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 T _ { \mathrm { eff } } < 5400 K , and the discrepancy ( \Delta \mathrm { Fe } = [ Fe ii/H ] - [ Fe i/H ] ) increases dramatically with decreasing T _ { \mathrm { eff } } , 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 over-ionization trends .
The Pleiades \Delta \mathrm { Fe } abundances are correlated with Ca ii infrared triplet and H \alpha 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 T _ { \mathrm { eff } } , and a comparison with the \Delta \mathrm { Fe } abundances suggests 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 abundances have low internal scatter ( < 0.11 dex ) , but the abundances of stars with T _ { \mathrm { eff } } < 5400 K are systematically higher compared to the warmer stars .
The cool star [ Fe i/H ] abundances can not be connected directly to over-excitation effects , but similarities with the \Delta \mathrm { Fe } and O i triplet trends suggest the abundances are dubious .
Using the [ Fe i/H ] abundances of five stars with T _ { \mathrm { eff } } > 5400 K , we derive a mean Pleiades cluster metallicity of [ Fe/H ] = +0.01 \pm 0.02 .