Beryllium in Lithium‐deficient F and G Stars

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Abstract

We present the results of an extensive search, conducted at the Canada-France-Hawaii 3.6-m telescope, for beryllium (Be) in the atmospheres of lithium-decient F and G dwarfs. We also report revised lithium (Li) estimates for the entire sample using previously published equivalent widths and updated, consistently calculated stellar parameters. Abundances derived from an LTE analysis of the Li and Be line-forming regions conrm the suspicion that F stars which deplete Li by factors of 10200 may also be beryllium decient. Photospheric Be concentrations range from near meteoritic levels in G dwarfs to factors of 10100 below this assumed initial abundance in hotter stars. Moreover, signicant Be deciencies appear in stars that populate a 600 K wide e ective temperature window centered on 6500 K. This Be abundance gap is reminiscent of the Li gap observed in open clusters. Also, the discovery of 12 probable "" 110 Herculis stars, objects that exhibit a depleted, but detected, surface concentration of both Li and Be, provides a powerful means of di erentiating between the possible physical processes responsible for observed light element abundance patterns. Indeed, the Be data presented here, in conjunction with the newly calculated Li abundances, lead to the following conclusions regarding the hypothesized, light element depletion scenarios : Mass loss cannot account for stars with severely depleted (but detected) Li and moderate Be deciencies. The predicted timescales for surface depletion due to microscopic di usion are too long for signicant Li and Be deciencies to develop in cool (T eff 6200) stars ; nevertheless, underabundances are observed in these stars. Di usion theory also predicts Li and Be depletion rates to be comparable, but it is evident that Li and Be depletion proceed at di erent speeds. Models of mixing induced by internal gravity waves cannot explain mild Be deciencies in cool dwarfs. A key meridional circulation prediction regarding the efficiency and severity of Li and Be dilution is shown to be fallible. However, rotationally induced mixing, a turbulent blending of material beneath the surface convection zone due to the onset of instabilities from supercial angular momentum loss, predicts both the observed light element depletion morphology as well as the existence of 110 Her analogs. These "" Yale mixing models provide, therefore, the most plausible explanation, of those presented, for the observed Li and Be abundances.

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