THE EVOLUTION OF THE LITHIUM ABUNDANCES OF SOLAR-TYPE STARS .3. THE PLEIADES

We report new measurements of lithium in more than 100 Pleiades F, G, and K dwarfs. Abundances were determined from spectrum synthesis fits to the data as well as from use of new curves of growth for the Li 670 8 angstrom feature (presented in an Appendix). We confirm the intrinsi c spread in lithium abundance within the Pleiades seen by Duncan & Jon es [ApJ, 271, 663 (1983)], but we establish more observational constra ints on Li in this cluster: First, for stars near 1.0 M. [about 0.60 t o 0.75 in (B-V)0], the scatter in the relation between log N(Li)(= N(L i) and T(eff) is consistent with our observational uncertainty. That m eans that most late-F and early-G dwarfs in the Pleiades are consisten t with the tight N(Li) versus mass relation seen in the Hyades in the same mass range. Second, at (B-V)0 almost-equal-to 0.8 (M almost-equal -to 0.9 M.), large and real star-to-star differences in N(Li) appear. The range in N(Li) at (B-V)0 almost-equal-to 0.8 is about 1 dex, and g rows to as much as 1.5 dex for less massive stars Third, the most Li-r ich stars have abundances at or near the primordial level for Populati on I (N(Li) almost-equal-to 3.2), and none exceed that level by a sign ificant amount. Fourth, at any given color the stars that rotate faste st have the most Li and have the strongest chromospheric activity. We consider the ways in which an apparent spread in N(Li) could arise fro m an intrinsically tight N(Li)-mass relation and conclude that the spr ead is probably real and is not an artifact of line formation conditio ns or inhomogeneous atmospheres on the stars. It is possible to produc e large apparent changes in N(Li) by covering a significant fraction o f a star's surface with cooler regions (''spots''), but doing so has o ther ramifications that conflict with the observations. Some current m odels lead to a spread in N(Li) in which the fastest rotators (those t hat have lost the least angular momentum) have the most Li, and that m echanism may account for what is seen. A comparison of the Pleiades to the alpha Persei cluster shows that most a Persei stars have Li abund ances comparable to their Pleiades counterparts, but there is a signif icant fraction (about 30%) of alpha Persei stars that lie below the Pl eiades in N(Li) by 1 dex or more. Some of these anomalous stars have e ven less Li than Hyades stars of the same T(eff). If these stars are b ona fide alpha Persei members (and they probably are), their Li abunda nces strain our understanding of Li depletion. The Pleiades, considere d together with alpha Persei and the Hyades, shows that stars with [Fe /H]greater-than-or-equal-to 0.0 and which are more massive than about 1.25 M. do not deplete Li prior to reaching the main sequence. Moreove r, solar-abundance stars ([Fe/H] = 0.0) with M greater than or similar to 1.1 M. do not experience pre-main-sequence depletion either. Pleia des dwarfs near T(eff) = 6700 K show evidence of being depleted in Li, indicating that an incipient Li ''chasm'' is present even at an age o f 70 Myr.