10:45 AM - 11:07 AM
[PEM10-07] Spectroscopic and photometric observations of superflares and filament eruption on cool stars
★Invited Papers
Keywords:stellar flares, superflares, stellar mass ejection, Seimei telescope, optical spectroscopic observations
Solar and stellar flares are caused by the release of magnetic energy on the surfaces. Solar flares produce strong emissions from radio to X-rays and mass ejections (known as filament eruptions or coronal mass ejections), which often affect the Earth's environment. In the case of stars other than the Sun, very large flares called `superflares' (more than 10 times the most energetic solar flares) have been observed not only on M-dwarfs and binaries but also on solar-type stars (G-type main-sequence stars). These can produce much larger X-ray/UV emission and huge mass ejection than those of solar flares, severely affecting the exoplanet habitability. Therefore, stellar superflares are getting more and more attention in terms of the exoplanet habitability as well as possible extreme space weather events on the current Sun (Maehara et al. 2012, Airapetian et al. 2020). The gigantic superflares are expected to share the same physics as solar flares, but the properties are not well known mostly because of the difficulty in observations.
Recently, we have started monitoring observations of stellar flares with our new 3.8m Seimei Telescope at Okayama Observatory in Japan (the largest optical and infrared telescope in East Asia), simultaneously with space/ground optical photometry (e.g. TESS). In this talk, I introduce interesting properties of the observed superflares on a solar-type star (G-dwarf) and an M-dwarf. First, we detected two superflares on an active young solar-type star named EK Draconis. Interestingly, one of them shows clear evidence for a stellar filament eruption associated with a superflare. After the superflare with radiated energy of 2.0×1033 erg, a blue-shifted Hα absorption component with a velocity of -510 km s-1 appeared. The temporal changes in the spectra greatly resemble those of solar filament eruptions and the erupted mass of 1.1×1018 g corresponds to those predicted from solar flare-energy/CME-mass relation. These discoveries imply that a huge stellar filament eruption occurred possibly in the same way as solar ones (Namekata et al. submitted). This fast eruption can lead to a coronal mass ejection, which enables further studies to estimate its impacts on planets and stellar mass loss.
We also detected one superflare on an active M-dwarf named AD Leonis. The time-resolved Hα line of the superflare shows symmetric line broadenings of 14 Å in association with the large white-light flare (2.0×1033 erg). The line broadening indicates very high electron density in the flaring chromosphere (cf. pressure or opacity broadening). The dense regions can be generated by flare heating of non-thermal high-energy particles that can penetrate into the lower chromosphere. We modeled the flaring M-dwarf atmosphere with the RADYN code (e.g. Kowalski et al. 2017) and found that the energy injection of high-energy particles with the flux of 1012 erg/s/cm2 can well reproduce the observed Hα spectra (Namekata et al. 2020b, PASJ). The energy flux is much higher than the solar ones, which indicates the very efficient particle acceleration in the superflare. On the other hand, the superflare on M-dwarf and another superflare on G-dwarf did not show any evidence of stellar filament eruptions. This may indicate the flare/mass-eruption association rates are not 100 % in stellar flares, although it should be noted that our detection is limited to the line-of-sight motions. Our studies provided how to investigate stellar mass eruption and high energy particles on stellar flares with the help of solar physics, and further solar-stellar connection will deepen our understanding of extreme events not only on active stars but also on the Sun.
Recently, we have started monitoring observations of stellar flares with our new 3.8m Seimei Telescope at Okayama Observatory in Japan (the largest optical and infrared telescope in East Asia), simultaneously with space/ground optical photometry (e.g. TESS). In this talk, I introduce interesting properties of the observed superflares on a solar-type star (G-dwarf) and an M-dwarf. First, we detected two superflares on an active young solar-type star named EK Draconis. Interestingly, one of them shows clear evidence for a stellar filament eruption associated with a superflare. After the superflare with radiated energy of 2.0×1033 erg, a blue-shifted Hα absorption component with a velocity of -510 km s-1 appeared. The temporal changes in the spectra greatly resemble those of solar filament eruptions and the erupted mass of 1.1×1018 g corresponds to those predicted from solar flare-energy/CME-mass relation. These discoveries imply that a huge stellar filament eruption occurred possibly in the same way as solar ones (Namekata et al. submitted). This fast eruption can lead to a coronal mass ejection, which enables further studies to estimate its impacts on planets and stellar mass loss.
We also detected one superflare on an active M-dwarf named AD Leonis. The time-resolved Hα line of the superflare shows symmetric line broadenings of 14 Å in association with the large white-light flare (2.0×1033 erg). The line broadening indicates very high electron density in the flaring chromosphere (cf. pressure or opacity broadening). The dense regions can be generated by flare heating of non-thermal high-energy particles that can penetrate into the lower chromosphere. We modeled the flaring M-dwarf atmosphere with the RADYN code (e.g. Kowalski et al. 2017) and found that the energy injection of high-energy particles with the flux of 1012 erg/s/cm2 can well reproduce the observed Hα spectra (Namekata et al. 2020b, PASJ). The energy flux is much higher than the solar ones, which indicates the very efficient particle acceleration in the superflare. On the other hand, the superflare on M-dwarf and another superflare on G-dwarf did not show any evidence of stellar filament eruptions. This may indicate the flare/mass-eruption association rates are not 100 % in stellar flares, although it should be noted that our detection is limited to the line-of-sight motions. Our studies provided how to investigate stellar mass eruption and high energy particles on stellar flares with the help of solar physics, and further solar-stellar connection will deepen our understanding of extreme events not only on active stars but also on the Sun.