CFBDS J005910.90-011401.3: reaching the T-Y brown dwarf transition?

Abstract

Aims. We report the discovery of CFBDS J005910.90-011401.3 (hereafter CFBDS0059), the coolest brown dwarf identified to date. Methods. We found CFBDS0059 using i' and z' 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. Results. A side-to-side comparison of the near-infrared spectra of CFBDS0059 and ULAS J003402.77-005206.7 (hereafter ULAS0034), previously the coolest known brown dwarf, indicates that CFBDS0059 is similar to 50 +/- 15 K cooler. We estimate a temperature of T(eff) ' 620K and gravity of log g similar to 4.75. Evolutionary models translate these parameters into an age of 1-5Gyr and a mass of 15-30 M(Jup). We estimate a photometric distance of similar to 13 pc, 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 ULAS J0034 show 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. Together, CFBDS0059 and ULAS J0034 would then be the first Y0 dwarfs.