We combine the results from two CCD surveys covering a large area of the custer at I and Z wavebands. We have obtained follow-up K photometry for many of the numerous brown dwarf candidates discovered in these surveys which we employ as a test for cluster membership. From these data we derive the mass function of the whole Pleiades cluster down to 0.04 M⊙. We emphasise the importance of a careful consideration of the spatial distribution within the cluster and find the core radius for brown dwarfs to be 2±1 parsecs. The contribution of brown dwarfs to the total mass of the cluster is about 1%.

Introduction

The Pleiades has long been recognised as one of the best places to search for brown dwarfs, e.g. Jameson & Skillen (1989), Stauffer et al. (1989, 1994), Simons & Becklin (1992), Rebolo et al. (1995), Cossburn et al. (1997), Zapatero Osorio et al. (1997), Bouvier et al. (1998), Festin (1998).

The cluster is both reasonably close (but not so close as to cover too large an area of the sky) and young, so that brown dwarfs are not too faint. Controversy still rages over the precise distance to the Pleiades, which Hipparcos places significantly closer (at 118 parsecs) than ground based measurements (at typically 133 parsecs). The Hipparcos results have been published by Van Leeuwen & Hansen Ruiz (1997) and Mermilliod et al. (1997) and critically discussed by Pinnsoneault et al. (1998).