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We report that our optical spectroscopic observation of the young solar-type star EK Draconis reveals the first evidence for a stellar filament eruption associated with a superflare on a young Sun-like star EK Draconis. We monitored EK Draconis for about 30 nights with the TESS’s optical photometry and ground-based spectroscopy, and finally detected a superflare in white-light and Hα. After the superflare with the radiated energy of 2.0×1033 erg, a blue-shifted Hα absorption component with a large velocity of -510 km s-1 appeared. The temporal changes in the spectra greatly resemble the Sun-as-a-star spectra of solar filament eruptions, indicating a common picture of solar and stellar filament eruptions. The comparison of this stellar filament eruption with solar filament eruptions in terms of the length scale and velocity strongly suggests that this would eventually become a stellar CME. The mass of erupted filament of 1.1×1018 g is surprisingly 10 times larger than those of the largest solar CMEs. The huge filament eruption and a possible associated CME on the young Sun-like star provide the first opportunity to evaluate how the young Sun-like star/young Sun can affect the environment of young exoplanets/young Earth as well as stellar mass and angular-momentum evolution., {"references":["Maehara, H. et al. (2012), Nature, 485, 478","Namekata, K. et al. (2021), Nature Astronomy, 6, 241","Namekata, K. et al. (2022), ApJ Letters, 926, 5","Namekata, K. et al. (2022), ApJ, 933, 209","Vida, K. et al. (2016), A&A, 590, A11"]}

The rotation rates of main-sequence stars slow over time as they gradually lose angular momentum to their magnetized stellar winds. The rate of angular momentum loss depends on the strength and morphology of the magnetic field, the mass-loss rate, and the stellar rotation period, mass, and radius. Previous observations revealed a shift in magnetic morphology between two F-type stars with comparable rotation rates but very different ages. We confirm a similar transition in several well-characterized solar analogs with ages between 2 and 7 Gyr. We present new spectropolarimetry of 18 Sco and 16 Cyg A & B from the Large Binocular Telescope, and we reanalyze previously published Zeeman Doppler images of HD 76151 and 18 Sco to confirm a shift in magnetic morphology near the middle of main-sequence lifetimes. We combine archival X-ray observations with updated distances from Gaia to estimate mass-loss rates, and we adopt precise stellar properties from asteroseismology and other sources. We then calculate the wind braking torque for each star in the evolutionary sequence, and we assess the uncertainties that arise from errors in the observational inputs. We conclude that the shift in magnetic morphology occurs before the age of the Sun, reinforcing the notion that the solar dynamo may be in a transitional evolutionary phase. We suggest that this magnetic transition may represent a disruption of the global dynamo arising from weaker differential rotation, and we outline our plans to probe this behavior in additional stars spanning a wide range of spectral types., Presentation title in the slides was updated to reflect the published journal article. Video of presentation is available at https://youtu.be/D7h99HIltFM, {"references":["Metcalfe, T. S. et al. (2022), ApJ, 933, L17"]}

The Sun and Sun-like stars commonly host the multi-million-K corona and the 10,000-K chromosphere. Because these extremely hot gases generate X-ray and extreme ultraviolet (XUV) emissions that may impact the erosion and chemistry of (exo)planetary atmospheres, influencing the climate and conditions for habitability, it is of crucial importance in solar and stellar physics to understand the responsible heating mechanisms. While, for the Sun, the magnetic field is thought to play a pivotal role in driving and transporting the energy from the surface upwards, it is not clear whether such a magnetically driven heating is commonly at work on other stars. Here we present the analysis of 10 years of multi-wavelength synoptic observations of the Sun and comparison with stellar data, providing the critical clues to the common nature of the heating mechanisms of coronae, transition regions, and chromospheres of the Sun and Sun-like stars. First, by analyzing the sequences of solar images of sunspot transit events, we derive the means to understand the magnetic and thermal environments of starspot magnetic fields that cannot be spatially resolved. Specifically, it is found that the surface magnetic flux can be inferred from the chromospheric light curves and that the thermal structures around the starspots can be inspected from the sub-MK UV light curves. Second, by investigating the power-law relationships between the surface magnetic flux and the luminosity of various emission lines with the formation temperatures from the corona to the chromosphere of the Sun, we discover that, in any temperature ranges, the solar scaling laws can be extended to the Sun-like stars with the ages of 50 Myr to 4.5 Gyr. This suggests that the magnetically-driven heating of the atmospheres is universal among the Sun and Sun-like stars, regardless of age or activity. Furthermore, by deriving the scaling laws between the magnetic flux and XUV spectrum, it is now possible to empirically reproduce the XUV spectra for various solar-type stars given the observationally-obtained magnetic fluxes. It is expected that the reproduced XUV spectra are used for photochemical calculations of (exo)planets of various solar-type stars, including the young Sun, and estimation of atmospheric formation and habitability of surface life., {"references":["Toriumi, S. & Airapetian, V. S. (2022), ApJ, 927, 179","Toriumi, S. et al. (2022), ApJS, 262, 46","Toriumi, S. & Wang, H. (2019), LRSP, 16, 3"]}

We use the ASH code to model the convective dynamo of solar-type stars. Based on a series of 15 3-D MHD simulations spanning 4 bins in rotation and mass, we show what mechanisms are at work in these stellar dynamos with and without magnetic cycles and how global stellar parameters affect the outcome. We also derive scaling laws for the differential rotation and magnetic field based on these simulations. We find a weaker trend between differential rotation and stellar rotation rate, ($\Delta \Omega \sim (|\Omega|/\Omega_{\odot})^{0.46}$) in the MHD solutions than in their HD counterpart $(|\Omega|/\Omega_{\odot})^{0.66})$, yielding a better agreement with the observational trends based on power laws. We find that for a fluid Rossby number between $0.15 \lesssim Ro_f \lesssim 0.65$ the solutions possess long magnetic cycle, if $Ro_f \lesssim 0.42$ a short cycle and if $Ro_f \gtrsim 1$ (anti-solar-like differential rotation) a statistically steady state. We show that short-cycle dynamos follow the classical Parker-Yoshimura rule whereas the long-cycle period ones do not. We further demonstrate that the Rossby number dependency of the large-scale surface magnetic field in the simulation ($B_{L,surf} \sim Ro_{f}^{-1.26}$) agrees better with observations ($B_{V} \sim Ro_{s}^{-1.4 \pm 0.1}$) and differs from dynamo scaling based on the global magnetic energy ($B_{bulk} \sim Ro_{f}^{-0.5}$). We also show that up to few percents of the stellar luminosity can be channelled into the star's magnetism, hence providing a large energy reservoir for possible surface eruptive events., Comment: 45 pages, 24 figures, 7 tables, 3 appendices, accepted by ApJ on 12/23/21, published on 02/03/22. (1) DAp/AIM, CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay \& Univ. de Paris, France, (2) CMPA, KU Leuven, Belgium, (3) Universidad Carlos III de Madrid, Spain, (4) Dept. de physique, Univ. de Montr\'eal, Canada, (5) JILA and Dept. of Astrophysical and Planetary Sciences, University of Colorado, USA

The rotation rates of main-sequence stars slow over time as they gradually lose angular momentum to their magnetized stellar winds. The rate of angular momentum loss depends on the strength and morphology of the magnetic field, the mass-loss rate, and the stellar rotation period, mass, and radius. Previous observations suggested a shift in magnetic morphology between two F-type stars with similar rotation rates but very different ages (88 Leo and rho CrB). In this Letter, we identify a comparable transition in an evolutionary sequence of solar analogs with ages between 2-7 Gyr. We present new spectropolarimetry of 18 Sco and 16 Cyg A & B from the Large Binocular Telescope, and we reanalyze previously published Zeeman Doppler images of HD 76151 and 18 Sco, providing additional constraints on the nature and timing of this transition. We combine archival X-ray observations with updated distances from Gaia to estimate mass-loss rates, and we adopt precise stellar properties from asteroseismology and other sources. We then calculate the wind braking torque for each star in the evolutionary sequence, demonstrating that the rate of angular momentum loss drops by more than an order of magnitude between the ages of HD 76151 and 18 Sco (2.6-3.7 Gyr) and continues to decrease modestly to the age of 16 Cyg A & B (7 Gyr). We suggest that this magnetic transition may represent a disruption of the global dynamo arising from weaker differential rotation, and we outline plans to probe this phenomenon in additional stars spanning a wide range of spectral types., 6 pages including 2 figures and 1 table. ApJ Letters (accepted June 16)

Asteroseismology of bright stars has become increasingly important as a method to determine the fundamental properties (in particular ages) of stars. The Kepler Space Telescope initiated a revolution by detecting oscillations in more than 500 main-sequence and subgiant stars. However, most Kepler stars are faint and therefore have limited constraints from independent methods such as long-baseline interferometry. Here we present the discovery of solar-like oscillations in alpha Men A, a naked-eye (V = 5.1) G7 dwarf in TESS's southern continuous viewing zone. Using a combination of astrometry, spectroscopy, and asteroseismology, we precisely characterize the solar analog alpha Men A (T-eff = 5569 +/- 62 K, R-star = 0.960 +/- 0.016 R-circle dot, M-star = 0.964 +/- 0.045 M-circle dot). To characterize the fully convective M dwarf companion, we derive empirical relations to estimate mass, radius, and temperature given the absolute Gaia magnitude and metallicity, yielding M-star = 0.169 +/- 0.006 M (circle dot), R-star = 0.19 +/- 0.01 R-circle dot, and T-eff = 3054 +/- 44 K. Our asteroseismic age of 6.2 +/- 1.4 (stat) +/- 0.6 (sys) Gyr for the primary places alpha Men B within a small population of M dwarfs with precisely measured ages. We combined multiple ground-based spectroscopy surveys to reveal an activity cycle of P = 13.1 +/- 1.1 yr for alpha Men A, a period similar to that observed in the Sun. We used different gyrochronology models with the asteroseismic age to estimate a rotation period of similar to 30 days for the primary. Alpha Men A is now the closest (d = 10 pc) solar analog with a precise asteroseismic age from space-based photometry, making it a prime target for next-generation direct-imaging missions searching for true Earth analogs., National Science Foundation under the Graduate Research Fellowship Program [DGE 1842402]; Alfred P. Sloan Foundation; National Aeronautics and Space Administration [80NSSC18K1585, 80NSSC19K0379]; National Science Foundation [AST-1717000]; NASA FINESST award [80NSSC19K1424]; MICINN project [PRPPID2019-108709GBI00]; Independent Research Fund Denmark [7027-00096B]; Carlsberg Foundation [CF19-0649, CF17-0760]; Australian Research Council [DP210103119]; UK Science and Technology Facilities Council (STFC) [ST/R0023297/1]; PLATO CNES grant; GOLF CNES grant; Danish National Research Foundation [DNRF106]; Spanish Ministry of Science and Innovation; Ramon y Cajal fellowship [RYC-201517697]; NASA [80NSSC20K0458, 80NSSC20K0515]; XSEDE [TG-AST090107]; NSF [ACI-1663696, AST1716436, PHY-1748958]; NASA Exoplanet Exploration Program; [PID2019-107187GB-I00], We acknowledge the traditional owners of the land on which the Anglo-Australian Telescope stands, the Gamilaraay people, and pay our respects to elders past, present, and emerging. The authors would like to thank the staff at the Gemini South Observatory for follow-up observations. A.C. acknowledges support from the National Science Foundation under the Graduate Research Fellowship Program (DGE 1842402). D.H. acknowledges support from the Alfred P. Sloan Foundation, the National Aeronautics and Space Administration (80NSSC18K1585, 80NSSC19K0379), and the National Science Foundation (AST-1717000). T.A.B. acknowledges support from a NASA FINESST award (80NSSC19K1424). A.S. is partially supported by MICINN project PRPPID2019-108709GBI00. V.S.A. acknowledges support from the Independent Research Fund Denmark (research grant 7027-00096B) and the Carlsberg Foundation (grant agreement CF19-0649). T.R.B. acknowledges support from the Australian Research Council (DP210103119). W.H.B., W.J.C., and M.B.N. thank the UK Science and Technology Facilities Council (STFC) for support under grant ST/R0023297/1. R.A.G. acknowledges the support of the PLATO and GOLF CNES grants. M.S.L. is supported by the Carlsberg Foundation (grant agreement No. CF17-0760). Funding for the Stellar Astrophysics Centre is provided by the Danish National Research Foundation (grant DNRF106). S.M. acknowledges support from the Spanish Ministry of Science and Innovation with Ramon y Cajal fellowship No. RYC-201517697 and from grant No. PID2019-107187GB-I00. T.S.M. acknowledges support from NASA grant 80NSSC20K0458. Computational time at the Texas Advanced Computing Center was provided through XSEDE allocation TG-AST090107. R.H. D.T. acknowledges support from NSF grants ACI-1663696, AST1716436, and PHY-1748958 and NASA grant 80NSSC20K0515. Some of the observations in the paper made use of the highresolution imaging instrument Zorro obtained under Gemini LLP proposal No. GN/S-2021A-LP-105. Zorro was funded by the NASA Exoplanet Exploration Program and built at the NASA AMES Research Center by Steve B. Howell, Nic Scott, Elliott P. Horch, and Emmett Quigley. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

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 ejections. Here we report an optical spectroscopic and photometric observation of a long-duration superflare on the young solar-type star EK Draconis (50-120 Myr age) with the Seimei telescope and $Transiting$ $Exoplanet$ $Survey$ $Satellite$ ($TESS$). The flare energy 2.6$\times$10$^{34}$ erg and white-light flare duration 2.2 hr are much larger than those of the largest solar flares, and this is the largest superflare on a solar-type star ever detected by optical spectroscopy. The H$\alpha$ emission profile shows no significant line asymmetry, meaning no signature of a filament eruption, unlike the only previous detection of a superflare on this star (Namekata et al. 2021, $Nat.Astron$). Also, it did not show significant line broadening, indicating that the non-thermal heating at the flare footpoints are not essential or that the footpoints are behind the limb. The time evolution and duration of the H$\alpha$ flare are surprisingly almost the same as those of the white-light flare, which is different from general M-dwarf (super-)flares and solar flares. This unexpected time evolution may suggest that different radiation mechanisms than general solar flares are predominant, as follows: (1) radiation from (off-limb) flare loops, and (2) re-radiation via radiative backwarming, in both of which the cooling timescales of flare loops could determine the timescales of H$\alpha$ and white light., Comment: 13 pages, 4 figures, 1 table. Accepted for publication in The Astrophysical Journal Letters

Until the last few decades, investigations of stellar interiors had been restricted to theoretical studies only constrained by observations of their global properties and external characteristics. However, in the last thirty years the field has been revolutionized by the ability to perform seismic investigations of stellar interiors. This revolution begun with the Sun, where helioseismology has been yielding information competing with what can be inferred about the Earth's interior from geoseismology. The last two decades have witnessed the advent of asteroseismology of solar-like stars, thanks to a dramatic development of new observing facilities providing the first reliable results on the interiors of distant stars. The coming years will see a huge development in this field. In this review we focus on solar-type stars, i.e., cool main-sequence stars where oscillations are stochastically excited by surface convection. After a short introduction and a historical overview of the discipline, we review the observational techniques generally used, and we describe the theory behind stellar oscillations in cool main-sequence stars. We continue with a complete description of the normal mode analyses through which it is possible to extract the physical information about the structure and dynamics of the stars. We then summarize the lessons that we have learned and discuss unsolved issues and questions that are still unanswered., Invited review article for Living Reviews in Solar Physics. 91 pages and 46 figures

Energetic flares and associated coronal mass ejections (CMEs) from young magnetically active solar-like stars can play a critical role in setting conditions for atmospheric escape as well aspenetration of accelerated particles into their atmospheres that promotes formation of biologically relevant molecules. We have used the observationally reconstructed magnetic field of the 0.7 Gyr-young Suns twin, k1 Ceti, to study the effects of CME deflections in the magnetic corona of the young Sun and their effects on the impact frequency on the early Venus, Earth and Mars. We find that the coronal magnetic field deflects the CMEs toward the astrospheric current sheet (ACS). This effect suggests that CMEs tend to propagate within a small cone about the ecliptic plane increasing the impact frequency of CMEs with planetary magnetospheres near this plane to ~ 30 percent or by a factor of 6 as compared to previous estimate by Airapetian et al. 2016. Our model has important implications for the rise of prebiotic chemistry on early terrestrial planets as well as terrestrial type exoplanets around young G-K dwarfs., Comment: 13 pages, 3 figures, accepted for publication in The Astrophysical Journal Letters

The Sun has a steady 11-year cycle in magnetic activity most well-known by the rising and falling in the occurrence of dark sunspots on the solar disk in visible bandpasses. The 11-year cycle is also manifest in the variations of emission in the Ca II H & K line cores, due to non-thermal (i.e. magnetic) heating in the lower chromosphere. The large variation in Ca II H & K emission allows for study of the patterns of long-term variability in other stars thanks to synoptic monitoring with the Mount Wilson Observatory HK photometers (1966-2003) and Lowell Observatory Solar-Stellar Spectrograph (1994-present). Overlapping measurements for a set of 27 nearby solar-analog (spectral types G0-G5) stars were used to calibrate the two instruments and construct time series of magnetic activity up to 50 years in length. Precise properties of fundamental importance to the dynamo are available from Hipparcos, the Geneva-Copenhagen Survey, and CHARA interferometry. Using these long time series and measurements of fundamental properties, we do a comparative study of stellar "twins" to explore the sensitivity of the stellar dynamo to small changes to structure, rotation, and composition. We also compare this sample to the Sun and find hints that the regular periodic variability of the solar cycle may be rare among its nearest neighbors in parameter space., Comment: 5 pages, 2 figures. To appear in the proceedings of "The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun" (ed. G. A. Feiden), Uppsala, Sweden, 06-10 June 2016

This thesis is divided in two main parts, oriented toward planet formation inside circumstellar disks around young solar analogs. First, I will present results obtained using Spitzer/IRS mid-infrared spectroscopic data. These spectra (5-35µm) display a large number of emission features, attributed to silicate dust grains, either in their amorphous or crystalline form. Based on large sample, I led two complementary statistical studies concerning the dust grains mineralogy. First, I show that crystalline grains are widespread inside circumstellar disks, and I show a so-called "crystallinity paradox" concerning the radial distribution of crystals, less crystals being detected in the inner regions (place of preferred formation) compared to outer regions. Amorphization processes can be responsible for these differences. Second, I also show a flattening of grain size distributions, taking place inside circumstellar disks (which was usually interpreted as grain growth). One explanation could be a change in the equilibrium between coagulation and fragmentation. In the second part of this thesis, I present results of pioneering interferometric observations, with the AMBER instrument, for two TTauri stars. These observations enable us to confirm, and refine, two existing models: i) the presence of a gap inside a transitional disk, with an inner disk close to the star, ii) the importance of scattered light when interpreting interferometric data. These works help us better understanding and constraining planet-forming regions.; Cette thèse se découpe en deux volets complémentaires, orientés sur la formation planétaire dans les disques autour d'étoiles jeunes. Dans une première partie je présente les résultats obtenus avec des données spectroscopiques du satellite Spitzer. Ces spectres (5-35 µm) montrent un grand nombre de raies en émission, attribuées à des poussière silicatées, amorphes ou cristallines. Sur un large échantillon, j'ai mené deux analyses statistiques complémentaires sur la minéralogie des grains de poussière. Je montre tout d'abord que les silicates cristallins sont très présents dans les disques proto-planétaires, et je mets en évidence un paradoxe de cristallinité sur la distribution radiale de ces cristallins, avec moins de cristaux dans les régions internes (lieu de leur formation préférentiel) que dans les régions externes. Des processus d'amorphisation peuvent être responsables de cet écart. Par la suite, j'ai montré qu'un applatissement des distributions en taille de grain a lieu dans les disques (ce qui était souvent interprété comme un grossissement de grains). Cela peut s'expliquer soit par un changement d'équilibre entre les processus de fragmentation et de coagulation, soit par une évacuation des petits grains par la pression de radiation ou les vents stellaires. Dans une seconde partie, je présente les résultats d'observations pionières, par la méthode d'interférométrie dans le proche infrarouge, avec l'instrument AMBER, pour deux étoiles TTauri. Ces observations ont permis de confirmer deux modèles existants dans la litérature : i) la présence d'un sillon dans un disque en transition, avec un disque interne proche de l'étoile, ii) l'importance de la lumière diffusée dans l'interprétation de données interférométriques. Ces travaux permettent de mieux connaître les régions de formation planétaire.

This thesis is divided in two main parts, oriented toward planet formation inside circumstellar disks around young solar analogs. First, I will present results obtained using Spitzer/IRS mid-infrared spectroscopic data. These spectra (5-35µm) display a large number of emission features, attributed to silicate dust grains, either in their amorphous or crystalline form. Based on large sample, I led two complementary statistical studies concerning the dust grains mineralogy. First, I show that crystalline grains are widespread inside circumstellar disks, and I show a so-called "crystallinity paradox" concerning the radial distribution of crystals, less crystals being detected in the inner regions (place of preferred formation) compared to outer regions. Amorphization processes can be responsible for these differences. Second, I also show a flattening of grain size distributions, taking place inside circumstellar disks (which was usually interpreted as grain growth). One explanation could be a change in the equilibrium between coagulation and fragmentation. In the second part of this thesis, I present results of pioneering interferometric observations, with the AMBER instrument, for two TTauri stars. These observations enable us to confirm, and refine, two existing models: i) the presence of a gap inside a transitional disk, with an inner disk close to the star, ii) the importance of scattered light when interpreting interferometric data. These works help us better understanding and constraining planet-forming regions., Cette thèse se découpe en deux volets complémentaires, orientés sur la formation planétaire dans les disques autour d'étoiles jeunes. Dans une première partie je présente les résultats obtenus avec des données spectroscopiques du satellite Spitzer. Ces spectres (5-35 µm) montrent un grand nombre de raies en émission, attribuées à des poussière silicatées, amorphes ou cristallines. Sur un large échantillon, j'ai mené deux analyses statistiques complémentaires sur la minéralogie des grains de poussière. Je montre tout d'abord que les silicates cristallins sont très présents dans les disques proto-planétaires, et je mets en évidence un paradoxe de cristallinité sur la distribution radiale de ces cristallins, avec moins de cristaux dans les régions internes (lieu de leur formation préférentiel) que dans les régions externes. Des processus d'amorphisation peuvent être responsables de cet écart. Par la suite, j'ai montré qu'un applatissement des distributions en taille de grain a lieu dans les disques (ce qui était souvent interprété comme un grossissement de grains). Cela peut s'expliquer soit par un changement d'équilibre entre les processus de fragmentation et de coagulation, soit par une évacuation des petits grains par la pression de radiation ou les vents stellaires. Dans une seconde partie, je présente les résultats d'observations pionières, par la méthode d'interférométrie dans le proche infrarouge, avec l'instrument AMBER, pour deux étoiles TTauri. Ces observations ont permis de confirmer deux modèles existants dans la litérature : i) la présence d'un sillon dans un disque en transition, avec un disque interne proche de l'étoile, ii) l'importance de la lumière diffusée dans l'interprétation de données interférométriques. Ces travaux permettent de mieux connaître les régions de formation planétaire.