K2 Ultracool Dwarfs Survey. II. The White Light Flare Rate of Young Brown Dwarfs

Abstract

We use Kepler K2 Campaign 4 short-cadence (one-minute) photometry to measure white light flares in the young, moving group brown dwarfs 2MASS J03350208+2342356 (2M0335+23) and 2MASS J03552337+1133437 (2M0355+11), and report on long-cadence (thirty-minute) photometry of a superflare in the Pleiades M8 brown dwarf CFHT-PL-17. The rotation period (5.24 hr) and projected rotational velocity (45 km s −1 ) confirm 2M0335+23 is inflated ( ) as predicted for a , 24 Myr old brown dwarf β Pic moving group member. We detect 22 white light flares on 2M0335+23. The flare frequency distribution follows a power-law distribution with slope over the range 10 31 to 10 33 erg. This slope is similar to that observed in the Sun and warmer flare stars, and is consistent with lower-energy flares in previous work on M6–M8 very-low-mass stars; taking the two data sets together, the flare frequency distribution for ultracool dwarfs is a power law over 4.3 orders of magnitude. The superflare ( erg) on CFHT-PL-17 shows higher-energy flares are possible. We detect no flares down to a limit of erg in the nearby L5 γ AB Dor moving group brown dwarf 2M0355+11, consistent with the view that fast magnetic reconnection is suppressed in cool atmospheres. We discuss two multi-peaked flares observed in 2M0335+23, and argue that these complex flares can be understood as sympathetic flares, in which fast-mode magnetohydrodynamic waves similar to extreme-ultraviolet waves in the Sun trigger magnetic reconnection in different active regions.