Oxygen from the λ7774 High‐Excitation Triplet in Open Cluster Dwarfs: Hyades

Oxygen abundances have been derived from the near-IR, high-excitation Lambda7774 O I triplet in high-resolution, high signal-to-noise spectra of 45 Hyadesdwarfs using standard one dimensional, plane-parallel LTE models. Effectivetemperatures of the stellar sample range from 4319-6301 K, and the derivedrelative O abundances as a function of T_eff evince a trichotomous morphology.At T_eff > 6100 K, there is evidence of an increase in the O abundances withincreasing T_eff, consistent with non-LTE (NLTE) predictions. At intermediateT_eff (5450 < T_eff < 6100 K), the O abundances are flat, and star-to-starvalues 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 abundancesfor stars with T_eff < 5450 K show a striking increase with decreasing T_eff,in stark contrast to expectations and canonical NLTE calculations. The coolHyades triplet results are compared to those recently reported for dwarfs inthe Pleiades cluster and the UMa moving group; qualitative differences betweenthe trends observed in these stellar aggregates point to a possible age-relateddiminution of triplet abundance trends in cool open cluster dwarfs.Correlations with age-related phenomena, i.e., chromospheric activity andphotospheric spots, faculae, and/or plages, are investigated. No correlationwith Ca II H+K chromospheric activity indicators is observed. Multi-componentLTE ``toy'' models have been constructed in order to simulate photospherictemperature inhomogeneities that could arise from the presence of starspots,and we demonstrate that photospheric spots are a plausible source of thetriplet trends among the cool dwarfs.Comment: 40 pages, 7 figures; Accepted for publication in Ap