Fe I and Fe II Abundances of Solar-Type Dwarfs in the Pleiades Open Cluster1

Abstract

We have derived Fe abundances of 16 solar-type Pleiades dwarfs by means of an equivalent-width analysis of Fe I and Fe II lines in high-resolution spectra obtained with the Hobby-Eberly Telescope and High Resolution Spectrograph. Abundances derived from Fe II lines are larger than those derived from Fe I lines (herein referred to as overionization) for stars with Teff < 5400 K, and the discrepancy (ΔFe = [Fe II/H]-[Fe I/H]) increases dramatically with decreasing Teff, reaching over 0.8 dex for the coolest stars of our sample. The Pleiades joins the open clusters M 34, the Hyades, IC 2602, and IC 2391, and the Ursa Major moving group, demonstrating ostensible overionization trends. The Pleiades ΔFe abundances are correlated with Ca II infrared triplet and Hα chromospheric emission indicators and relative differences therein. Oxygen abundances of our Pleiades sample derived from the high-excitation O I triplet have been previously shown to increase with decreasing Teff, and a comparison with the ΔFe abundances suggests that the overexcitation (larger abundances derived from highexcitation lines relative to low-excitation lines) and overionization effects that have been observed in cool open cluster and disk field main-sequence (MS) dwarfs share a common origin. Curiously, a correlation between the Pleiades O I abundances and chromospheric emission indicators does not exist. Star-to-star Fe I abundances have low internal scatter (<0:11 dex), but the abundances of stars with Teff < 5400 K are systematically higher compared to the warmer stars. The cool star [Fe I/H] abundances cannot be connected directly to overexcitation effects, but similarities with the ΔFe and O I triplet trends suggest the abundances are dubious. Using the [Fe I/H] abundances of five stars with T eff> 5400 K, we derive a mean Pleiades cluster metallicity of [Fe/H] = +0:01 ± 0:02. © 2010. The Astronomical Society of the Pacific. All rights reserved.