Publications (51)
The Preliminary Mauve Science Programme: Science themes identified for the first year of operations
Mauve Science Collaboration, Marcel Agueros, Don Dixon +22
Mauve is a low-cost small satellite developed and operated by Blue Skies Space Ltd. The payload features a 13 cm telescope connected with a fibre that feeds into a UV-Vis spectrome…
Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -
Yuta Notsu, Hiroyuki Maehara, Satoshi Honda +7
We report the latest view of Kepler solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5m telescope spectroscopic ob…
Detection of a high-velocity prominence eruption leading to a CME associated with a superflare on the RS CVn-type star V1355 Orionis
Shun Inoue, Hiroyuki Maehara, Yuta Notsu +5
Stellar coronal mass ejections (CMEs) have recently received much attention for their impacts on exoplanets and stellar evolution. Detecting prominence eruptions, the initial phase…
Coronal Mass Ejections from Young Suns: Insights from Solar and Stellar Observations and Models
Kosuke Namekata
Recent discoveries have revealed exoplanets orbiting young Sun-like stars, offering a window into the early solar system. These young stars frequently produce extreme magnetic expl…
Sun-as-a-star Analyses of Various Solar Active Events Using H Spectral Images Taken by SMART/SDDI
Takato Otsu, Ayumi Asai, Kiyoshi Ichimoto +2
Sun-as-a-star analyses, in which observational data is spatially integrated, are useful for interpreting stellar data. For future applications to stellar observations, we performed…
The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. II. Hydrogen Broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope
Adam F. Kowalski, Joel C. Allred, Mats Carlsson +5
Red-shifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resol…
Time resolved spectroscopic observations of an M-dwarf flare star EV Lacertae during a flare
Satoshi Honda, Yuta Notsu, Kosuke Namekata +5
We have performed 5 night spectroscopic observation of the Halpha line of EV Lac with a medium wavelength resolution (R~ 10,000) using the 2m Nayuta telescope at the Nishi-Harima A…
Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. III. Comparison between Starspot Mapping, Zeeman Doppler Imaging, and Multiwavelength Variability
Kai Ikuta, Kosuke Namekata, Pascal Petit +12
Recent simultaneous multiwavelength observations of a nearby young solar-type star EK Dra in the optical, H spectrum, and X-ray, have provided evidence for stellar prominence e…
Starspot activity and superflares on solar-type stars
Hiroyuki Maehara, Yuta Notsu, Shota Notsu +5
We analyze the correlation between starspots and superflares on solar-type stars using observations from the Kepler mission. The analysis shows that the observed fraction of stars…
A Superflare on YZ Canis Minoris Observed by Seimei Telescope and TESS: Red Asymmetry of H Emission Associated with White-Light Emission
Keiichi Namizaki, Kosuke Namekata, Hiroyuki Maehara +4
Active M-type stars are known to often produce superflares on the surface. Radiation from stellar (super-)flares is important for the exoplanet habitability, but the mechanisms are…
Rising Near-Ultraviolet Spectra in Stellar Megaflares
Adam F. Kowalski, Rachel A. Osten, Yuta Notsu +5
Flares from M-dwarf stars can attain energies up to times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and…
High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. I. Discovery of Rapid and Short-Duration Prominence Eruptions
Yuto Kajikiya, Kosuke Namekata, Yuta Notsu +3
M-dwarfs show frequent flares and associated coronal mass ejections (CMEs) may significantly impact close-in habitable planets. M-dwarf flares sometimes show red/blue asymmetries i…
A Semi-Empirical Estimate of Solar EUV Evolution from 10 Myr to 10 Gyr
Kevin France, Girish Duvvuri, Cynthia Froning +9
The extreme-ultraviolet (EUV; 100 -- 911 Ã ) spectra of F, G, K, and M stars provide diagnostics of the stellar chromosphere through the corona, with line and continuum formation t…
APO & SMARTS flare star campaign observations I. Blue wing asymmetries in chromospheric lines during mid M dwarf flares from simultaneous spectroscopic and photometric observation data
Yuta Notsu, Adam F. Kowalski, Hiroyuki Maehara +12
We conducted the time-resolved simultaneous optical spectroscopic and photometric observations of mid M dwarf flare stars YZ CMi, EV Lac, and AD Leo. Spectroscopic observations wer…
Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. I. Discovery of Prominence Eruptions Associated with Superflares
Kosuke Namekata, Vladimir S. Airapetian, Pascal Petit +14
Young solar-type stars frequently produce superflares, serving as a unique window into the young Sun-Earth environments. Large solar flares are closely linked to coronal mass eject…
Comparison of solar multifrequency microwave data with other solar indices for understanding solar and stellar microwave data
Masumi Shimojo, Kosuke Namekata, Kazumasa Iwai +2
Thermal microwave emissions detected from stellar atmospheres contain information on stellar activity. However, even for the Sun, the relationship between multifrequency microwave…
Starspot mapping with adaptive parallel tempering. II. Application to TESS data for M-dwarf flare stars, AU Microscopii, YZ Canis Minoris, and EV Lacertae
Kai Ikuta, Kosuke Namekata, Yuta Notsu +5
Starspots and stellar flares are indicators of stellar magnetic activity.The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not co…
Universal Scaling Laws for Solar and Stellar Atmospheric Heating: Catalog of Power-law Index between Solar Activity Proxies and Various Spectral Irradiances
Shin Toriumi, Vladimir S. Airapetian, Kosuke Namekata +1
The formation of extremely hot outer atmospheres is one of the most prominent manifestations of magnetic activity common to the late-type dwarf stars, including the Sun. It is wide…
Systematic NICER study of the low-ionized Fe K line on RS Canum Venaticorum type stars
Shun Inoue, Teruaki Enoto, Yuta Notsu +4
The Fe K fluorescence line ( keV) has been observed during solar and stellar flares. Two emission mechanisms of the Fe K line, photoionization and collisional ion…
Multiwavelength observation of an active M-dwarf star EV Lac and its stellar flare accompanied by a delayed prominence eruption
Shun Inoue, Teruaki Enoto, Kosuke Namekata +9
We conducted 4-night multiwavelength observations of an active M-dwarf star EV Lac on 2022 October 2427 with simultaneous coverage of soft X-rays (NICER; 0.212 …
High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. II. Statistical Properties of Blue/Red Asymmetries in the H Line
Yuto Kajikiya, Kosuke Namekata, Yuta Notsu +4
M-dwarfs frequently produce flares, and their associated coronal mass ejections (CMEs) may threaten the habitability of close-in exoplanets. M-dwarf flares sometimes show prominenc…
Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER
Kosuke Namekata, Hiroyuki Maehara, Ryo Sasaki +24
We report multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with Seimei Telescope (6150--7930 Ã ), SCAT (Spectroscopic Chuo-university Astronomical Telesc…
Time evolution of white-light flare accompanied by probable postflare loops on M-type dwarf EV Lacertae
Shinnosuke Ichihara, Daisaku Nogami, Kosuke Namekata +6
White-light flares are explosive phenomena accompanied by brightening of continuum from near-ultraviolet to optical, which occur on the Sun and stars. In order to investigate the m…
Temporal Variation of Flare Occurrence Rates via the Spot Evolution on the Sun and Solar-type Stars
Takato Tokuno, Kosuke Namekata, Hiroyuki Maehara +1
The spot evolution on the Sun and solar-type stars is important for understanding the nature of consequential flaring activity. This study statistically investigates the variance o…
Spectral and photometric variability of SS 433 observed with XRISM and simultaneous optical and near-infrared telescopes
Yusuke Sakai, Shinya Yamada, Yuta Okada +36
We present results from coordinated multiwavelength observations of the SS 433, obtained with XRISM, optical telescopes, and near-infrared camera during 2024 April and 2025 March.…
Discovery of multi-temperature coronal mass ejection signatures from a young solar analogue
Kosuke Namekata, Kevin France, Jongchul Chae +14
Coronal mass ejections (CMEs) on the early Sun may have profoundly influenced the planetary atmospheres of early Solar System planets. Flaring young solar analogues serve as excell…
Probable detection of an eruptive filament from a superflare on a solar-type star
Kosuke Namekata, Hiroyuki Maehara, Satoshi Honda +20
Solar flares are often accompanied by filament/prominence eruptions ( K and cm), sometimes leading to coronal mass ejections (CMEs) that direct…
Statistical Properties of Superflares on Solar-type Stars: Results Using All of the Kepler Primary Mission Data
Soshi Okamoto, Yuta Notsu, Hiroyuki Maehara +5
We report the latest statistical analyses of superflares on solar-type (G-type main-sequence; effective temperature is 5100 - 6000 K) stars using all of the primary missio…
Superoutburst of WZ Sge-type Dwarf Nova Below the Period Minimum: ASASSN-15po
Kosuke Namekata, Keisuke Isogai, Taichi Kato +38
We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed…
Do Young Suns Produce Frequent, Massive CMEs? Results from Five-year Dedicated Optical Observations of EK Draconis and V889 Hercules
Kosuke Namekata, Hiroyuki Maehara, Yuta Notsu +4
We report results from a five-year (132-night) dedicated observational campaign targeting two nearby young solar-type stars, EK Draconis (50-125 Myr age) and V889 Hercules ($…
Spectroscopic and Photometric Observations of Dwarf Nova Superoutbursts by the 3.8 m Telescope Seimei and the Variable Star Network
Yusuke Tampo, Keisuke Isogai, Naoto Kojiguchi +59
We present spectroscopic and photometric observations of 17 dwarf-nova superoutbursts obtained by KOOLS-IFU mounted on the 3.8 m telescope Seimei at Okayama Observatory of Kyoto Un…
Reconstructing the XUV Spectra of Active Sun-like Stars Using Solar Scaling Relations with Magnetic Flux
Kosuke Namekata, Shin Toriumi, Vladimir S. Airapetian +3
Kepler Space Telescope and Transiting Exoplanet Survey Satellite unveiled that Sun-like stars frequently host exoplanets. These exoplanets are subject to fluxes of ionizing radiati…
X-ray and Hα superflare on an RS CVn-type star, UX Arietis: Constraint on the flare location from radial velocity change during the flare
Sota Urabe, Yohko Tsuboi, Kosuke Namekata +5
We report on a giant stellar flare from the RS CVn-type binary UX Arietis, detected with the Monitor of All-sky X-ray Image (MAXI) and followed by a 12-day optical spectroscopic ca…
Temporal Evolution of Spatially-Resolved Individual Star Spots on a Planet-Hosting Solar-type Star: Kepler 17
Kosuke Namekata, James R. A. Davenport, Brett M. Morris +9
Star spot evolution is visible evidence of the emergence/decay of the magnetic field on stellar surface, and it is therefore important for the understanding of the underlying stell…
Statistical Study of Solar White-light Flares and Comparison with Superflares on Solar-type Stars
Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe +9
Recently, many superflares on solar-type stars were discovered as white-light flares (WLFs). A correlation between the energies (E) and durations (t) of superflares is derived as $…
Statistical properties of superflares on solar-type stars with Kepler data
Yuta Notsu, Hiroyuki Maehara, Takuya Shibayama +5
Superflares are flares that release total energy 1010 times greater than that of the biggest solar flares with energy of 10 erg. We searched superflares on…
Spectroscopic observations of active solar-analog stars having high X-ray luminosity, as a proxy of superflare stars
Yuta Notsu, Satoshi Honda, Hiroyuki Maehara +4
Recent studies of solar-type superflare stars have suggested that even old slowly rotating stars similar to the Sun can have large starspots and superflares. We conducted high disp…
Assessing the capability of a model-based stellar XUV estimation
Munehito Shoda, Kosuke Namekata, Shinsuke Takasao
Stellar XUV (X-ray and extreme ultraviolet) emission drives the heating and chemical reactions in planetary atmospheres and protoplanetary disks, and therefore, a proper estimation…
Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. II. Understanding Prominence Eruption through Data-Driven Modeling and Observed Magnetic Environment
Kosuke Namekata, Kai Ikuta, Pascal Petit +15
EK Draconis, a nearby young solar-type star (G1.5V, 50-120 Myr), is known as one of the best proxies for inferring the environmental conditions of the young Sun. The star frequentl…
Doppler imaging combined with high-cadence photometry. I. Revisiting the surface of a pre-main-sequence flare star
Sanghee Lee, Engin Bahar, Hakan Volkan Åenavcı +11
Latitude distribution of stellar magnetic activity is not well constrained by observations, despite its importance for a better understanding of stellar dynamos. We aim to obtain a…
Impact of Stellar Superflares on Planetary Habitability
Yosuke A. Yamashiki, Hiroyuki Maehara, Vladimir Airapetian +15
High-energy radiation caused by exoplanetary space weather events from planet-hosting stars can play a crucial role in conditions promoting or destroying habitability in addition t…
The Exospace Weather Frontier
R. O. Parke Loyd, Evgenya L. Shkolnik, Joseph Lazio +26
Space weather is among the most powerful and least understood forces shaping planetary atmospheres. In our Solar System, we observe its effects directly: atmospheric escape, chemic…
Lifetimes and Emergence/Decay Rates of Star Spots on Solar-type Stars Estimated by Kepler Data in Comparison with Those of Sunspots
Kosuke Namekata, Hiroyuki Maehara, Yuta Notsu +7
Active solar-type stars show large quasi-periodic brightness variations caused by stellar rotations with star spots, and the amplitude changes as the spots emerge and decay. The Ke…
Stellar Forcing of (exo)Planetary Environments
Julián D. Alvarado-Gómez, Erika Palmerio, Manuel Güdel +13
The environments of exoplanets are fundamentally shaped by the magnetic activity of their host stars through radiative, plasma, and particle-driven processes. This article presents…
Discovery of a Long-Duration Superflare on a Young Solar-Type Star EK Draconis with Nearly Similar Time Evolution for H and White-Light Emissions
Kosuke Namekata, Hiroyuki Maehara, Satoshi Honda +17
Young solar-type stars are known to show frequent "superflares", which may severely influence the habitable worlds on young planets via intense radiations and coronal mass ejection…
Validation of a Scaling Law for the Coronal Magnetic Field Strengths and Loop Lengths of Solar and Stellar Flares
Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe +2
Shibata & Yokoyama (1999, 2002) proposed a method of estimating the coronal magnetic field strengths () and magnetic loop lengths () of solar and stellar flares, on the basis…
Sun-as-a-star Analysis of H Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening
Kosuke Namekata, Kiyoshi Ichimoto, Takako T. Ishii +1
Stellar flares sometimes show red/blue asymmetries of H line, which can indicate chromospheric dynamics and prominence activations. However, the origin of asymmetries is not co…
Time-Resolved Connection between Starspots and Flares in Nearby Young Solar-type Stars Observed by TESS
Daijiro Hitotsuyanagi, Hiroto Yamada, Daisuke Yamashiki +3
The paper analyzes ~7 years of TESS photometry of three young solar-type stars to show that larger starspot areas are associated with higher flare frequencies, indicating that supe…
Starspot mapping with adaptive parallel tempering I: Implementation of computational code
Kai Ikuta, Hiroyuki Maehara, Yuta Notsu +7
Starspots are thought to be regions of locally strong magnetic fields, similar to sunspots, and they can generate photometric brightness modulations. To deduce stellar and spot pro…
Hunting for stellar coronal mass ejections
Kosuke Namekata, Hiroyuki Maehara, Satoshi Honda +3
Solar flares are often accompanied by filament/prominence eruptions, sometimes leading to coronal mass ejections (CMEs). By analogy, we expect that stellar flares are also associat…
Statistical Studies of Solar White-Light Flares and Comparisons with Superflares on Solar-type Stars
Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe +9
Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies () and duratio…