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

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 discrepancy (deltaFe = [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 over-ionization trends. The Pleiades deltaFe abundances are correlated with Ca II infrared triplet and Halpha 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 deltaFe 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. Star-to-star Fe I abundances have low internal scatter, 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 deltaFe 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.Comment: 32 pages, 7 figures, 7 tables; accepted by PAS

Rotation, inflation, and lithium in the Pleiades

The rapidly rotating cool dwarfs of the Pleiades are rich in lithium relative to their slowly rotating counterparts. Motivated by observations of inflated radii in young, active stars, and by calculations showing that radius inflation inhibits pre-main sequence (pre-MS) Li destruction, we test whether this pattern could arise from a connection between stellar rotation rate and radius inflation on the pre-MS. We demonstrate that pre-MS radius inflation can efficiently suppress lithium destruction by rotationally induced mixing in evolutionary models, and that the net effect of inflation and rotational mixing is a pattern where rotation correlates with lithium abundance for $M_{*} < {\rm M}_{\odot}$, and anti-correlates with lithium abundance for $M_{*} > {\rm M}_{\odot}$, similar to the empirical trend in the Pleiades. Next, we adopt different prescriptions for the dependence of inflation on rotation, and compare their predictions to the Pleiades lithium/rotation pattern. We find that if a connection between rotation and radius inflation exists, then the important qualitative features of this pattern naturally and generically emerge in our models. This is the first consistent physical model to date that explains the Li--rotation correlation in the Pleiades. We discuss plausible mechanisms for inducing this correlation and suggest an observational test using granulation.Comment: 17 pages, 9 figures. A short video summarizing the results of our paper can be found here: https://www.youtube.com/watch?v=pnXFBCRQgd