LITHIUM DEPLETION IN FULLY CONVECTIVE PREÈMAIN-SEQUENCE STARS

We present an analytic calculation of the thermonuclear depletion of lithium in contracting, fully con- vective, preEmain-sequence stars of mass Previous numerical work relies on still uncertain M ( 0.5 M _ . physics (atmospheric opacities and convection, in particular) to calculate the e†ective temperature as a unique function of stellar mass. We assume that the starIs e†ective temperature, is -xed during T eff , Hayashi contraction and allow its actual value to be a free parameter constrained by observation. Using this approximation, we compute lithium burning analytically and explore the dependence of lithium depletion on M, and composition. Our calculations yield the radius, age, and luminosity of a preE T eff , main-sequence star as a function of lithium depletion. This allows for more direct comparisons with observations of lithium-depleted stars. Our results agree with those numerical calculations that explicitly determine stellar structure during Hayashi contraction. In agreement with Basri, Marcy, & Graham, we show that the absence of lithium in the Pleiades star HHJ 3 implies that it is older than 100 Myr. We also suggest a generalized method for dating Galactic clusters younger than 100 Myr (i.e., those with contracting stars of and for constraining the masses of lithium-depleted stars. M Z 0.08 M _ ) Subject headings: nuclear reactions, nucleosynthesis, abundances E stars: evolution E stars: interiors E stars: preEmain-sequence