Superflare occurrence and energies on G-, K- and M-type dwarfs

TL;DR

This study analyzes Kepler data to understand how stellar parameters like temperature and rotation influence superflare occurrence and energy on G-, K-, and M-type stars, suggesting superflares could occur on solar-type stars.

Contribution

It provides new insights into the relationship between stellar rotation, spot coverage, and superflare activity, supported by dynamo theory modeling.

Findings

Superflare rate decreases with increasing effective temperature.

Superflare occurrence rate increases with rotation rate up to a critical point, then decreases.

Dynamo modeling shows similar energy fluctuation patterns as observed in superflares.

Abstract

Kepler data from G-, K- and M-type stars are used to study conditions that lead to superflares with energies above $10^{34} {\rm erg}$. From the 117,661 stars included, 380 show superflares with a total of 1690 such events. We study whether parameters, like effective temperature or the rotation rate, have any effect on the superflare occurrence rate or energy. With increasing effective temperature we observe a decrease in the superflare rate, which is analogous to the previous findings of a decrease in dynamo activity with increasing effective temperature. For slowly rotating stars, we find a quadratic increase of the mean occurrence rate with the rotation rate up to a critical point, after which the rate decreases linearly. Motivated by standard dynamo theory, we study the behavior of the relative starspot coverage, approximated as the relative brightness variation. For faster rotating stars, an increased fraction of stars shows higher spot coverage, which leads to higher superflare rates. A turbulent dynamo is used to study the dependence of the Ohmic dissipation as a proxy of the flare energy on the differential rotation or shear rate. The resulting statistics of the dissipation energy as a function of dynamo number is similar to the observed flare statistics as a function of the inverse Rossby number and shows similarly strong fluctuations. This supports the idea that superflares might well be possible for solar-type G stars.