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The Astrophysical Journal


The American Astronomical Society


Oxygen abundances have been derived from the near-IR, high-excitation O i k7774 triplet in high-resolution, high signal-to-noiseratiospectraof45Hyadesdwarfsusingstandardone-dimensional,plane-parallelLTEmodels.Effective temperatures of the stellar sample range from 4319 to 6301 K, and the derived relative O abundances as a function of Teff evince a trichotomous morphology. At TeA > 6100 K, there is evidence of an increase in the O abundances with increasing Teff, consistent with non-LTE (NLTE) predictions. At intermediate Teff (5450 K TeA 6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of ½O/H¼þ0:25 Æ0:02; however, systematic errors at the P0.10 dex level might exist. The O abundances for stars with TeA 5450 K show a striking increase with decreasing Teff, 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.