LITHIUM IN BROWN DWARF CANDIDATES - THE MASS AND AGE OF THE FAINTEST PLEIADES STARS

We present high-resolution optical spectroscopy and infrared photometr y of one of the lowest luminosity Pleiades stars, PPL 15. Its cluster membership is strengthened by both its measured radial velocity and H alpha strength. Its reported mass is 0.06 M., based on its I-band lumi nosity and the Pleiades age of 75 Myr as reported by Stauffer, Hamilto n, & Probst in 1994. We confirm its luminosity with JHK photometry. Su ch a low mass for PPL 15 implies that it should currently retain lithi um, unlike all low-mass Pleiades stars tested so far. Our Keck HIRES s pectrum of PPL 15 indeed exhibits the lithium absorption feature with an equivalent width of 0.5 Angstrom. We estimate the likelihood this d etection is spurious to be less than 1%. Thus, PPL 15 passes the lithi um test for brown dwarf status. Calculations of the luminosity as a fu nction of mass and age for very low mass stars, along with the history of lithium depletion, have been provided by Nelson, Rappaport, & Chia ng in 1993. Lithium is depleted in HHJ 3, which is only a little brigh ter than PPL 15. The self-consistent interpretation with both observat ions and theory is that the age of the Pleiades is similar to 115 Myr. If so, the derived mass for PPL 15 increases to similar to 0.078 M., The canonical 75 Myr age was derived from the upper main-sequence turn off, but it substantially increases if core convective overshoot is in cluded. Such mixing could bring the two methods of age determination i nto agreement. It is therefore possible that the ages of young cluster s have generally been underestimated. The luminosity of brown dwarfs i n these clusters would thus have been overestimated.