Poster Presentation

A Study of the Dependence of the Superflare Frequencies on the Starspots Sizes of Solar-type Stars

Li-Ching Huang, Wing-Huen Ip, and Chi-Ju Wu

The continuous long-term observations of the KEPLER Space Telescope have provided a wealth of information on flare activities of different types of stars. Of particular interest has to do with the occurrence of superflares among some solar-type stars as first reported by Maehara et al.(2012). The radiative energy in optical emission of the largest superflares can reach as much as 10^38 ergs, namely, one million times of the largest solar flares ever observed. Such phenomenon has important implications on the formation and evolution of planetary systems and biospheres of exoplanets and our own Sun. Notsu et al. (2013) have subsequently pointed out that the energy of a superflare could be related to the magnetic energy stored in the high-field region of the star spot responsible for its generation. The flare energy – spot size relation is given by E(flare)~7*10^32[erg]*(f/0.1)*{[(B/(10^3)*G]^2}*(A(spot)/3*10^19cm^2)^3/2, where B is the average spot magnetic field strength and A is the amplitude of the stellar lightcurve due to the star spot. We have examined the dependence of superflare frequency distribution on the spot size and found that stars with smaller A values tend to have less activity of superflares, which is consistent with the model of Notus et al. (2013). However, in a sample of non-flaring solar-type stars we have also found a small fraction of them with very large A values but without superflares. It is interesting to note that similar behavior has been found in the case of the host star of the hot Jupiter Kepler-17b.

Maehara, Shibayama, Notsu et al., 2012, Nature, doi: 10.1038
Notsu, Shibayama, Maehara et al., 2013, ApJ, 771, 127
Shibata, Isobe, Hillier et al., 2013, PASJ, vol. 65, No. 3