New Brown Dwarfs in the Pleiades Cluster

We present intermediate and low resolution optical spectroscopy (650-915 nm) of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a CCD-based IZ survey covering an area of 1 sq. deg in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs, having spectral types in the range M6-M9. Five out of the seven objects can be considered as Pleiades members on the basis of their radial velocities, Halpha emission and other gravity sensitive atomic features like the NaI doublet at 818.3 and 819.5 nm. According to current evolutionary models the masses of these new objects range from roughly 80 MJup for the hottest in the sample down to 45 MJup for Roque 4, the coolest and faintest confirmed member. These observations prove that the cloud fragmentation process extends well into the brown dwarf realm, suggesting a rise in the initial mass function below the substellar limit.