Oral Presentation

Superflares on solar-type stars

Hiroyuki Maehara (University of Tokyo), Takuya Shibayama (Kyoto University), Yuta Notsu (Kyoto University), Shota Notsu (Kyoto University), Satoshi Honda (University of Hyogo), Daisaku Nogami (Kyoto University), Kazunari Shibata (Kyoto University)

We present the results of a search for superflares (stellar flares releasing >10^33 ergs of energy) on solar-type (G-type main sequence) stars using the data obtained by the Kepler space telescope. We found more than 1500 flares on solar-type stars. The energy of detected flares ranges from 10^33 to 10^36 erg.
The occurrence frequency distribution of superflares can be fitted by a power-law function with the index of about -2 which is similar to that of solar flares. Moreover, the frequency distribution of superflares on Sun-like stars (solar-type stars with rotation period >10 days and with effective temperature of 5600-6000K) and that of solar flares are roughly on the same power-law line.
Most of superflare stars show quasi-periodic light variations, which are thought to be caused by the rotation of the star with starspots. The frequency of superflares roughly decreases as the square of rotation period for a period range above a few days. On the other hand, the maximum energy of superflares observed in a given period
bin does not depend the rotation period. The energy of superflares correlates with the amplitude of quasi-periodic light variations which corresponds to the total area of starspots.
These results suggest superflares can occur not only on rapidly rotating stars but also on slowly rotating stars like our Sun and the presence of large starspot groups is one of the necessary condition for the generation of superflares.