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Astron. Astrophys. 322, 455-459 (1997)

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1. Introduction

Brown dwarfs (BDs) are stellar-like objects. The only difference from ordinary stars is that the mass is too low to bring up the central temperature to the level of stable hydrogen burning, thus the BD luminosity decreases with time. As an example, from 70 Myr to 10 Gyr, a 0.08 [FORMULA] object at the hydrogen burning limit would decrease a factor 15 in luminosity, while a 0.06 [FORMULA] would go a factor 700 (Burrows et al. 1993). The ideal target for a BD search thus is a fairly young, nearby and rich star cluster. The Pleiades is the obvious choice in the northern hemisphere, being at [FORMULA] pc and 70-120 Myr old. Several recent authors have proposed an age above 100 Myr. In this paper 120 Myr is adopted. The mean distance modulus of the cluster used is 5.53 (see Basri et al. 1996 for a discussion).

Whether BDs could be a significant part of the local dark matter is a subject of controversy. Several recent photometric surveys have found a significant drop in the luminosity function from [FORMULA] to [FORMULA], leading to a turnover in the initial mass function (IMF, [FORMULA] (m = mass; n = IMF-index)) at [FORMULA] 0.3 [FORMULA] (see e.g. Gould et al. 1996; Tinney 1993) or a continued rise towards the hydrogen burning limit (see e.g. Kirkpatrick et al. 1994) depending on the choice of mass-luminosity relation. Kroupa (1995) showed that the difference between the nearby stellar luminosity function (LF), measured by parallax and the more distant LF, measured by photometry alone, can be explained by undetected binary companions in the distant sample. From a recently derived mass-luminosity relation (Chabrier et al. 1996) and the well-known photometric LF (see e.g. Gould et al. 1996), Mera et al. (1996) concluded that the IMF for low mass stars continues to rise to the hydrogen burning limit. However the number of known field stars at the low mass end of the main sequence is small. To clearify this item it is necessary to discover more low mass stars and BDs, preferrably at a known distance and age.

In Sect. 2, observations, reductions and a short discussion on photometry and completeness limits is given. In Sect. 3, the extraction of Pleiades members is described. Sect. 4 discusses contamination and overall observing strategy. In Sect. 5 the implications of this paper on the IMF-index is discussed and compared to other authors.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998