Abstract
We have obtained intermediate resolution spectra of 11 candidate brown dwarf members of the Pleiades open cluster using the Keck II telescope and Low-Resolution Imaging Spectrograph. Our primary goal was to determine the location of the depletion 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-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 signal-to-noise spectra in the region of the lithium 6708 A 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.
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