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Flares are frequent energetic explosions in the stellar atmosphere, and are thought to occur by impulsive releases of magnetic energy stored around starspots. Large flares (so called “superflares”) generate strong high energy emissions and coronal mass ejections (CMEs), which can greatly affect the planetary environments. For haracterizing stellar CMEs, understanding of their sources (e.g., flares and erupting prominences) is important. Recent Kepler/TESS photometric data have revealed the statistical properties of superflares on G, K, M-type stars. Superflare stars are well characterized by the existence of large starspots on the surface, and their magnetic fluxes can explain well superflare energies. Flare frequency/energy depends on stellar rotation period and stellar temperature. Young rapidly-rotating stars and cooler stars tend to have frequent flares. Recent spectroscopic observation campaigns have detected some possible candidates of stellar prominence eruptions from G-M stars using blueshifts of chromospheric lines (e.g., H-alpha). They could be the start point for quantitatively discussing physical properties and time evolution of stellar CMEs, and their effects on the environment and habitability of various planets. In the early part of the talk, I briefly overview the recent statistical results of superflares from Kepler/TESS data. Then in the latter part, I discuss the recent reports of stellar prominence eruptions and how these results can help our understanding of stellar CMEs., Invited talk at the Splinter Session "Solar and stellar coronal mass ejections"

M dwarfs are considered one of the most likely places to find extraterrestrial life in part due to their large numbers in the nearby solar neighborhood. However, they have much more intense flaring events than stars like our Sun, which could negatively impact the habitability of close-in exoplanets. Our current understanding of the multi-wavelength connections of M dwarf flaring events is surprisingly far from complete, both in wavelength coverage and temporal resolution. To rectify this, our team collected multi-wavelength data of the dM1e flare star AU Mic over 7-days using a variety of telescopes. Here, we focus on data from XMM-Newton and the Las Cumbres Observatory Global Telescope (LCOGT) network. We discuss the Neupert effect among the X-ray, UV, and optical response in a sample of high-energy flares and present cumulative flare frequency distribution (CFFD) statistics. We find that AU Mic’s U-band CFFD is consistent with other M dwarfs in the literature and that the Neupert effect (i.e. the X-ray derivative peak and NUV peak timings overlap) is not present in all characterized flares. We propose a new Neupert classification system that includes Quasi-Neupert (response in X-ray and NUV, but the timings do not match) and Non-Neupert (missing a response from either X-ray or NUV). Future work on this project includes adding existing AU Mic radio and H-alpha observations to our analysis and using our RADYN flare modeling program to determine the electron beam heating, proton beam heating, and magnetic mirroring needed to reproduce the full range of multi-wavelength responses we see in observations., {"references":["Veronig, Astrid M. et al. (2005). Physics of the Neupert Effect: Estimates of the Effects of Source Energy, Mass Transport, and Geometry Using RHESSI and GOES Data.","Smith, David S. et al. (2004). Importance of Biologically Active Aurora-like Ultraviolet Emission: Stochastic Irradiation of Earth and Mars by Flares and Explosions.","Walker, A. R. (1981). Flare activity of proxima Cen.","Lacy, C. H. (1976). UV Ceti stars: statistical analysis of observational data.","McTiernan, James M. et al. (1999). The Solar Flare Soft X-Ray Differential Emission Measure and the Neupert Effect at Different Temperatures.","Veronig, Astrid M. et al. (2002). Investigation of the Neupert effect in solar flares. I. Statistical properties and the evaporation model.","Carlsson & Stein, (1997). Formation of Solar Calcium H and K Bright Grains.","Allred, Joel C. et al. (2015). A Unified Computational Model for Solar and Stellar Flares.","Gershberg, R. E. (1972). Some results of the cooperative photometric observations of the UV Cet-type flare stars in the years 1967 71.","Mitra-Kraev, U. et al. (2005). Relationship between X-ray and ultraviolet emission of flares from dMe stars observed by XMM-Newton","Kowalski, Adam F. et al (2013). Time-resolved Properties and Global Trends in dMe Flares from Simultaneous Photometry and Spectra."]}

Flares are releases of magnetic energy in the solar/stellar atmosphere, and they have strong emissions from radio to X-rays. During some M dwarf flares, chromospheric line profiles show blue asymmetries (Eason et al. 1992; Honda et al. 2018), although red asymmetries are more commonly observed in solar flares. Similar enhancements of the blue wings of Balmer lines may provide clues for investigating the early phases of stellar coronal mass ejections (CMEs) during flares (cf. Vida et al. 2016&2019), but this is still controversial. Thus, we need more flare spectroscopic observations with high time resolution to understand the relationship between mass ejections and flaring events. The latter is helpful for estimating the impact on planets from flares. We have conducted several simultaneous spectroscopic and photometric observations of mid M dwarf flare stars using APO 3.5m/ARCES, SMARTS 1.5m/CHIRON, TESS, and ground- based 0.4-1m photometric telescopes. During 34 nights of observations, we detected 48 flares in Balmer lines (e.g. H-alpha). Among them, at least 7 flare events show clear blue asymmetries. Blue asymmetry durations are different among the 7 events (20min ~ 2hr). These results suggest upward flows of chromospheric plasma during flare events. By assuming that the blue asymmetries were caused by prominence eruptions, we estimate the mass and kinetic energy of the upward-moving material to be 1015 - 1018 g and 1029 - 1032 erg, respectively. The estimated masses are comparable to expectations from the empirical relation between the flare X-ray energy and mass of upward-moving material for stellar flares and solar CMEs. In contrast, the estimated kinetic energies for these non-white-light flares are roughly 2-3 orders of magnitude smaller than that expected from the relation between flare X-ray energy and kinetic energy for solar CMEs. This could be understood by the difference in the velocity between CMEs and prominence eruptions., {"references":["Eason, E.~L.~E., Giampapa, M.~S., Radick, R.~R., et al.\\ 1992, \\aj, 104, 1161. doi:10.1086/116305","Honda, S., Notsu, Y., Namekata, K., et al.\\ 2018, \\pasj, 70, 62. doi:10.1093/pasj/psy055","Vida, K., Kriskovics, L., Ol{\\'a}h, K., et al.\\ 2016, \\aap, 590, A11. doi:10.1051/0004-6361/201527925","Vida, K., Leitzinger, M., Kriskovics, L., et al.\\ 2019, \\aap, 623, A49. doi:10.1051/0004-6361/201834264","Fuhrmeister, B., Czesla, S., Schmitt, J.~H.~M.~M., et al.\\ 2018, \\aap, 615, A14. doi:10.1051/0004-6361/201732204","Maehara, H., Notsu, Y., Namekata, K., et al.\\ 2021, \\pasj, 73, 44. doi:10.1093/pasj/psaa098","Gopalswamy, N., Shimojo, M., Lu, W., et al.\\ 2003, \\apj, 586, 562. doi:10.1086/367614"]}