110-HERCULIS - A POSSIBLE PROTOTYPE FOR SIMULTANEOUS LITHIUM AND BERYLLIUM DEPLETION, AND IMPLICATIONS FOR STELLAR INTERIORS

Standard stellar evolution predicts that late F stars should have reta ined their initial surface lithium abundance because their convection zones are too shallow to destroy it at their base. Yet observations re veal Li depletions, sometimes quite severe (the ''Boesgaard Li gap''). Several physical mechanisms, which are not usually included in stella r evolution calculations, have been proposed to account for the Li dep letions. These mechanisms include mass loss, microscopic diffusion, an d various types of slow mixing driven by either waves or rotation. Ide ntifying which of these (if any) might really be at work not only is o f vital interest to advancing our knowledge of stellar interiors but a lso may have serious implications in other contexts, such as cosmology . We bring attention to beryllium (Be) observations in late F stars, a nd propose that they are crucial for discriminating between scenarios. Particularly important is the star 110 Her, which is depleted in surf ace Be by about a factor of 5-10, and yet enough surface Li has surviv ed to be detected. The Li depletion is stronger than that of Be, about a factor of 100-200. Depleting surface Be without having depleted all of the surface Li (and depleting Li more strongly than Be) requires s pecific circumstances; we discuss how this Li/Be depletion pattern, if representative, would severely constrain or eliminate most of the pro posed mechanisms. One mechanism, rotationally induced mixing, predicts relative Li and Be depletions that agree well with what is observed. It is important to establish whether or not the Li/Be depletion patter n observed in 110 Her is representative.