BERYLLIUM ABUNDANCES IN LATE-TYPE HYADES DWARF STARS

We have observed, with high spectral resolution, the Be II resonance d oubler at lambda 3131 Angstrom in five G and K-type main-sequence star s belonging to the Hyades open cluster, as well as in four K-type prob able members of the Ursa Major Group. We carried out a detailed abunda nce analysis via spectral synthesis fitting and studied the utility of the Be II lines for deriving beryllium abundances in late-type dwarf stars, Reliable abundances were derived for three Hyades stars with ef fective temperatures in the range 5700-5200 K. For stars cooler than s imilar to 5200 K the observed feature in the position of the weaker Be ll line is strongly affected by a line of other element which we tenta tively identify as Mn I lambda 3131.037 Angstrom. Due to this fact, we could establish only upper limits to the abundances in these very coo l stars. Lithium and beryllium abundances in Hyades stars with effecti ve temperatures in the range 6000-5200 g show a complete different beh aviour. While lithium is rapidly depleted towards decreasing temperatu res, reaching a relative difference of similar to 3 orders of magnitud e, beryllium remains fairly constant for the same stars, with an abund ance similar to that exhibited by hotter stars of the cluster and clos e to the solar value. Standard evolutionary models cannot account for the present Li and Be observations in Hyades late-type stars. Rotation al mixing associated with angular momentum loss can explain the observ ed abundances if the initial angular momenta of the stars were progres sively larger for decreasing stellar mass. On the other hand, mixing i nduced by internal gravity waves satisfies the observational constrain ts.