Oxygen abundances have been derived from the near-IR , high-excitation \lambda 7774 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 T _ { \mathrm { eff } } evince a trichotomous morphology .
At T _ { \mathrm { eff } } > 6100 \mathrm { K } , there is evidence of an increase in the O abundances with increasing T _ { \mathrm { eff } } , consistent with non-LTE ( NLTE ) predictions .
At intermediate T _ { \mathrm { eff } } ( 5450 \leq T _ { \mathrm { eff } } \leq 6100 \mathrm { K } ) , the O abundances are flat , and star-to-star values are in good agreement , having a mean value of \mathrm { [ O / H ] } = +0.25 \pm 0.02 ; however , systematic errors at the \lesssim 0.10 \mathrm { dex } level might exist .
The O abundances for stars with T _ { \mathrm { eff } } \leq 5450 \mathrm { K } show a striking increase with decreasing T _ { \mathrm { 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 .
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 .