Oxygen in Open Cluster Dwarfs: Pleiades and M34

We analyze the high-excitation O i l7774 triplet in high-resolution, moderate signal-to-noise ratio spectra of 15 Pleiades and eight M34 open cluster dwarfs over the effective temperature range of 5048–6172 K. Relative O abundances have been derived using model atmospheres interpolated from four different sets of ATLAS9 grids. In contrast to existing non-LTE (NLTE) predictions, a dramatic increase in the O i triplet abundance with decreasing temperature is seen for both clusters, regardless of the atmospheric model. S i abundances of three Pleiades stars derived from the high-excitation l6053 feature mimic the O i abundance behavior. O abundances have also been derived from the [O i] l6300 feature in three Pleiades stars; the abundances exhibit a much lower mean value than do the cool dwarf triplet results. Inasmuch as the [O i] abundances are presumed to be free from NLTE effects, their mean values ( ) provide the current best estimate of the Pleiades O abundance. [O/H] p 0.14 6300 Spreads in at a given temperature are also seen in both clusters; no correlation is found between the [O/H]Trip spreads and the chromospheric emission measures. Subject headings: open clusters and associations: individual (M34, Pleiades) — stars: abundances — stars: atmospheres — stars: late-type — stars: Population II On-line material: machine-readable table