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
