DSCOVR/EPIC. We determined the representative components that can be interpreted as cloud and ocean. Additionally, we found two components that resembled the distribution of land. One of the components captures the Sahara Desert, and the other roughly corresponds to vegetation, although their spectra are still contaminated by clouds. Sparse modeling significantly improves the geographic retrieval, in particular, of clouds and leads to higher resolutions for other components when compared with spin–orbit unmixing using Tikhonov regularization., American Astronomical Society
  The Astrophysical Journal, 01 May 2022
• Dippers from TESS Full-frame Images. II. Spectroscopic Characterization of Four Young Dippers
  Yui Kasagi; Takayuki Kotani; Hajime Kawahara; Tomoyuki Tajiri; Takayuki Muto; Masataka Aizawa; Michiko S. Fujii; Kohei Hattori; Kento Masuda; Munetake Momose; Ryou Ohsawa; Satoshi Takita, Abstract
  
  Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasiperiodic dimming around young stellar objects. The dimming mechanisms of these objects, the so-called “dippers,” are thought to be related to either the accretion property or the structure of protoplanetary disks especially in regions close to the host star. Recently, we have created a catalog of dippers from one year of TESS full-frame image data. In this paper, we report on the spectral features of four newly found dippers in that catalog and show that they potentially shed light on the dimming mechanisms. We found that all of the targets exhibit the Hα emission line, which is an indicator of ongoing accretion. Based on their line profiles and/or their variability, we characterized the properties of the disks of each source, which can support dimming mechanisms via a dusty disk wind or an accretion-driven inner-disk warp. Also, we found an interesting dipper (TIC 317873721), a “close-in binary dipper,” showing a complex variability of the line profile and a large radial velocity variation. Because the dimming intervals are similar to the orbital period of the binary, we suggest that the dips are caused by dust in the accretion warp from a circumbinary disk onto stars. Such a close-in (<0.1 au) binary dipper has been rarely reported thus far; further investigation will reveal new aspects of disk evolution and planetary formation., American Astronomical Society
  The Astrophysical Journal Supplement Series, 23 Mar. 2022
• Dippers from the TESS Full-frame Images. I. Results of the First One Year Data and Discovery of a Runaway Dipper
  Tomoyuki Tajiri; Hajime Kawahara; Masataka Aizawa; Michiko S. Fujii; Kohei Hattori; Yui Kasagi; Takayuki Kotani; Kento Masuda; Munetake Momose; Takayuki Muto; Ryou Ohsawa; Satoshi Takita, Abstract
  
  We present a comprehensive catalog of the dippers—young stellar objects that exhibit episodic dimming—derived from one year’s worth of data of the Transiting Exoplanet Survey Satellite (TESS) full-frame images. In the survey, we found 35 dippers using the convolutional neural network, most of them newly discovered. Although these dippers are widely distributed over the first half-hemisphere that TESS surveyed, we identified the majority’s membership with the nearest association Scorpius–Centaurus, Velorum OB2, and the nearby Orion molecular cloud complex. However, several dippers are likely to be located in the field. We also found three old dippers whose age exceeds 10 million years, which is considered to be the disk dissipation time. The color–color diagram indicates that these old dippers are likely to have an extreme debris disk. In particular, we found a runaway old dipper having a large three-dimensional velocity of 72 km s⁻¹. The dippers in the field, which were probably escaping from their birth molecular clouds or were born outside the current area of star-forming regions, are more common than previously considered., American Astronomical Society
  The Astrophysical Journal Supplement Series, 23 Nov. 2020
• Search for Alignment of Disk Orientations in Nearby Star-forming Regions: Lupus, Taurus, Upper Scorpius, ρ Ophiuchi, and Orion
  Masataka Aizawa; Yasushi Suto; Yoko Oya; Shiro Ikeda; Takeshi Nakazato, Abstract
  
  Spatial correlations among protoplanetary disk orientations carry unique information on physics of multiple-star formation processes. We select five nearby star-forming regions that comprise a number of protoplanetary disks with spatially resolved images with ALMA and Hubble Space Telescope, and we search for the mutual alignment of the disk axes. Specifically, we apply the Kuiper test to examine the statistical uniformity of the position angle (PA: the angle of the major axis of the projected disk ellipse measured counterclockwise from the north) distribution. The disks located in the star-forming regions, except the Lupus clouds, do not show any signature of the alignment, supporting the random orientation. Rotational axes of 16 disks with spectroscopic measurement of PA in the Lupus III cloud, a subregion of the Lupus field, however, exhibit a weak and possible departure from the random distribution at a 2σ level, and the inclination angles of the 16 disks are not uniform as well. Furthermore, the mean direction of the disk PAs in the Lupus III cloud is parallel to the direction of its filament structure and approximately perpendicular to the magnetic field direction. We also confirm the robustness of the estimated PAs in the Lupus clouds by comparing the different observations and estimators based on three different methods, including sparse modeling. The absence of the significant alignment of the disk orientation is consistent with the turbulent origin of the disk angular momentum. Further observations are required to confirm/falsify the possible disk alignment in the Lupus III cloud., American Astronomical Society
  The Astrophysical Journal, 01 Aug. 2020
• Global Mapping of an Exo-Earth Using Sparse Modeling
  Masataka Aizawa; Hajime Kawahara; Siteng Fan, Abstract
  
  We develop a new retrieval scheme for obtaining two-dimensional surface maps of exoplanets from scattered light curves. In our scheme, the combination of the L1-norm and total squared variation, which is one of the techniques used in sparse modeling, is adopted to find the optimal map. We apply the new method to simulated scattered light curves of the Earth, and find that the new method provides a better spatial resolution of the reconstructed map than those using Tikhonov regularization. We also apply the new method to observed scattered light curves of the Earth obtained during the two-year Deep Space Climate Observatory/Earth Polychromatic Imaging Camera observations presented by Fan et al. The method with Tikhonov regularization enables us to resolve North America, Africa, Eurasia, and Antarctica. In addition to that, the sparse modeling identifies South America and Australia, although it fails to find Antarctica, maybe due to low observational weights on the poles. Besides, the proposed method is capable of retrieving maps from noise-injected light curves of a hypothetical Earthlike exoplanet at 5 pc with a noise level expected from coronagraphic images from a 8 m space telescope. We find that the sparse modeling resolves Australia, Afro-Eurasia, North America, and South America using 2 yr observation with a time interval of one month. Our study shows that the combination of sparse modeling and multiepoch observation with 1 day or 5 days per month can be used to identify main features of an Earth analog in future direct-imaging missions such as the Large UV/Optical/IR Surveyor., American Astronomical Society
  The Astrophysical Journal, 01 Jun. 2020
• Systematic Search for Rings around Kepler Planet Candidates: Constraints on Ring Size and Occurrence Rate
  Masataka Aizawa; Kento Masuda; Hajime Kawahara; Yasushi Suto, Abstract
  
  We perform a systematic search for rings around 168 Kepler planet candidates with sufficient signal-to-noise ratios that are selected from all of the short-cadence data. We fit ringed and ringless models to their light curves and compare the fitting results to search for the signatures of planetary rings. First, we identify 29 tentative systems, for which the ringed models exhibit statistically significant improvement over the ringless models. The light curves of those systems are individually examined, but we are not able to identify any candidate that indicates evidence for rings. In turn, we find several mechanisms of false positives that would produce ringlike signals, and the null detection enables us to place upper limits on the size of the rings. Furthermore, assuming the tidal alignment between axes of the planetary rings and orbits, we conclude that the occurrence rate of rings larger than twice the planetary radius is less than 15%. Even though the majority of our targets are short-period planets, our null detection provides statistical and quantitative constraints on largely uncertain theoretical models of the origin, formation, and evolution of planetary rings., American Astronomical Society
  The Astronomical Journal, 24 Apr. 2018
• Toward Detection of Exoplanetary Rings via Transit Photometry: Methodology and a Possible Candidate
  Masataka Aizawa (逢澤正嵩); Sho Uehara (上原翔); Kento Masuda (増田賢人); Hajime Kawahara (河原創); Yasushi Suto (須藤靖), Abstract
  
  The detection of a planetary ring of exoplanets remains one of the most attractive, but challenging, goals in the field of exoplanetary science. We present a methodology that implements a systematic search for exoplanetary rings via transit photometry of long-period planets. This methodology relies on a precise integration scheme that we develop to compute a transit light curve of a ringed planet. We apply the methodology to 89 long-period planet candidates from the Kepler data so as to estimate, and/or set upper limits on, the parameters of possible rings. While the majority of our samples do not have sufficient signal-to-noise ratios (S/Ns) to place meaningful constraints on ring parameters, we find that six systems with higher S/Ns are inconsistent with the presence of a ring larger than 1.5 times the planetary radius, assuming a grazing orbit and a tilted ring. Furthermore, we identify five preliminary candidate systems whose light curves exhibit ring-like features. After removing four false positives due to the contamination from nearby stars, we identify KIC 10403228 as a reasonable candidate for a ringed planet. A systematic parameter fit of its light curve with a ringed planet model indicates two possible solutions corresponding to a Saturn-like planet with a tilted ring. There also remain two other possible scenarios accounting for the data; a circumstellar disk and a hierarchical triple. Due to large uncertain factors, we cannot choose one specific model among the three., American Astronomical Society
  The Astronomical Journal, 31 Mar. 2017
• TRANSITING PLANET CANDIDATES BEYOND THE SNOW LINE DETECTED BY VISUAL INSPECTION OF 7557 KEPLER OBJECTS OF INTEREST
  Sho Uehara; Hajime Kawahara; Kento Masuda; Shin’ya Yamada; Masataka Aizawa, ABSTRACT
  
  We visually inspected the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) that were possibly due to long-period giant planets. We identified 28 STEs in 24 KOIs, among which 14 events are newly reported in this paper. We estimate the radius and orbital period of the objects causing STEs by fitting the STE light curves simultaneously with the transits of the other planets in the system or with prior information on the host star density. As a result, we found that STEs in seven of those systems are consistent with Neptune- to Jupiter-sized objects of orbital periods ranging from a few to ∼ $20\;\mathrm{years}$ . We also estimate that $\gtrsim 20\%$ of the compact multi-transiting systems host cool giant planets with periods $\gtrsim 3\;\mathrm{years}$ on the basis of their occurrence in the KOIs with multiple candidates, assuming the small mutual inclination between inner and outer planetary orbits., American Astronomical Society
  The Astrophysical Journal, 25 Apr. 2016