LITHIUM IN THE YOUNG CLUSTER NGC-2264

We use moderate-resolution, moderate signal-to-noise ratio spectroscop y to derive Li abundances for six F and G stars in the young (few Myr) cluster NGC 2264. These are combined with consistently determined abu ndances of five NGC 2264 G and K stars having published Li data. The m ean non-LTE abundance, calculated with photometric temperatures, is lo g N(Li) = 3.27 +/- 0.05. This is essentially identical to the meteorit ic value, providing no evidence of Galactic Li enrichment over the pas t 4.6 Gyr-at least to the extent that the meteoritic value represents a typical ''cosmic'' value at that time. The scatter of 0.16 dex is we ll within the expected uncertainties and does not indicate any unexpec ted differential Li depletion. Our mean Li abundance is 0.2-0.3 dex la rger than that in the hotter stars of IC 2602 (30 Myr) and the Pleiade s (100 Myr), which have consistently determined abundances. This might indicate prior modest differential enrichment, very recent Galactic L i enrichment that is not a global process, or the increasing effect of Li depletion at the young ages of these clusters; such depletion cann ot be satisfactorily understood in terms of extant standard or rotatio nal stellar models. Li is not overabundant in the cluster short-period binary W134, a result consistent with the predictions of tidal theory and rotational stellar models. The flatness of Li with T-eff (mass) p ersists to 4000 K (similar to 0.5 M.), a morphology in agreement with both standard and rotational stellar models having ages less than or s imilar to 4 Myr. We note that some spectral type-based T-eff scales le ad to Li abundances in all of the five cooler cluster stars that are 0 .3-0.8 dex larger than abundances in the six hotter stars. Interesting ly, such behavior also is inferred for the near-initial LI-T-eff (mass ) morphology of IC 2602 and the Pleiades when model depletion factors are mapped onto their observed current abundances. No physical origin of such an abundance pattern, which might suggest an initial cluster L i abundance over a factor of 2 larger than meteoritic, is identified. Rather, we believe that it is caused by lingering deficiencies in the model depletion predictions and errant spectral type-based relative T- eff values; comparison of independent spectral classifications and the effects of their differences on the derived Li abundances provide dir ect support for the latter. This underscores the need for accurate rel ative spectroscopic T-eff values derived for a larger number of cluste r stars from higher quality data. Finally, radial velocities are deriv ed for our NGC 2264 stars. Our heliocentric estimate of 24 km s(-1) is in fine agreement with recent determinations from early-type cluster stars, but shows ample scatter. Some candidate pre-main-sequence spect roscopic binaries are noted, including a multiple-lined star not inclu ded in the Li study.