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 (R ≥ 0.20R ) as predicted for a 0.06M , 26-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 −α = −1.8 ± 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 × 10 34 erg) on CFHT-PL-17 shows higher energy flares are possible. We detect no flares down to a limit of 2 × 10 30 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 a fast-mode MHD waves similar to EUV waves in the Sun trigger magnetic reconnection in different active regions.

EXTraS discovery of an X-ray superflare from an L dwarf

“…However, in view of the similarities of the J0331-27 X-ray flare and the X-ray flares on late-M dwarfs described above, we can use the observed optical-to-X-ray energy ratio of LP412-32 (E opt,F ≈ E X,F ) to estimate an optical counterpart of E opt,F 10 33 erg for the X-ray superflare on J0331-27. Remarkably, other white-light superflares observed on L dwarfs (without simultaneous X-ray data) show flare energies of the same order (Jackman et al 2019, Gizis et al 2017. We note that for a flare on an early M dwarf observed simultaneously with XMM-Newton and Kepler (KIC 8454353; see Pizzocaro et al 2019) we also find that the X-ray and optical flare energy are within a factor of two of each other.…”

K2 Ultracool Dwarfs Survey – V. High superflare rates on rapidly rotating late-M dwarfs

“…In order to assess the habitability of M dwarf planets, it is important to constrain the flare rates of M dwarfs as a function of their masses and ages. Because of this, we have been studying the flare rates of various mid and late-M dwarfs and early-L dwarfs which were observed by the K2 mission (Howell et al 2014) in various campaigns (see for e.g., Gizis et al 2017b;Paudel et al 2018a). Here we present our results on superflares which we have observed on three late-M dwarfs: 2MASS J08315742+2042213 (hereafter 2M0831+2042), 2MASS J08371832+2050349 (hereafter 2M0837+2050) and 2MASS J08312608+2244586 (hereafter 2M0831+2244).…”

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

“…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. Observations of an L5 dwarf (Gizis et al 2017b) not sensitive to late spectral types, this requirement limits observations to a small number of nearby early L dwarfs. Wide-field surveys (e.g.…”