K2 Ultracool Dwarfs Survey. IV. Monster Flares Observed on the Young Brown Dwarf CFHT-BD-Tau 4

Abstract

We present photometric measurements of two superflares observed on a very young brown dwarf CFHT-BD-Tau 4, observed during Campaign 13 of the Kepler K2 mission. The stronger of the two superflares brightened by a factor of ~48 relative to the quiescent photospheric level, with an increase in Kepler magnitude ΔK~p = -4.20. It has an equivalent duration of ~107 hour, a flare duration of 1.7 day, and an estimated total bolometric (ultraviolet/optical/infrared) energy up to 2.1 ✕10³⁸ erg. The weaker of the two superflares is a complex (multipeaked) flare with an estimated total bolometric (UV/optical/IR) energy up to 4.7 ✕10³⁶ erg. They are the strongest flares observed on any brown dwarf so far. The flare energies are strongly dependent on the value of visual extinction parameter Aᵥ used for extinction correction. If we apply a solar flare-model to interpret the two superflares, we fi nd that the magnetic fi elds are required to be stronger by as much as an order of magnitude than previous reports of fi eld measurements in CFHT-BD-Tau 4 by Reiners et al. (2009b). On the other hand, if we interpret our data in terms of accretion, we find that the requisite rate of accretion for the stronger superflare exceeds the rates which have been reported for other young brown dwarfs.