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Near-infrared Detection of WD 0806-661 B with the Hubble Space Telescope

WD 0806-661 B is one of the coldest known brown dwarfs (T=300-345 K) based on previous mid-infrared photometry from the Spitzer Space Telescope. In addition, it is a benchmark for testing theoretical models of brown dwarfs because its age and distance are well-constrained via its primary star (2+/-0.5 Gyr, 19.2+/-0.6 pc). We present the first near-infrared detection of this object, which has been achieved through F110W imaging (~Y+J) with the Wide Field Camera 3 on board the Hubble Space Telescope. We measure a Vega magnitude of m110=25.70+/-0.08, which implies J~25.0. When combined with the Spitzer photometry, our estimate of J helps to better define the empirical sequence of the coldest brown dwarfs in M4.5 versus J-[4.5]. The positions of WD 0806-661 B and other Y dwarfs in that diagram are best matched by the cloudy models of Burrows et al. and the cloudless models of Saumon et al., both of which employ chemical equilibrium. The calculations by Morley et al. for 50% cloud coverage differ only modestly from the data. Spectroscopy would enable a more stringent test of the models, but based on our F110W measurement, such observations are currently possible only with Hubble, and would require at least ~10 orbits to reach a signal-to-noise ratio of ~5.