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 k7774 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 T eff evince a trichotomous morphology. At T eA > 6100 K, there is evidence of an increase in the O abundances with increasing T eff , consistent with non-LTE (NLTE) predictions. At intermediate T eff (5450 K T eA 6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of ½O/H ¼ þ0:25 AE 0:02; however, systematic errors at the P0.10 dex level might exist. The O abundances for stars with T eA 5450 K show a striking increase with decreasing T 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. 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.

Consistent metallicity scale for cool dwarfs and giants

“…We present here a comparison of our results with selected previous analyses of Hyades stars: Paulson et al (2003), Schuler et al (2006) and Carrera & Pancino (2011). These works were chosen because they have a larger number of stars in common with our analysis.…”

“…Spectroscopic studies of FGK-type dwarfs in the Hyades find a metallicity of about +0.13 dex (Cayrel et al 1985;Boesgaard & Friel 1990;Paulson et al 2003;Schuler et al 2006). Regarding the giants, the metallicity values range from +0.10 up to +0.20 dex, where this scatter is usually attributed to the star HIP 20455, a spectroscopic binary (Schuler et al 2006;Carrera & Pancino 2011).…”

“…For the sample in common, the average difference in T eff between Paulson et al (2003) and this work is 58 ± 59 K. The metallicity reported by Paulson et al (2003) is 0.13 ± 0.01 dex, which is within a 2σ range of the values found with M1 (0.18 ± 0.03 dex), but it is in excellent agreement with our metallicity derived according the same spectroscopic method (M2) of 0.14 ± 0.03 dex. Schuler et al (2006) analyzed the giants and dwarfs of the Hyades using a method similar to M1. The T eff was obtained with the IRFM by Blackwell & Lynas-Gray (1994.…”

EVOLUTION OF THE SOLAR ACTIVITY OVER TIME AND EFFECTS ON PLANETARY ATMOSPHERES. II. κ1Ceti, AN ANALOG OF THE SUN WHEN LIFE AROSE ON EARTH

“…Authors do not generally agree whether this offset is due to NLTE effects in cool stars, in the sense that for stars with T eff ∼ 5000 K the offsets are large, while for T eff ∼ 6000 K good agreement is found (Ramírez et al 2007); this discrepancy is revealed in this case either by a disagreement between chemical abundances derived from different lines of the same species (atomic or molecular) or by the non-realization of the Fe i/Fe ii ionization equilibrium. Another interpretation is the presence of high chromospheric activity, an enhanced non-local UV radiation field, and a resulting photospheric overionization (Schuler et al 2006). In a classical spectroscopic analysis, forcing agreement between Fe i and Fe ii abundances under LTE, but in the presence of an overionizing radiation field, leads naturally to a higher T eff .…”

On the Iron Abundance Anomaly in K-dwarf and Hyades Stars

“…A long-standing problem in astrophysics stems from the work of Yong et al (2004) and Schuler et al (2004Schuler et al ( , 2006, who noticed startling discrepancies involving iron and oxygen abundances, leading to questions regarding the viability of standard 1D-LTE model atmospheres. Yong et al (2004) noticed that the overall iron abundance as inferred from the measurement of singly ionized iron lines rapidly increased in Hyades stars with decreasing T eff , starting at around 5000 K. Schuler et al (2006), likewise, found a similar trend with the inferred oxygen abundance from their measurements of the O I 777 nm triplet lines in dwarf stars of the Hyades, Pleiades, and M34 open clusters.…”