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

## TL;DR

This study measures white light flare rates in young brown dwarfs using Kepler K2 data, revealing a power-law distribution of flare energies and insights into magnetic activity suppression in cool atmospheres.

## Contribution

It provides the first detailed flare frequency distribution for young ultracool dwarfs, including detection of a superflare and analysis of complex flares, advancing understanding of magnetic activity in these objects.

## Key findings

- 22 flares detected on 2M0335+23 with a power-law distribution slope of -1.8
- A superflare of 2.6×10^{34} erg observed on CFHT-PL-17
- No flares detected on 2M0355+11 down to 2×10^{30} erg, indicating suppressed magnetic reconnection

## 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 ($R \ge 0.20 R_\odot$) as predicted for a $0.06M_\odot$, 26-Myr old brown dwarf $\beta $Pic moving group member. We detect 22 white light flares on 2M0335+23. The flare frequency distribution follows a power-law distribution with slope $-\alpha = -1.8 \pm 0.2$ 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; taken the two datasets together, the flare frequency distribution for ultracool dwarfs is a power law over 4.3 orders of magnitude. The superflare ($2.6\times10^{34}$ erg) on CFHT-PL-17 shows higher energy flares are possible. We detect no flares down to a limit of $2 \times 10^{30}$ erg in the nearby L$5\gamma$ 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 a fast-mode MHD waves similar to EUV waves in the Sun trigger magnetic reconnection in different active regions.