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

• Published in: The Astrophysical journal Vol. 845; no. 1; pp. 33 - 44
• Main Authors: Gizis, John E., Paudel, Rishi R., Mullan, Dermott, Schmidt, Sarah J., Burgasser, Adam J., Williams, Peter K. G.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10-08-2017; IOP Publishing
• Subjects: Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Brown dwarf stars; brown dwarfs; DISTRIBUTION; DWARF STARS; EXTREME ULTRAVIOLET RADIATION; Flare stars; Fluid flow; Frequency distribution; HYDROMAGNETIC WAVES; Low mass stars; MAGNETIC RECONNECTION; Magnetohydrodynamic waves; MAGNETOHYDRODYNAMICS; MASS; PHOTOMETRY; Power law; ROTATION; Slopes; stars: activity; stars: flare; stars: individual (2MASS J03350208+2342356, 2MASS J03552337+1133437); STARSPOTS; STELLAR ATMOSPHERES; Stellar flares; VELOCITY; White light
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa7da0

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 βPic 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 1031 to 1033 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 L5γ 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.