KECK SPECTRA OF PLEIADES BROWN DWARF CANDIDATES AND A PRECISE DETERMINATION OF THE LITHIUM DEPLETION EDGE IN THE PLEIADES

We have obtained intermediate resolution spectra of 11 candidate brown dwarf members of the Pleiades open cluster using the Keck II telescop e and Low-Resolution Imaging Spectrograph. Our primary goal was to det ermine the location of the ''lithium depletion edge'' in the Pleiades and hence to derive a precise age for the cluster. All but one of our 11 program objects have radial velocities appropriate for Pleiades mem bers, have moderately strong H alpha emission, and have spectral types M6-M8.5 as expected from their (R-I)(c) colors. We have constructed a color-magnitude diagram for the faint end of the Pleiades main sequen ce, including only stars for which high signal-to-noise spectra in the region of the lithium 6708 Angstrom absorption line have been obtaine d. These data allow us to accurately determine the Pleiades single-sta r lithium depletion edge at I-CO = 17.80, (R-I)(CO) = 2.20, spectral t ype M6.5. By reference to theoretical evolutionary models, this conver ts fairly directly into an age for the Pleiades of tau = 125 Myr. This is significantly older than the age that is normally quoted, but does agree with some other recent estimates.