Towards the true number of flaring giant stars in the Kepler field. Are there flaring specialities associated with the giant nature?

TL;DR

This study estimates the number of flaring giant stars in the Kepler field, investigates their flare characteristics, and finds that flaring is rare among giants with no strong link to their giant status, suggesting common physics in stellar flares.

Contribution

It provides the first reliable estimate of flaring giant stars in Kepler data and explores their flare properties and potential specialities related to their giant nature.

Findings

Only 61 out of 706 giants confirmed as flaring stars.

Flaring giants constitute about 0.3% of the giant population in Kepler.

No strong correlation between stellar properties and flaring characteristics.

Abstract

We aim to give a reliable estimate of the number of flaring giant stars in the Kepler field. By analyzing the flaring activity of these stars we explore their flare statistics and the released flare energies. The role of oscillation in suppressing magnetic activity is also investigated. On a sample of flaring giant stars we search for flaring specialities which may be associated with the giant nature. We search for flares using the full K1 data on a sample of 706 stars compiled from two lists of flaring giants found in the literature. In the end, we confirm only 61 stars as flaring giants. Among these 61 flaring giants we found only six which also show oscillations; we suggest that a large fraction of the 61 flaring giants are members of spectroscopic binaries which are proven already for 11. Tests are carried out to correct the detection bias at low flare energies for a subsample of 19 further studied, frequently flaring stars. For these 19 stars flare energy distributions and flare frequency diagrams (FFDs) are constructed. The number of detected flares on giant stars correlate only weakly with the rotational periods. The 61 confirmed flaring giant stars make up only about 0.3% of the entire giant star population in the Kepler database, in contrast with previous estimates of about an order higher percentage. No strong correlation was found between the stellar properties and the flaring characteristics. The wide scale of the flaring specialities are hardly related to the giant nature, if at all. This, together with the finding that the observed flare durations correlate with flare energies, regardless of the flare energy level and stellar luminosity class, suggest common background physics in flaring stars, or in other words, a general scaling effect behind.