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Astron. Astrophys. 327, L25-L28 (1997)

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1. Introduction

The DEep Near-Infrared Survey (DENIS) is a southern sky survey (Copet et al. 1997), which will provide full coverage of the southern hemisphere in two near-infrared bands (J and K [FORMULA]) and one optical band (I). The approximate 3- [FORMULA] limits of the survey are I=18.5, J=16.5, K=13.5. The major DENIS products will be databases of calibrated images, extended sources, and small objects. The survey started in January 1996 and is expected to be completed within five years. As of April 1997 some 30% of the sky has been observed. One area of research in which such a survey will clearly have a profound impact is the study of brown dwarfs. With a 50% completeness limit of I=18.5, and coverage of the whole southern sky, DENIS is uniquely sensitive to this class of object. Moreover, the optical-infrared I-J and I-K colours provided by DENIS are sensitive probes of effective temperature for very cool objects, so that they can be easily selected from the DENIS catalogues.

Over the last two years a brown dwarf has been found orbiting the nearby star Gl 229 (Gliese 229B, Nakajima et al. 1995), and a number of free floating ones have been identified in the Pleiades cluster (Rebolo et al. 1995; Basri et al. 1996). So brown dwarfs have moved from the realm of abstract theoretical construction, into observational astronomy. However, important issues remain to be addressed.

The two coolest known dwarfs, GD 165B (Becklin & Zuckerman 1988) and GL 229B, have been found as companions to brighter nearby stars. The technique of "looking for things around other things" has therefore been very successful, but unfortunately does not directly determine the local brown dwarf density. The relation between the mass distributions in binaries and in the field is a matter of lively debate (e.g. Kroupa 1995, and Reid & Gizis 1997). Low mass brown dwarfs have also been found in the Pleiades, where recent surveys (Zapaterio Osorio et al. 1997a; 1997b) suggest a rising mass function, [FORMULA] (Martín et al. 1997a). Brown dwarfs may be thus a numerous and dynamically important galactic disk population. However, the IMF in a particular young cluster may not be representative of the disk altogether. Moreover, their present mass function may also have been affected by cluster evaporation, which is a mass sensitive process. It is clearly essential to search for brown dwarfs in the field, which is precisely what DENIS will do.

We present here three objects discovered by DENIS, which are significantly cooler than the coolest known isolated field dwarf star - 2MASP J0345 - which was found with the 2 Micron All Sky Survey proto-type camera (Kirkpatrick et al. 1997a). Soon after the discovery of the three objects discussed in this paper was first announced (Delfosse et al. 1997b), Ruiz (1997) presented observations of Kelu 1, a similar field brown dwarf.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998