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 Teff ~ 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 <Teff <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 Teff <120K relative to 2MASS J0415-09i.e.
630 <Teff <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 <Teff <700K. Measurement of the
parallax will allow an additional check of the inferred low temperature.
Despite the low inferred Teff 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
MJup 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.
- brown dwarfs
- stars: low-mass