K2 Ultracool Dwarfs Survey. III. White Light Flares Are Ubiquitous in M6-L0 Dwarfs

• Published in: The Astrophysical journal Vol. 858; no. 1; pp. 55 - 68
• Main Authors: Paudel, Rishi R., Gizis, John E., Mullan, D. J., Schmidt, Sarah J., Burgasser, Adam J., Williams, Peter K. G., Berger, Edo
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-05-2018; IOP Publishing
• Subjects: Astrophysics; Bolometers; Brightening; Frequency distribution; Low mass stars; Photosphere; Power law; Slopes; Stars; stars: activity; stars: flare; stars: individual (2MASS J12321827-0951502, TRAPPIST-1); Stellar flares; White light
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aab8fe

We report the white light flare rates for 10 ultracool dwarfs using Kepler K2 short-cadence data. Among our sample stars, two have spectral type M6, three are M7, three are M8, and two are L0. Most of our targets are old low-mass stars. We identify a total of 283 flares in all of the stars in our sample, with Kepler energies in the range log EKp ∼ (29-33.5) erg. Using the maximum-likelihood method of line fitting, we find that the flare frequency distribution (FFD) for each star in our sample follows a power law with slope − in the range −(1.3-2.0). We find that cooler objects tend to have shallower slopes. For some of our targets, the FFD follows either a broken power law, or a power law with an exponential cutoff. For the L0 dwarf 2MASS J12321827-0951502, we find a very shallow slope (− = −1.3) in the Kepler energy range (0.82-130) × 1030 erg: this L0 dwarf has flare rates which are comparable to those of high-energy flares in stars of earlier spectral types. In addition, we report photometry of two superflares: one on the L0 dwarf 2MASS J12321827-0951502 and another on the M7 dwarf 2MASS J08352366+1029318. In the case of 2MASS J12321827-0951502, we report a flare brightening by a factor of ∼144 relative to the quiescent photospheric level. Likewise, for 2MASS J08352366+1029318, we report a flare brightening by a factor of ∼60 relative to the quiescent photospheric level. These two superflares have bolometric (ultraviolet/optical/infrared) energies 3.6 × 1033 erg and 8.9 × 1033 erg respectively, while the full width half maximum timescales are very short, ∼2 min. We find that the M8 star TRAPPIST-1 is more active than the M8.5 dwarf 2M03264453+1919309, but less active than another M8 dwarf (2M12215066-0843197).