REEXAMINING THE LITHIUM DEPLETION BOUNDARY IN THE PLEIADES AND THE INFERRED AGE OF THE CLUSTER

Moderate-dispersion (R similar to 5400), optical spectroscopy of seven brown dwarf candidate members of the Pleiades was. obtained using the Echellette Spectrograph and Imager on the Keck II telescope. The proper motion and photometrically selected sample lies on the single-star main sequence of the cluster and effectively brackets the established lithium depletion boundary. The brown dwarf candidates range in spectral type from M6 to M7, implying effective temperatures between similar to 2800 and 2650 K. All sources exhibit Ha emission, consistent with enhanced chromospheric activity that is expected for young, very low-mass stars and brown dwarfs. Li I lambda 6708 absorption is confidently detected in the photospheres of two of the seven sources. A revised lithium depletion boundary is established in the near-infrared where the effects of extinction and variability are minimized. This lithium depletion edge occurs near K-o = 14.45 or M-K = 8.78 mag (UKIRT Infrared Deep Sky Survey), assuming the most accurate and precise distance estimate for the cluster of 136.2 pc. From recent theoretical evolutionary models, a revised age of tau = 112 +/- 5 Myr is determined for the Pleiades. Accounting for the effects of magnetic activity on the photospheres of these very low-mass stars and brown dwarfs, however, would imply an even younger age for the cluster of similar to 100 Myr.