Spectroscopic abundances in cool Pleiades dwarfs and NGC 2264 stars

We derive parameters and abundances of several elements in two cool Pleiades dwarfs, four cool NGC 2264 pre-main-sequence stars, and a probable NGC 2264 nonmember from high-resolution, moderate signal-to-noise ratio, Keck/HIRES spectra. Our Pleiades Fe abundance agrees with previous spectroscopic and photometric values of hotter stars and does not resolve the 0.3 mag distance modulus discrepancy between main-sequence fitting and Hipparcos parallaxes. Abundances of Cr-Ca-Ti-Al are subsolar, mimicking the pattern of interstellar medium abundances. While modest temperature errors may contribute, the results (particularly for Al) could suggest an association with ionization potential; such effects might be related to the Pleiades Li scatter. The cluster Fe scatter and its relation to Li scatter is discussed. Three NGC 2264 members suggest [Fe/H] = -0.15 and near-solar ratios of other elements. Mildly supersolar abundances for another object support its probable nonmembership. A fourth member exhibits an Mg-Si-Fe-Ni and Cr-Ti-Ca-Al dichotomy opposite to that of the Pleiades stars; a relation to ionization potential is again suggested. A 0.15-0.20 dex scatter or steep decline, neither well accommodated by stellar models, in the NGC 2264 Li abundances with T-eff is indicated. We note the surprising presence of the lambda 7774 O I triplet in our Pleiades stars, one of the cool NGC 2264 stars, and the K6 field dwarf GL 241. The inferred LTE O abundances are enhanced by 0.23-0.85 dex over solar, suggesting that even non-LTE calculations of the O I triplet are incomplete and perhaps implicating the influence of an overlying chromosphere. Our results demonstrate the utility of cluster abundances besides Fe and Li in addressing fundamental issues concerning stellar evolution and systematic errors in the analysis of cool young stars.