Superflares on Solar-Type Stars Observed with Kepler II. Photometric Variability of Superflare-Generating Stars : A Signature of Stellar Rotation and Starspots

TL;DR

This study analyzes Kepler data to link superflare activity on solar-type stars with stellar rotation and starspots, revealing that larger starspots and slower rotation still produce energetic superflares, similar to solar flares.

Contribution

It demonstrates that superflare energies correlate with starspot coverage and rotation period, providing insights into magnetic energy storage in stellar atmospheres.

Findings

Superflares are associated with starspots and stellar rotation.

Slower rotating stars can produce energetic superflares.

Superflare energy correlates with starspot coverage.

Abstract

We performed simple spot-model calculations for quasi-periodic brightness variations of solar-type stars showing superflares, by using Kepler photometric data. Most of superflare stars show quasi-periodic brightness modulations with the typical period of one to a few tens of days. Our results indicate that these brightness variations of superflare stars can be explained by the rotation of the star with fairly large starspots. Using the result of the period analysis, we investigated the relation between the energy and frequency of superflares and the rotation period. Stars with relatively slower rotation rates can still produce flares that are as energetic as those of more rapidly rotating stars, although the average flare frequency is lower for more slowly rotating stars. We found that the energy of superflares are related to the total coverage of starspots. The correlation between the spot coverage and the flare energy in superflares is similar to that in solar flares. These results suggest that the energy of superflares can be explained by the magnetic energy stored around starspots.