Free access article

A&A 440, L55-L58 (2005)
DOI: 10.1051/0004-6361:200500167

Letter

The first wide ultracool binary dwarf in the field: DENIS-J055146.0-443412.2 (M8.5 + L0)

M. Billères¹, X. Delfosse², J.-L Beuzit², T. Forveille^{3, 2}, L. Marchal² and E. L. Martín<sup>4

1</sup>  European Southern Observatory, Casilla 19001, Santiago 19, Chile
    e-mail: mbillere@eso.org
²  Laboratoire d'Astrophysique de Grenoble, Observatoire de Grenoble, BP 53, 38041 Grenoble, France
³  Canada-France-Hawaii Telescope Corporation, 65-1238 Mamaloha Highway, Kamuela, HI 96743, USA
⁴  Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain

(Received 18 July 2005 / Accepted 29 July 2005)

Abstract
We present observations of a new very low mass field binary, discovered during an infrared imaging survey of 250 DENIS L and very late-M dwarfs. DENIS-J055146.0-443412.2 is an M8.5 + L0 pair, with a physical separation of over 200 AU. This makes it the widest very low mass binary known in the field, by an order of magnitude. Such a system is fragile, and it would not have survived a close encounter with a third body. Its existence demonstrate that some very low mass stars/brown dwarfs form without ejection from a multiple system, or any other strong dynamical interaction.

Key words: stars -- brown dwarfs -- binary system -- formation

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