Statistical properties of superflares on solar-type stars based on 1-min cadence data

We searched for superflares on solar-type stars using Kepler data with 1 min sampling in order to detect superflares with short duration. We found 187 superflares on 23 solar-type stars whose bolometric energy ranges from the order of $10^{32}$ erg to $10^{36}$ erg. Some superflares show multiple peaks with the peak separation of the order of $100$-$1000$ seconds which is comparable to the periods of quasi-periodic pulsations in solar and stellar flares. Using these new data combined with the results from the data with 30 min sampling, we found the occurrence frequency ($dN/dE$) of superflares as a function of flare energy ($E$) shows the power-law distribution ($dN/dE \propto E^{-\alpha}$) with $\alpha \sim -1.5$ for $10^{33}<E<10^{36}$ erg which is consistent with the previous results. The average occurrence rate of superflares with the energy of $10^{33}$ erg which is equivalent to X100 solar flares is about once in 500-600 years. The upper limit of energy released by superflares is basically comparable to a fraction of the magnetic energy stored near starspots which is estimated from the photometry. We also found that the duration of superflares ($\tau$) increases with the flare energy ($E$) as $\tau \propto E^{0.39\pm 0.03}$. This can be explained if we assume the time-scale of flares is determined by the Alfv$\acute{\rm e}$n time.Comment: Accepted for for publication in Earth, Planets and Spac

Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -

We report the latest view of Kepler solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5m telescope spectroscopic observations and Gaia-DR2 data. First, we newly conducted APO3.5m spectroscopic observations of 18 superflare stars found from Kepler 1-min time cadence data. More than half (43 stars) are confirmed to be "single" stars, among 64 superflare stars in total that have been spectroscopically investigated so far in this APO3.5m and our previous Subaru/HDS observations. The measurements of $v\sin i$ (projected rotational velocity) and chromospheric lines (Ca II H\&K and Ca II 8542\AA) support the brightness variation of superflare stars is caused by the rotation of a star with large starspots. We then investigated the statistical properties of Kepler solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates. As a result, the maximum superflare energy continuously decreases as the rotation period $P_{\mathrm{rot}}$ increases. Superflares with energies $\lesssim 5\times10^{34}$ erg occur on old, slowly-rotating Sun-like stars ($P_{\mathrm{rot}}\sim$25 days) approximately once every 2000--3000 years, while young rapidly-rotating stars with $P_{\mathrm{rot}}\sim$ a few days have superflares up to $10^{36}$ erg. The maximum starspot area does not depend on the rotation period when the star is young, but as the rotation slows down, it starts to steeply decrease at $P_{\mathrm{rot}}\gtrsim$12 days for Sun-like stars. These two decreasing trends are consistent since the magnetic energy stored around starspots explains the flare energy, but other factors like spot magnetic structure should also be considered.Comment: 71 pages, 31 figures, 10 tables. Accepted for publication in The Astrophysical Journal (on March 29, 2019