Evolution of the lithium abundances of solar-type stars. IX. High-resolution spectroscopy of low-mass stars in NGC 2264

We have obtained Keck HIRES spectra of 35 objects in the pre-main-sequence cluster NGC 2264 in order to determine their radial velocities, rotation ra tes, activity levels, and Li abundances. Our precise radial velocities indi cate that the eight objects we observed that lie below a 5 Myr isochrone ar e nonmembers. This means that the age spread within NGC 2264 is only about 4 Myr, a factor of 2 less than found previously, although this is a lower l imit to the true age spread because star formation is still taking place in the cluster. After correction for non-LTE effects, our Li abundances are c onsistent with no depletion having yet occurred and a uniform value of log N(Li) = 3.2, provided that the uncertainty in T-eff values is about 250 K. This is high, but this cluster appears to have varying extinction, which co uld lead to large temperature errors. Stars in NGC 2264 have spun up compar ed with the younger Orion Nebula cluster. Current models suggest disk-locki ng timescales of 1-2 Myr to account for the most rapidly rotating stars. By combining rotation periods and v sin i values, we show that the distributi on of sin i is not in accord with a random distribution of rotation axes. T he reason for this is not known, but it is probably due to a selection agai nst equator-on objects that still have obscuring material around them.