Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars

Abstract

We present high-resolution optical spectroscopy and infrared photometry of one of the lowest luminosity Pleiades stars, PPL 15. Its cluster membership is strengthened by both its measured radial velocity and Hα strength. Its reported mass is 0.06 Msun, based on its I-band luminosity and the Pleiades age of 75 Myr as reported by Stauffer, Hamilton, & Probst in 1994. We confirm its luminosity with JHK photometry. Such a low mass for PPL 15 implies that it should currently retain lithium, unlike all low-mass Pleiades stars tested so far. Our Keck HIRES spectrum of PPL 15 indeed exhibits the lithium absorption feature with an equivalent width of 0.5 Å. We estimate the likelihood this detection is spurious to be less than 1%. Thus, PPL 15 passes the lithium test for brown dwarf status. Calculations of the luminosity as a function of mass and age for very low mass stars, along with the history of lithium depletion, have been provided by Nelson, Rappaport, & Chiang in 1993. Lithium is depleted in HHJ 3, which is only a little brighter than PPL 15. The self-consistent interpretation with both observations and theory is that the age of the Pleiades is ∼115 Myr. If so, the derived mass for PPL 15 increases to ∼0.078 Msun. The canonical 75 Myr age was derived from the upper main-sequence turnoff, but it substantially increases if core convective overshoot is included. Such mixing could bring the two methods of age determination into agreement. It is therefore possible that the ages of young clusters have generally been underestimated. The luminosity of brown dwarfs in these clusters would thus have been overestimated.