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 eleven candidate brown dwarf members of the Pleiades open cluster using the Keck II telescope and LRIS spectrograph. Our primary goal was to determine 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 members, have moderately strong Hα emission, and have spectral types M6 to 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 sequence, including only stars for which high S/N spectra in the region of the lithium λ6708Å absorption line have been obtained. These data allow us to accurately determine the Pleiades single-star lithium depletion edge at Ic0 = 17.80, (R-I)c0 = 2.20, spectral type = M6.5. By reference to theoretical evolutionary models, this converts fairly directly into an age for the Pleiades of τ = 125 Myr. This is significantly older than the age that is normally quoted, but does agree with some other recent estimates. Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology