A New Parallax Measurement for the Coldest Known Brown Dwarf
  WISE J085510.83-071442.5 was recently discovered as the coldest known brown
dwarf based on four epochs of images from the Wide-field Infrared Survey
Explorer and the Spitzer Space Telescope. We have improved the accuracy of its
parallax measurement by obtaining two additional epochs of Spitzer astrometry.
We derive a parallactic distance of 2.31+/-0.08 pc, which continues to support
its rank as the fourth closest known system to the Sun when compared to WISE
J104915.57-531906.1 AB (2.02+/-0.02 pc) and Wolf 359 (2.386+/-0.012 pc). The
new constraint on the absolute magnitude at 4.5um indicates an effective
temperature of 235-260 K based on four sets of theoretical models. We also show
the updated positions of WISE J085510.83-071442.5 in two color-magnitude
diagrams. Whereas Faherty and coworkers cited its location in MW2 versus J-W2
as evidence of water clouds, we find that those data can be explained instead
by cloudless models that employ non-equilibrium chemistry.
