Free access article

A&A 482, 961-971 (2008)
DOI: 10.1051/0004-6361:20079317

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

P. Delorme¹, X. Delfosse¹, L. Albert², E. Artigau³, T. Forveille^{1, 4}, C. Reylé⁵, F. Allard⁶, D. Homeier⁷, A. C. Robin⁵, C. J. Willott⁸, M. C. Liu⁴, and T. J. Dupuy<sup>4

1</sup>  Laboratoire d'Astrophysique de Grenoble, Observatoire de Grenoble, Université Joseph Fourier, CNRS, UMR 5571 Grenoble, France
    e-mail: Philippe.Delorme@obs.ujf-grenoble.fr
²  Canada-France-Hawaii Telescope Corporation, 65-1238 Mamaloha Highway, Kamuela, HI 96743, USA
³  Gemini Observatory, Southern Operations Center, Association of Universities for Research in Astronomy, Inc., Casilla 603, La Serena, Chile
⁴  Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822-1839, USA
⁵  Observatoire de Besançon, Institut UTINAM, University of Franche-Comté, CNRS-UMR 6213, BP 1615, 25010 Besançon Cedex, France
⁶  Centre de Recherche Astrophysique de Lyon, UMR 5574 CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 07, France
⁷  Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁸  Physics Department, University of Ottawa, 150 Louis Pasteur, MacDonald Hall, Ottawa, ON K1N 6N5, Canada

(Received 21 December 2007 / Accepted 5 March 2008)

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 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 ~50 15 K cooler. We estimate a temperature of 620 K and gravity of log g ~ 4.75. Evolutionary models translate these parameters into an age of 1-5 Gyr and a mass of 15-30 . We estimate a photometric distance of ~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.

Key words: techniques: spectroscopic -- surveys -- stars: atmospheres -- infrared: stars -- stars: low-mass, brown dwarfs

© ESO 2008

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