Beryllium in Lithium‐deficient F and G Stars

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-deficient 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 confirm the suspicion that F stars which deplete Li by factors of 10-200 may also be beryllium deficient. Photospheric Be concentrations range from near meteoritic levels in G dwarfs to factors of 10-100 below this assumed initial abundance in hotter stars. Moreover, significant Be deficiencies appear in stars that populate a 600 K wide effective 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 differentiating 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 deficiencies. The predicted timescales for surface depletion due to microscopic diffusion are too long for significant Li and Be deficiencies to develop in cool (Teff ≤ 6200) stars; nevertheless, underabundances are observed in these stars. Diffusion theory also predicts Li and Be depletion rates to be comparable, but it is evident that Li and Be depletion proceed at different speeds. Models of mixing induced by internal gravity waves cannot explain mild Be deficiencies 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 superficial 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.