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We present the first detection of photometric variability of aspectroscopically-confirmed Y dwarf. The Infrared Array Camera on board theSpitzer Space Telescope was used to obtain times series photometry at 3.6 and4.5 microns over a twenty four hour period at two different epochs separated by149 days. Variability is evident at 4.5 um in the first epoch and at 3.6 and4.5 um in the second epoch which suggests that the underlying cause or causesof this variability change on the timescales of months. The second-epoch [3.6]and [4.5] light curves are nearly sinusoidal in form, in phase, have periods ofroughly 8.5 hours, and have semi-amplitudes of 3.5%. We find that a simplegeometric spot model with a single bright spot reproduces these observationswell. We also compare our measured semi-amplitudes of the second epoch lightcurves to predictions of the static, one-dimensional, partly cloudy and hotspot models of Morley and collaborators and find that neither set of models canreproduce the observed [3.6] and[4.5] semi-amplitudes simultaneously. Moreadvanced two- or three-dimensional models that include time-dependent phenomenalike vertical mixing, cloud formation, and thermal relaxation are thereforesorely needed in order to properly interpret our observations.

The First Detection of Photometric Variability in a Y Dwarf: WISE J140518.39+553421.3