A study of the white light flare rates of M and L dwarfs
Date
2019
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
We have used the NASA Kepler K2 mission short cadence (~1 minute) and long cadence (~30 minutes) data to study the white light flare (WLF) rates of (sub)stellar objects with spectral types later than M4. Most of our targets are ultracool dwarfs. We analyzed WLFs of 42 targets using short cadence data and ~350 targets using long cadence data. We identified a total of 1105 WLFs on 22 targets which were observed in short cadence mode. We observe a higher flare rate in the mid-M dwarfs where the stars are believed to become fully convective. The total estimated (UV/visible/IR) energies of flares observed in short and long cadence mode are in the range of log E (erg) ~(29.5-36). The flare rate decreases as the effective temperature decreases. In an energy range of 1029.8 1033.8 erg, the flare energy distributions follow a power law with slopes -α in range -(1.3 - 2.0). The cooler targets tend to have shallower slopes. We suggest that such shallower slopes can be attributed to the reduced electrical conductivity of the atmospheres in the cooler targets. We find that the slopes are independent of the kinematic ages of the targets, and have a mean value of -α equal to -1.7±0.2. Comparing our results with those in literature, we find that the flare energy distributions of ultracool dwarfs have similar properties to those of flares in the Sun and also in stars with spectral types earlier than M5. This is a strong indication that solar and (sub)stellar flares are caused by a universal phenomenon presumably related to the magnetic field strength B in active regions on each star: the total flare energy involves the product of magnetic energy density B2/8π and a volume. We find that there is no significant difference in the maximum energies of flares produced by targets of various spectral types in our sample. This suggests that the upper limit to the flare energy is independent of the effective temperature of the targets. ☐ We find that targets with spectral type as late as L5 can produce white light flares. We observed one large flare on 2MASS J08585891+1804463 (L2) and two superflares (flares with energy in excess of 10$^{33}$ erg) on VVV BD001 (L5). The largest flare on VVV BD001 had an amplitude $>$300 times the quiescent photospheric level: this was the largest amplitude event among all the flares studied here. The estimated energy of this flare is equal to 10$^{34.7}$ erg. The occurrence of superflares on targets with spectral types as late as L5 also suggests that either they have strong magnetic fields comparable to those in the warmer targets or the volumes associated with flares are larger on cooler targets, or a combination thereof. For the superflares observed on an M7 and an L0 dwarf using short cadence data, we find that they have very short full-width-half-maximum (FWHM) timescales of ~2 minutes.