We report the discovery of CFBDS J005910.90-011401.3 ( hereafter CFBDS0059 ) , the coolest brown dwarf identified to date .
We found CFBDS0059 using i ^ { \prime } and z ^ { \prime } images from the Canada-France-Hawaii Telescope ( CFHT ) , and present optical and near-infrared photometry , Keck laser guide star adaptive optics imaging , and a complete near-infrared spectrum , from 1.0 to 2.2 \mu m. A side to side comparison of the near-infrared spectra of CFBDS0059 and ULASÂ J003402.77-005206.7 ( hereafter ULAS0034 ) , previously the coolest known brown dwarf , indicates that CFBDS0059 is \sim 50 ~ { } \pm~ { } 15 K cooler .
We estimate a temperature of T _ { \mathrm { eff } } \sim 620 K and gravity of \log g \sim 4.75 .
Evolutionary models translate these parameters into an age of 1-5 Gyr and a mass of 15 - 30 M _ { Jup } .
We estimate a photometric distance of \sim 13pc , which puts CFBDS0059 within easy reach of accurate parallax measurements .
Its large proper motion suggests membership in the older population of the thin disk .
The spectra of both CFBDS0059 and ULASÂ J0034 shows probable absorption by a wide ammonia band on the blue side of the H -band flux peak .
If , as we expect , that feature deepens further for still lower effective temperatures , its appearance will become a natural breakpoint for the transition between the T spectral class and the new Y spectral type .
CFBDS0059 and ULASÂ J0034 would then be the first Y0 dwarfs .