A very cool brown dwarf in UKIDSS DR1

We report the discovery of a very cool brown dwarf, ULAS J003402.77-005206.7 (ULAS J0034-00), identified in the UKIRT Infrared Deep Sky Survey First Data Release. We provide optical, near-infrared, and mid-infrared photometry of the source, and two near-infrared spectra. Comparing the spectral energy distribution of ULAS J0034-00 to that of the T8 brown dwarf 2MASS J04151954-0935066 (2MASS J0415-09), the latest type and coolest well-studied brown dwarf to date, with effective temperature T_eff ~ 750K, we find evidence that ULAS J0034-00 is significantly cooler. First, the measured values of the near-infrared absorption spectral indices imply a later classification, of T8.5. Secondly, the H - [4.49] colour provides an empirical estimate of the effective temperature of 540 < T_eff < 660K (+/- 2σ range). Thirdly, the J- and H-band peaks are somewhat narrower in ULAS J0034-00, and detailed comparison against spectral models calibrated to 2MASS J0415-09 yields an estimated temperature lower by 60 < Δ T_eff < 120K relative to 2MASS J0415-09i.e. 630 < T_eff < 690K (+/- 2σ), and lower gravity or higher metallicity according to the degenerate combination -0.5 < Δ(logg - 2[m/H]) < -0.25 (+/- 2σ). Combining these estimates, and considering systematics, it is likely the temperature lies in the range 600 < T_eff < 700K. Measurement of the parallax will allow an additional check of the inferred low temperature. Despite the low inferred T_eff we find no evidence for strong absorption by NH₃ over the wavelength range 1.51-1.56μm. Evolutionary models imply that the mass and age are in the ranges 15-36 M_Jup and 0.5-8Gyr, respectively. The measured proper motion, of 0.37 +/- 0.07 arcsec yr^-1, combined with the photometrically estimated distance of 14-22 pc, implies a tangential velocity of ~30 kms^-1, a value consistent with expectation for the inferred age. ULAS J0034-00 is significantly bluer than 2MASS J0415-09 in Y - J, so future searches should allow for the possibility that cooler T dwarfs are bluer still.