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

Abstract

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 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 K ≤ 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. 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.