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Stellar magnetic activity affects exoplanet transit observations. In particular, unocculted spots and flares pose a problem in accurately measuring the depth of planetary transits in each wavelength, which result in misinterpretations of the planetary transmission spectra. Therefore, it is essential to evaluate stellar activities before attempting dedicated planetary characterization. Here, we focus on a young M4.5 dwarf K2-25. It is a well-studied system that shows brightness variations in the K2 light curves due to spots with a rotation period of 1.88 days. We conducted monitoring observations of K2-25 using the LCOGT worldwide ground-based telescopes in g- and z- bands for five months, including the TESS observation period. The derived monitoring light curves showed 1.88 days modulations in all bands. They showed clear differences in the modulation amplitudes, which are useful to constrain spot temperature and spot covering fractions. At the same time as the monitoring observations, we observed six transits of its planet K2-25b with the four-optical-band photometry instruments MuSCAT2 and 3. Although the transit depth variations due to the stellar activity were not detected, considering their uncertainties, the transit depth at each wavelength was useful to rule out the extreme stellar activity., {"references":["Rackham et al. (2018) ApJ 853 122","Mann et al. (2016) ApJ 818 46","Narita et al. (2018) JATIS 015001","Narita et al. (2020) SPIE Proc. 114475K","Kreidberg (2015) PASP 127 1161","Foreman-Mackey et al. (2017) AJ 154 220","Foreman-Mackey et al. (2013) PASP 125 306","Morris et al. (2018), ApJ, 857, 39"]}

Recent Kepler-space-telescope observations found more than 1000 superflares on 300 solar-type stars (e.g., Maeahra+2012 Nature; Shibayama+2013 ApJS). Many of the superflare stars show quasi-periodic brightness variations with the typical period of 1-30 days and the typical amplitude of 0.1-10 percent. We conducted spectroscopic observations of these superflare stars using Subaru/HDS and APO 3.5m telescope (Notsu+2015a&b, 2018 in prep). The projected rotation velocity (v sin i) values are consistent with brightness variation period, and there is a good correlation between Kepler brightness variation amplitude and the intensity of Ca II lines (Ca II H&K, Ca II 8542Å). These results support that the above brightness variations are caused by stellar rotation with large starspots, and existence of large starspots should be a key to understand superflares. More detailed spectroscopic studies (e.g., activity cycle) of superflare stars are important, but Kepler target stars are faint and not appropriate for such detailed studies. TESS satellite, launched in April 2018, brings us a large sample of brighter (e.g., V < 12 mag) superflare stars. We have started spectroscopic monitoring observations of nearby active solar-type stars (superflare candidate stars) in the TESS field. These results can have good collaborations with multi-wavelength project observations (e.g., X-ray, UV, polarimetry) of young solar-type stars., {"references":["Notsu et al. (2015, PASJ, 67, 32)","Notsu et al. (2015, PASJ, 67, 33)"]}

Recent space-based observations (e.g., Kepler mission) found thousands of ``superflares’’ on G-, K-, and M-type stars. These superflare stars show quasi-periodic brightness variations caused by the rotation of the star with starspots. We analyzed the statistical properties of starspots on G-, K-, and M-type stars and their correlation with the flare activity. The analysis shows that the fraction of stars showing large-amplitude rotational variations, which are thought to be the signature of the large starspots, decreases as the rotation period increases. We found that there is a good correlation between the bolometric energy of the largest superflares and area of starspots estimated from the amplitude of rotational light variations. The bolometric energy released by the largest flare on the star is consistent with the magnetic energy stored near the starspots. We also found that the frequency of superflares correlates with the starspot area. The average frequency of flares with a given bolometric energy is roughly proportional to the area of starspots. Our results suggest that flare activity level (e.g., energy of the largest flares, occurrence frequency) of the late-type stars can be determined by the area of starspots., {"references":["Berdyugina, S. V. (2005), Living Reviews in Solar Physics, 2, 8","Candelaresi, S., Hillier, A., Maehara, H., Brandenburg, A., & Shibata, K. (2014), ApJ, 792, 67","Kopparapu, R. K., Ramirez, R., Kasting, J. F., et al. (2013), ApJ, 765, 131","Maehara, H., Shibayama, T., Notsu, S., et al. (2012), Nature, 485, 478","McQuillan, A., Mazeh, T., & Aigrain, S. (2014), ApJS, 211, 24","Shibayama, T., Maehara, H., Notsu, S., et al. (2013), ApJS, 209, 5","Shibata, K., Isobe, H., Hillier, A., et al. (2013), PASJ, 65, 49"]}