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Superflares on solar-type stars

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Abstract

Solar flares are caused by the sudden release of magnetic energy stored near sunspots. They release 1029 to 1032 ergs of energy on a timescale of hours1. Similar flares have been observed on many stars, with larger ‘superflares’ seen on a variety of stars2,3, some of which are rapidly rotating4,5 and some of which are of ordinary solar type3,6. The small number of superflares observed on solar-type stars has hitherto precluded a detailed study of them. Here we report observations of 365 superflares, including some from slowly rotating solar-type stars, from about 83,000 stars observed over 120 days. Quasi-periodic brightness modulations observed in the solar-type stars suggest that they have much larger starspots than does the Sun. The maximum energy of the flare is not correlated with the stellar rotation period, but the data suggest that superflares occur more frequently on rapidly rotating stars. It has been proposed that hot Jupiters may be important in the generation of superflares on solar-type stars7, but none have been discovered around the stars that we have studied, indicating that hot Jupiters associated with superflares are rare.

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Figure 1: Light curve of typical superflares.
Figure 2: Frequency distribution of superflares on G-type main-sequence stars.
Figure 3: Relations between the brightness variation period and the properties of flares on G-type main-sequence stars.

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Acknowledgements

Kepler was selected as NASA’s tenth Discovery mission. Funding for the mission is provided by the NASA Science Mission Directorate. The data presented in this paper were obtained from the Multimission Archive at STScI. This work was supported by the Grant-in-Aid for the Global COE Program ‘The Next Generation of Physics, Spun from Universality and Emergence’ and the Grant-in-Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We are grateful to K. Sekiguchi and A. Hillier for suggestions.

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Authors and Affiliations

Authors

Contributions

H.M. was responsible for planning, coordination and data analysis. He also wrote the majority of the text. T.S. performed the data analysis for many stellar flares observed by Kepler, and S.N. and Y.N. analysed the rotation of stars observed by Kepler. T.N. and S.K. helped with data analysis and interpretation. S.H. and D.N. contributed to the interpretation of the stellar brightness variation and helped with the data analysis from a general stellar astronomical point of view. K.S. provided theoretical interpretation of the observations and gave advice on the paper’s content.

Corresponding author

Correspondence to Hiroyuki Maehara.

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The authors declare no competing financial interests.

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This file contains Supplementary Discussion and Data, Supplementary Figures 1-8, Supplementary Table 1 and additional references. (PDF 2603 kb)

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Maehara, H., Shibayama, T., Notsu, S. et al. Superflares on solar-type stars. Nature 485, 478–481 (2012). https://doi.org/10.1038/nature11063

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