Person:

Williams, Peter

We report on the results of fifteen months of monitoring the nearby field L1 dwarf WISEP J190648.47+401106.8 (W1906+40) with the Kepler mission. Supporting observations with the Karl G. Jansky Very Large Array and Gemini North telescope reveal that the L dwarf is magnetically active, with quiescent radio and variable Hα emission. A preliminary trigonometric parallax shows that W1906+40 is at a distance of 16.35+0.36 −0.34 pc, and all observations are consistent with W1906+40 being an old disk star just above the hydrogen-burning limit. The star shows photometric variability with a period of 8.9 hours and an amplitude of 1.5%, with a consistent phase throughout the year. We infer a radius of 0.92 ± 0.07RJ and sin i > 0.57 from the observed period, luminosity (10−3.67±0.03L⊙), effective temperature (2300 ± 75K) , and v sin i (11.2 ± 2.2 km s−1 ). The light curve may be modeled with a single large, high latitude dark spot. Unlike many L-type brown dwarfs, there is no evidence of other variations at the & 2% level, either non-periodic or transient periodic, that mask the underlying rotation period. We suggest that the long-lived surface features may be due to starspots, but the possibility of cloud variations cannot be ruled out without further multi-wavelength observations. During the Gemini spectroscopy, we observed the most powerful flare ever seen on an L dwarf, with an estimated energy of ∼ 1.6 × 1032 ergs in white light emission. Using the Kepler data, we identify similar flares and estimate that white light flares with optical/ultraviolet energies of 1031 ergs or more occur on W1906+40 as often as 1-2 times per month.

We present Kepler, Spitzer Space Telescope, Gemini-North, MMT, and Kitt Peak observations of the L1 dwarf WISEP J190648.47+401106.8. We find that the Kepler optical light curve is consistent in phase and amplitude over the nearly two years of monitoring with a peak-to-peak amplitude of 1.4%. Spitzer Infrared Array Camera 3.6 µm observations are in phase with Kepler with similar light curve shape and peak-to-peak amplitude 1.1%, but at 4.5 µm, the variability has amplitude < 0.1%. Chromospheric Hα emission is variable but not synced with the stable Kepler light curve. A single dark spot can reproduce the light curve but is not a unique solution. An inhomogeneous cloud deck, specifically a region of thick cloud cover, can explain the multi-wavelength data of this ultracool dwarf and need not be coupled with the asynchronous magnetic emission variations. The long life of the cloud is in contrast with weather changes seen in cooler brown dwarfs on the timescale of hours and days.