2006ApJ...636..432S

Oxygen abundances have been derived from the near-IR, high-excitation O I λ7774 triplet in high-resolution, high signal-to-noise ratio spectra of 45 Hyades dwarfs using standard one-dimensional, plane-parallel LTE models. Effective temperatures of the stellar sample range from 4319 to 6301 K, and the derived relative O abundances as a function of T_effevince a trichotomous morphology. At T_eff>6100 K, there is evidence of an increase in the O abundances with increasing T_eff, consistent with non-LTE (NLTE) predictions. At intermediate T_eff(5450 K≤T_eff≤6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of [O/H]=+0.25±0.02; however, systematic errors at the ≲0.10 dex level might exist. The O abundances for stars with T_eff≤5450 K show a striking increase with decreasing T_eff, in stark contrast to expectations and canonical NLTE calculations. The cool Hyades triplet results are compared to those recently reported for dwarfs in the Pleiades cluster and the UMa moving group; qualitative differences between the trends observed in these stellar aggregates point to a possible age-related diminution of triplet abundance trends in cool open cluster dwarfs. Correlations with age-related phenomena, i.e., chromospheric activity and photospheric spots, faculae, and/or plages, are investigated. No correlation with Ca II H+K chromospheric activity indicators is observed. Multicomponent LTE ``toy'' models have been constructed in order to simulate photospheric temperature inhomogeneities that could arise from the presence of starspots, and we demonstrate that photospheric spots are a plausible source of the triplet trends among the cool dwarfs.