An optical and near-infrared search for brown dwarfs in the Pleiades cluster

We have carried out a brown dwarf search over an area of 14 x 23 arcmin(2) near the central portion of the Pleiades open cluster in five optical and near-infrared bands (i ', Z , J , H , Ks ) with 10sigma detection limits of i 'similar to 22.0, J similar to 20.0 and Ks similar to 18.5 mag. The surveyed area has large extinction in excess of A(V) = 3 in the Pleiades region. We detected four new brown dwarf candidates from the colour-colour (J -K , i '-J ) and the colour-magnitude (J , i '-K ) diagrams. We estimated their masses as 0.046 M-circle dot down to 0.028 M-circle dot . The least massive one is estimated to have a mass smaller than Roque25 or int-pl-IZ-69, and possibly the lowest-mass object found so far in the Pleiades cluster.