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Publications of the Astronomical Society of the Pacific


Astronomical Society of the Pacific


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; this (near) coevality can explain the indistinguishable distributions of UMa and Hyades dwarfs in the chromospheric emission versus color plane. 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 λ7774-based O I abundance with declining Teff is markedly subdued in UMa compared to the Pleiades, suggesting a dependence on age or metallicity. Recent photometric metallicity estimates for several UMa dwarfs are markedly low compared to the group’s canonical metallicity, and sim-ilar deviants are seen among cool Hyads as well. Our spectroscopy does not con-firm these curious photometric estimates, which seem to be called into question for cool dwarfs. 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, sug-gesting the action of a fourth parameter beyond age, mass, and [Fe/H] controlling Li depletion. The UMa-Coma cool star Li abundances may show a slight 0.2 dex difference; however, this may be mass-independent, and thus more consistent with a modest initial Li abundance difference.


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