Flares in Open Clusters with K2. I. M45 (Pleiades), M44 (Praesepe) and M67

TL;DR

This study investigates how stellar flare rates and energies vary with age and temperature in open clusters, aiming to assess if flares can serve as reliable indicators of stellar age.

Contribution

The paper introduces an automated flare detection pipeline with synthetic injection testing and applies it to K2 data across three open clusters of different ages, providing new insights into flare activity evolution.

Findings

Flaring activity depends on stellar effective temperature and age.

Flare frequency distributions have similar slopes across clusters, indicating a universal process.

No flares detected in the oldest cluster M67, suggesting activity diminishes with age.

Abstract

The presence and strength of a stellar magnetic field and activity is rooted in a star's fundamental parameters such as mass and age. Can flares serve as an accurate stellar "clock"? To explore if we can quantify an activity-age relation in the form of a flaring-age relation, we measured trends in the flaring rates and energies for stars with different masses and ages. We investigated the time-domain photometry provided by Kepler's follow-up mission K2 and searched for flares in three solar metallicity open clusters with well-known ages, M45 (0.125 Gyr), M44 (0.63 Gyr), and M67 (4.3 Gyr). We updated and employed the automated flare finding and analysis pipeline Appaloosa, originally designed for Kepler. We introduced a synthetic flare injection and recovery subroutine to ascribe detection and energy recovery rates for flares in a broad energy range for each light curve. We collected a sample of 1 761 stars, mostly late-K to mid-M dwarfs and found 751 flare candidates with energies ranging from $4\cdot10^{32}$ erg to $6\cdot10^{34}$ erg, of which 596 belong to M45, 155 to M44, and none to M67. We find that flaring activity depends both on $T_\mathrm{eff}$, and age. But all flare frequency distributions have similar slopes with $\alpha \approx2.0-2.4$, supporting a universal flare generation process. We discuss implications for the physical conditions under which flares occur, and how the sample's metallicity and multiplicity affect our results.