Flares in open clusters with K2^⋆

I. M 45 (Pleiades), M 44 (Praesepe), and M 67

¹ Leibniz-Institut für Astrophysik Potsdam, Germany
e-mail: eilin@aip.de
² University of Washington, USA
e-mail: jrad@uw.edu

Received: 8 October 2018
Accepted: 14 December 2018

Abstract

Context. 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”?

Aims. 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.

Methods. 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, M 45 (0.125 Gyr), M 44 (0.63 Gyr), and M 67 (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 sub-routine to ascribe detection and energy recovery rates for flares in a broad energy range for each light curve.

Results. We collect a sample of 1761 stars, mostly late-K to mid-M dwarfs and found 751 flare candidates with energies ranging from 4 × 10³² erg to 6 × 10³⁴ erg, of which 596 belong to M 45, 155 to M 44, and none to M 67. We find that flaring activity depends both on T_eff, and age. But all flare frequency distributions have similar slopes with α ≈ 2.0−2.4, 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.