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Astron. Astrophys. 327, L29-L32 (1997)

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Letter to the Editor

Keck HIRES spectra of the brown dwarf DENIS-P J1228.2-1547 *

E.L. Martín 1, 2, G. Basri 1, X. Delfosse 3 and T. Forveille 3

1 Astronomy Department, University of California, Berkeley, CA 94720
2 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
3 Laboratoire d'Astrophysique, Observatoire de Grenoble, 414 rue de la piscine, B.P.53X, F-38041 Grenoble cedex, France

Received 31 July 1997 / Accepted 19 August 1997

Abstract

We report Keck high-resolution echelle spectroscopic observations of two very cool field dwarfs discovered by the near-IR photometric survey DENIS. DENIS-P J1228.2-1547 shows a conspicuous LiI resonance line that confirms it as an incontrovertible brown dwarf (BD). From the presence of Li, and its low surface temperature, we estimate from theoretical models a mass and an age upper limit of [FORMULA] 60 jupiters and [FORMULA] 109 years. The other DENIS object shows no detectable LiI line in our data, and thus we infer a mass [FORMULA] 60 jupiters for it. It could be a high-mass BD or very low-mass star. Both objects have modest radial velocities that suggest they are kinematically young. They show the strongest and broadest resonance line profiles from low ionization species ever seen. The extreme breadth of the KI lines are good further indicators of the expected high gravity in very low-mass dwarfs. We suggest a new spectral class, "L", for objects cooler than M-type (as these are) that do not show TiO molecular bands. The confirmation of at least one field BD in only [FORMULA] 1% of the final DENIS survey is a strong indication of the presence of a numerous population of these objects in the solar neighborhood.

Key words: stars: abundances, late-type, low-mass, brown dwarfs, luminosity function, mass function

* Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the Californian Institute of Technology.

Send offprint requests to: E.L. Martín

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Online publication: April 6, 1998
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