CORRELATED DEPLETION OF LITHIUM AND BERYLLIUM IN F-STARS

It has been known for over a decade that Hyades F stars have severely depleted their Li abundances (the ''Li gap''), in sharp contrast to th e predictions of the standard stellar evolution theory. We began a Li and Be survey aimed at identifying the physical mechanism that creates the Li gap. We present here the first results of that survey, which i nclude high-resolution (R = 48,000-120,000) and high signal-to-noise r atio observations in 24 stars of the Li I lambda 6707.8 and/or the Be II lambda 3131 doublets taken at the University of Hawaii 2.2 m, Canad a-France-Hawaii 3.6 m, and Keck I 10 m telescopes. Our program stars w ith detections in both Li and Be define a clear trend that suggests (1 ) the surface Li and Be abundance depletions are correlated and (2) su rface Li diminishes more rapidly than surface Be. Our results suggest that correlated Li and Be depletion is a normal process that F stars u ndergo. The Li-Be trend argues strongly against the mass-loss and diff usion mechanisms and strongly supports slow mixing as the cause of the surface light-element deficiencies. Moreover, models with rotationall y induced mixing are in better agreement with the data than are models with wave-driven mixing.