The Coldest Brown Dwarf (Or Free Floating Planet)?: The Y Dwarf WISE
  1828+2650
  We have monitored the position of the cool Y dwarf WISEPA J182831.08+265037.8
using a combination of ground- and space-based telescopes and have determined
its distance to be 11.2$_{-1.0}^{+1.3}$ pc. Its absolute H magnitude,
M$_H=22.21^{+0.25}_{-0.22}$ mag, suggests a mass in the range 0.5-20 M$_{Jup}$
for ages of 0.1-10 Gyr with an effective temperature in the range 250-400 K.
The broad range in mass is due primarily to the unknown age of the object.
Since the high tangential velocity of the object, 51$\pm5$ km s$^{-1}$, is
characteristic of an old disk population, a plausible age range of 2-4 Gyr
leads to a mass range of 3-6 M$_{Jup}$ based on fits to the (highly uncertain)
COND evolutionary models. The range in temperature is due to the fact that no
single model adequately represents the 1-5 $\mu$m spectral energy distribution
(SED) of the source, failing by factors of up to 5 at either the short or long
wavelength portions of the spectral energy distribution. The appearance of this
very cold object may be affected by non-equilibrium chemistry or low
temperature condensates forming clouds, two atmospheric processes that are
known to be important in brown dwarf atmospheres but have proven difficult to
model. Finally, we argue that there would have to be a very steep upturn in the
number density of late type Y-dwarfs to account for the putative population of
objects suggested by by recent microlensing observations. Whether WISE
1828+2650 sits at the low mass end of the brown dwarf population or is the
first example of a large number of "free-floating" planets is not yet known.
