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Evolution of the Lithium Abundances of Solar-Type Stars. IX. High-Resolution Spectroscopy of Low-Mass Stars in NGC 2264
The Astronomical Journal
  • David R Soderblom, Space Telescope Science Institute
  • Jeremy R King, Clemson University
  • Lionel Siess, Space Telescope Science Institute
  • Burton F Jones, University of California
  • Debra Fischer, San Francisco State University
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The American Astronomical Society

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 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 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 limit 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 consistent with no depletion having yet occurred and a uniform value of log N(Li) = 3.2, provided that the uncertainty in Teff values is about 250 K. This is high, but this cluster appears to have varying extinction, which 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 distribution 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.

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