Observational evidence for enhanced magnetic activity of superflare stars.

Christoffer Karoff,Antonio Frasca,Yong Zhang,Jianning Fu,Wei Zhang,Jesper Olsen,Yuefei Wang,Peter De Cat,Jianrong Shi,Alfio Bonanno,Mads Faurschou Knudsen,Fadil Inceoglu,Yonghui Hou,Alexandra Fogtmann-Schulz

doi:10.1038/ncomms11058

Abstract

Superflares are large explosive events on stellar surfaces one to six orders-of-magnitude larger than the largest flares observed on the Sun throughout the space age. Due to the huge amount of energy released in these superflares, it has been speculated if the underlying mechanism is the same as for solar flares, which are caused by magnetic reconnection in the solar corona. Here, we analyse observations made with the LAMOST telescope of 5,648 solar-like stars, including 48 superflare stars. These observations show that superflare stars are generally characterized by larger chromospheric emissions than other stars, including the Sun. However, superflare stars with activity levels lower than, or comparable to, the Sun do exist, suggesting that solar flares and superflares most likely share the same origin. The very large ensemble of solar-like stars included in this study enables detailed and robust estimates of the relation between chromospheric activity and the occurrence of superflares.

Highlights

Superflares are large explosive events on stellar surfaces one to six orders-of-magnitude larger than the largest flares observed on the Sun throughout the space age
As we have no X-ray measurements of the Carrington event, it is not clear how large it was compared with the largest flares observed during the space age, which are classified according to their peak X-ray flux
A Kolmogorov–Smirnov test shows that the two distributions are different at the 99.999999983% significance level, comparable to 6s. This result shows that superflare stars generally exhibit stronger activity levels than other solar-like stars, indicating that the superflare stars have larger dynamo actions and that the superflares may be caused by a mechanism similar to the one generating solar flares

Summary

Introduction

Superflares are large explosive events on stellar surfaces one to six orders-of-magnitude larger than the largest flares observed on the Sun throughout the space age. The nature of these has been investigated by a number of authors who analysed periodic brightness modulations in their Kepler light curves[15,16,17,18,19] All these studies show that fast rotating stars are more likely to host superflares, but in general, the measured rotation periods have to be handled with great caution as: it has been shown using simulations that Kepler observations cannot be used to extract rotation periods for stars where the spot life-time is comparable to or shorter than the rotation period, as is the case for the Sun[20]; the Kepler light curves are heavily affected by artefacts on time scales longer than 20 days[21]; a large fraction of the apparently rapid rotating (a few days) stars are likely ellipsoidal binaries[22,23]. The emission in the Ca II H and K lines is measured by the canonical S index, which intensive research over the last 4 decades has shown is related to magnetic activity[26,27]

Methods

Results

Conclusion