Date : 6/27 (Wed) [15:00 - ] @F313
Speaker: Mr. Yuta Notsu (Kyoto University)
Title : Superflares on solar­-type stars
Abstract: Superflares are flares that release total energy 10-10⁴ times greater than that of the largest solar flares ever observed (~10³² erg). Recent Kepler-space-telescope observations found more than 1000 superflares on a few hundred solar-type (G-type main sequence) stars (e.g., Maehara+2012 Nature; Shibayama+2013 ApJS; Maehara+2015 EpS; Davenport 2016 ApJ). Statistical analyses of such large number of superflares indicate that the frequency distribution to the flare energy of the superflare show power-law distribution (dN/dE ∝ E^a with a~-2) and this distribution is consistent with that of the Sun. Many of the superflare stars show quasi-periodic brightness variations with the typical period of from one to a few tens of days and the typical amplitude of from 0.1 to 10% (Notsu+2013 ApJ). Spectroscopic measurements of the projected rotation velocity (v sin i) and the intensity of chromospheric lines (e.g., Ca II H&K, Ca II 8542Å, Hα6563Å) support that these variations are caused by the rotation of superflare stars with large starspots (Notsu+2015a&2015b PASJ; Karoff+2016 Nature Communications; Notsu+ in prep). On the basis of these results, we can estimate rotation period and starspot coverage from these brightness variations, and discuss stellar properties of superflare stars, as summarized in the following (cf. Shibata+2013 PASJ; Notsu+2013 ApJ; Candelaresi+2014 ApJ; Maehara+2015 EpS; Maehara+2017 PASJ). First, the bolometric energy released by flares is consistent with the magnetic energy stored around such large starspots. The occurrence frequency of superflares depends on the rotation period, and that the flare frequency increases as the rotation period decreases. However, the energy of the largest flares observed in a given period bin does not show any clear correlation with the rotation period, and even Sun-like stars rotating as slow as the Sun (Rotation Period > 20 days) have large starspots and superflares. Moreover, the size distribution of starspots shows the power-law distribution that is on the same line of that of relatively large sunspots. The frequency-energy distributions for flares originating from spots with different sizes are the same for solar-type stars with superflares and the Sun. We also found that the duration of superflares increases with the flare energy as E^(0.39+/-0.03), and similar relation is also found in solar white light flares (Namekata+2017 ApJ in press). This can be explained if we assume the time-scale of flares is determined by the Alfven time. These results suggest that the magnetic activity on solar-type superflare stars and the Sun are caused by the same basic physical processes. Future long-term monitoring of the chromospheric activity and research on possible coronal mass ejections accompanying with superflares will give us an insight on the effects of superflares on (exo)planetary environments, and possible extreme space weather events on the Earth (e.g., Airapetian+2016 Nature geosciences; Lingam&Loeb 2017 ApJ). In this talk, after brief introduction on solar flares, I will briefly review the above recent results and also would like to discuss future research plans with new space and ground-based telescopes.

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