High-Resolution Spectroscopy of Ursa Major Moving Group Stars

Abstract

We use new and extant literature spectroscopy to address abundances and membership for UMa moving group stars. We first compare the UMa, Coma, and Hyades H-R diagrams via a homogeneous set of isochrones, and find that these three aggregates are essentially coeval. Our spectroscopy of cool UMa dwarfs reveals striking abundance anomalies--trends with Teff, ionization state, and excitation potential--like those recently seen in young cool M34, Pleaides, and Hyades dwarfs. In particular, the trend of rising 7774 Ang-based OI abundance with declining Teff is markedly subdued in UMa compared to the Pleiades, suggesting a dependence on age or metallicity. Despite disparate sources of Li data,our homogeneous analysis indicates that UMa members evince remarkably small scatter in the Li-Teff plane for Teff>5200 K. Significant star-to-star scatter suggested by previous studies is seen for cooler stars. Comparison with the consistently determined Hyades Li-Teff trend reveals differences qualitatively consistent with this cluster's larger [Fe/H] (and perhaps slightly larger age). However, quantitative comparison with standard stellar models indicates the differences are smaller than expected, suggesting the action of a fourth parameter beyond age, mass, and [Fe/H] controlling Li depletion.