Beryllium abundances in F and G dwarfs in the Coma cluster and the URSA major moving group from Keck HIRES observations

The study of both light elements Li and Be in open clusters of known properties can reveal the internalstructure and the mechanisms of mixing in main-sequence stars as a function of age and composition. In previous work, we have investigated the older Hyades cluster and the younger Pleiades and alpha Per clusters. The Coma Berenices cluster and the UMa moving group are intermediate in age between the Hyades and Pleiades and provide a good linchpin for the influence of stellar age on light-element abundances; there are dips in the mid-F stars in both Li and Be in the Hyades but no Be dip and only a minor Li dip in the Pleiades. We have made observations of the resonance doublet of Be II near 3130 Angstrom in 13 Coma and six UMa stars with the Keck I telescope and the High Resolution Echelle Spectrometer. The Be abundances were determined by spectrum synthesis. In the F dwarfs in Coma, there are both Li and Be deficiencies, indicating that the depletions occur during the main-sequence phase of evolution but do not become evident until an age of 200 300 Myr. For both UMa and Coma stars, the Li depletion is greater than the Be depletion at all temperatures, but there is little, if any, Be depletion in stars with T-eff < 6000 K. In the four clusters studied for Be, the mean Be abundance for stars with temperatures less than 6000 K is log N(Be/H) = 12.00 = 1.27, independent of age or metal content. For the hotter stars (5850-6680 K), the Li and Be abundances are correlated, indicating that the depletion probably occurs simultaneously; this matches the results for the field stars and the Hyades and the predictions of Li and Be depletion by rotationally induced mixing.