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 the 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 include high-resolution (R = 48,000-120,000) and high signal-to-noise ratio observations in 24 stars of the Li I λ6707.8 and/or the Be II λ3131 doublets taken at the University of Hawaii 2.2 m, Canada-France-Hawaii 3.6 m, and Keck I 10 m telescopes. Our program stars with detections in both Li and Be define a clear trend that suggests (1) the surface Li and Be abundance depletions are correlated and (2) surface Li diminishes more rapidly than surface Be. Our results suggest that correlated Li and Be depletion is a normal process that F stars undergo. The Li-Be trend argues strongly against the mass-loss and diffusion mechanisms and strongly supports slow mixing as the cause of the surface light-element deficiencies. Moreover, models with rotationally induced mixing are in better agreement with the data than are models with wave-driven mixing.