A very cool brown dwarf in UKIDSS DR1
Abstract
(abridged) We report the discovery of a very cool brown dwarf, ULAS J003402.77-005206.7 (ULAS J0034-00), identified in UKIDSS DR1. 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 J0415-09, the latest-type and coolest well-studied brown dwarf to date, with Teff~750 K, 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. Second, the H-[4.49] colour provides an empirical estimate of the temperature of 540<Teff<660 K (+/-2sig range). Third, 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 60-120 K relative to 2MASS J0415-09 i.e. 630<Teff<690 K (+/-2sig), and lower gravity or higher metallicity according to the degenerate combination -0.5<delta(log g-2[m/H])<-0.25 (+/-2sig). Combining these estimates, and considering systematics, it is likely the temperature lies in the range 600<Teff<700 K. Despite the low inferred Teff we find no evidence for strong absorption by NH3 over the wavelength range 1.51-1.56 um. Evolutionary models imply that the mass and age are in the ranges 15-36 M(Jup) and 0.5-8 Gyr, respectively. The measured proper motion, of (0.37+/-0.07)arcsec/yr, combined with the photometrically estimated distance of 14-22 pc, implies a tangential velocity of ~30 km/s. 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.
Keywords
Cite
@article{arxiv.0708.0655,
title = {A very cool brown dwarf in UKIDSS DR1},
author = {S. J. Warren and D. J. Mortlock and S. K. Leggett and D. J. Pinfield and D. Homeier and S. Dye and R. F. Jameson and N. Lodieu and P. W. Lucas and A. J. Adamson and F. Allard and D. Barrado y Navascues and M. Casali and K. Chiu and N. C. Hambly and P. C. Hewett and P. Hirst and M. J. Irwin and A. Lawrence and M. C. Liu and E. L. Martin and R. L. Smart and L. Valdivielso and B. P. Venemans},
journal= {arXiv preprint arXiv:0708.0655},
year = {2008}
}
Comments
To appear in MNRAS