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 premain-sequence cluster NGC 2264 in order to determine their radial velocities, rotation rates, activity levels, and Li abundances. Our precise radial velocities indicate that the eight objects we observed that lie below a 5 Myr isochrone are non- members. 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 limit to the true age spread because star formation is still taking place in the cluster. After correction for non-LTE eUects, our Li abundances are consistent with no depletion having yet occurred and a uniform value of log N(Li) \ 3.2, provided that the uncertainty in values is about 250 K. This is high, but this cluster appears to have varying extinction, which T eff could lead to large temperature errors. Stars in NGC 2264 have spun up compared with the younger Orion Nebula cluster. Current models suggest disk-locking 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 distribu- tion of sin i is not in accord with a random distribution of rotation axes. The reason for this is not known, but it is probably due to a selection against equator-on objects that still have obscuring material around them.