110 Herculis: A Possible Prototype for Simultaneous Lithium and Beryllium Depletion, and Implications for Stellar Interiors

Abstract

Standard stellar evolution predicts that late F stars should have retained their initial surface lithium abundance because their convection zones are too shallow to destroy it at their base. Yet observations reveal Li depletions, sometimes quite severe (the "Boesgaard Li gap"). Several physical mechanisms, which are not usually included in stellar evolution calculations, have been proposed to account for the Li depletions. These mechanisms include mass loss, microscopic diffusion, and various types of slow mixing driven by either waves or rotation. Identifying which of these (if any) might really be at work not only is of vital interest to advancing our knowledge of stellar interiors but also may have serious implications in other contexts, such as cosmology. We bring attention to beryllium (Be) observations in late F stars, and propose that they are crucial for discriminating between scenarios. Particularly important is the star 110 Her, which is depleted in surface Be by about a factor of 5-10, and yet enough surface Li has survived 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 specific circumstances; we discuss how this Li/Be depletion pattern, if representative, would severely constrain or eliminate most of the proposed 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 pattern observed in 110 Her is representative.