Publications of the Astronomical Society of the Pacific
Astronomical Society of the Pacific
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 over-ionization) for stars with Teﬀ < 5400 K, and the dis-crepancy (∆Fe = [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 M34, the Hyades, IC2602, and IC2391, and the Ursa Major moving group, demonstrating ostensible over-ionization trends. The Pleiades ∆Fe abun-dances 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 ∆Fe abundances sug-gests that the over-excitation (larger abundances derived from high excitation lines relative to low excitation lines) and over-ionization 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 abun-dances 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 over-excitation eﬀects, but similarities with the ∆Fe and O I triplet trends suggest the abundances are dubious. Using the [Fe I/H] abundances of ﬁve stars with Teﬀ > 5400 K, we derive a mean Pleiades cluster metallicity of [Fe/H] = +0.01 ± 0.02.
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