Superflares investigated with Kepler&TESS photometric data and recent multi-wavelength campaign observations
Abstract
Solar flares are frequent energetic explosions in the solar atmosphere, and are thought to occur by impulsive releases of magnetic energy stored around sunspots. Large solar flares sometimes can have large impacts on our Earth and society (e.g., magnetic storms). Stars other than the Sun also show flares. Many young stars, active M-dwarfs known as flare stars, and close binary stars have ``superflares'', which are flares that have a total energy 10--10$^{6}$ times larger than that of the largest solar flares on the Sun ($\sim$10$^{32}$ erg). These stars generally rotate very fast (Period $\sim$ 2--3 days). In contrast, the Sun slowly rotates (Period $\sim$25 days). Then it had been thought that superflares cannot occur on slowly-rotating G-type main-sequence stars like the Sun. Recently, more than 1000 superflares on solar-type stars (G-type main sequence stars) have been found using the photometric data of Kepler spece telescope (and also TESS satellite). Using these large number of data, it is now possible to do statistical studies of superflares on solar-type stars. First, superflare stars are well characterized by the existence of large starspots on the surface, and their magnetic fluxes can explain well superflare energies. Then, maximum superflare energy continuously decreases as the rotation period increases. Superflares with their energy $\le \sim$ 5$\times$10$^{34}$ erg (a few hundred times larger than the largest solar flares) would occur on old slowly-rotating Sun-like stars (Rotation Period $\sim$25 days) once every 2000-3000 years, while young rapidly-rotating stars with Rotation Period $\sim$ a few days have superflares up to 10$^{36}$ erg. These results presented in this work support that even slowly-rotating stars similar to the Sun can have superflares, considering long-term activity level changes. In addition to these photometric observation results of solar-type superflare stars, cool M-dwarf superflare stars have been investigated in more detail by recent multi-wavelength surveys. For example, Hydrogen chromospheric lines during flares show a lot of blue-shifted profiles, which can give us some hints on dynamics or mass ejections during superflares. In the main part of this review talk, I review the recent statistical results of superflares from Kepler\&TESS data. Then in the latter part, I also briefly discuss the results of recent multi-wavelength campaign observations of M-dwarf superflares, and finally show the future prospects of superflare studies, which are closely related with solar physics and studies of effects on planets.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1750N