Discovery of a Candidate for the Coolest Known Brown Dwarf
  We have used multi-epoch images from the Infrared Array Camera on board the
Spitzer Space Telescope to search for substellar companions to stars in the
solar neighborhood based on common proper motions. Through this work, we have
discovered a faint companion to the white dwarf WD 0806-661. The comoving
source has a projected separation of 130", corresponding to 2500 AU at the
distance of the primary (19.2 pc). If it is physically associated, then its
absolute magnitude at 4.5um is ~1 mag fainter than the faintest known T dwarfs,
making it a strong candidate for the coolest known brown dwarf. The combination
of M_4.5 and the age of the primary (1.5 Gyr) implies an effective temperature
of ~300 K and a mass of ~7 M_Jup according to theoretical evolutionary models.
The white dwarf's progenitor likely had a mass of ~2 M_sun, and thus could have
been born with a circumstellar disk that was sufficiently massive to produce a
companion with this mass. Therefore, the companion could be either a brown
dwarf that formed like a binary star or a giant planet that was born within a
disk and has been dynamically scattered to a larger orbit.
