Lithium in the Young Cluster NGC 2264

Abstract

We use moderate-resolution, moderate signal-to-noise ratio spectroscopy to derive Li abundances for six F and G stars in the young (few Myr) cluster NGC 2264. These are combined with consistently determined abundances of five NGC 2264 G and K stars having published Li data. The mean non-LTE abundance, calculated with photometric temperatures, is log N(Li) = 3.27 ± 0.05. This is essentially identical to the meteoritic value, providing no evidence of Galactic Li enrichment over the past 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 well within the expected uncertainties and does not indicate any unexpected differential Li depletion. Our mean Li abundance is 0.2–0.3 dex larger than that in the hotter stars of IC 2602 (30 Myr) and the Pleiades (100 Myr), which have consistently determined abundances. This might indicate prior modest differential enrichment, very recent Galactic Li enrichment that is not a global process, or the increasing effect of Li depletion at the young ages of these clusters; such depletion cannot be satisfactorily understood in terms of extant standard or rotational 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 Teff (mass) persists to 4000 K (~0.5 M⊙), a morphology in agreement with both standard and rotational stellar models having ages 4 Myr. We note that some spectral type–based Teff scales lead 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. Interestingly, such behavior also is inferred for the near-initial Li-Teff (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 Li 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 Teff values; comparison of independent spectral classifications and the effects of their differences on the derived Li abundances provide direct support for the latter. This underscores the need for accurate relative spectroscopic Teff values derived for a larger number of cluster stars from higher quality data. Finally, radial velocities are derived 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 spectroscopic binaries are noted, including a multiple-lined star not included in the Li study.