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

Abstract

We report the white light flare rates for 10 ultracool dwarfs (UCDs) 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 E Kp ∼(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 -α in 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 (-α = -1.3) in the Kepler energy range (0.82-130)×10 30 erg: this L0 dwarf has flare rates which are comparable to the rates 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 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 (UV/optical/infrared) energies 3.6 × 10 33 erg and 8.9 × 10 33 erg respectively, while the FWHM time scales are very short, ∼2 minutes. 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).

Detection of a giant white-light flare on an L2.5 dwarf with the Next Generation Transit Survey

“…From this we estimate ∆V ∼ −10 and ∆U ∼ −15. This is consequently the second largest observed white-light flare from an L dwarf, following ASASSN-16ae (∆V=-11 Schmidt et al 2016), and larger than those observed with Kepler (Gizis et al 2013(Gizis et al , 2017aPaudel et al 2018).…”

“…Many ultracool dwarfs which show activity such as radio emission and flaring also tend to be fast rotators, with rotation periods on the order of hours. However, neither the L1 dwarf SDSSp J005406.55-003101.8 (Gizis et al 2017a) nor the L0 dwarf J12321827-0951502 (Paudel et al 2018) showed any sign of rapid rotation when observed by K2, despite showing large amplitude white-light flares. Consequently, we do not attempt to predict whether ULAS J2249-0112 is a fast rotator.…”

“…While seen regularly on GKM stars, observations of white-light flares on L dwarfs remain rare, with only a handful of stars showing them to date (e.g. Paudel et al 2018). However, those observed have included some of the largest amplitude flares ever recorded, reaching up to ∆V ≈ −11 (Schmidt et al 2016).…”

“…This became possible with the Kepler (Borucki et al 2010) and K2 (Howell et al 2014) missions, which allowed continuous high precision monitoring of chosen objects (Gizis et al 2013(Gizis et al , 2017aPaudel et al 2018). Through this, large amplitude flares (∆V ≈ −8, ∆K p = −5.4) were detected on an L0 and L1 dwarf (Gizis et al 2017a;Paudel et al 2018), along with smaller flares on two early L dwarfs (Gizis et al 2013;Paudel et al 2018). Spectroscopy of an L5 dwarf by Liebert et al (2003) showed an Hα flare, although this event lacked the continuum enhancement associated with white light flares.…”

A Catalog of M-dwarf Flares with ASAS-SN

“…Despite the difficulty of characterizing flares on the lowest-mass stars, studies have found them to be common (e.g. Schmidt et al 2014Schmidt et al , 2016Gizis et al 2017;Vida et al 2017;Paudel et al 2018Paudel et al , 2019.…”