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

Oxygen abundances have been derived from the near-IR, high-excitation λ7774 O I triplet in high-resolution, high signal-to-noise spectra of 45 Hyades dwarfs using standard one dimensional, plane-parallel LTE models. Effective temperatures of the stellar sample range from 4319 − 6301 K, and the derived relative O abundances as a function of Teff evince a trichotomous morphology. At Teff > 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 ≤ Teff ≤ 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 Teff ≤ 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. Multi-component 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. Subject headings: open clusters and associations: individual(Hyades) — stars: abundances — stars: atmospheres — stars: late-type — stars: spots