Document Type

Article

Publication Date

12-10-1997

Publication Title

The Astrophysical Journal

Publisher

The American Astronomical Society

Abstract

We present the results of an extensive search, conducted at the Canada-France-Hawaii 3.6-m tele-scope, for beryllium (Be) in the atmospheres of lithium-deÐcient 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 conÐrm the suspicion that F stars which deplete Li by factors of 10È200 may also be beryllium deÐcient. Photospheric Be concentrations range from near meteoritic levels in G dwarfs to factors of 10È100 below this assumed initial abundance in hotter stars. Moreover, signiÐcant Be deÐ-ciencies 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 HerculisÏÏ 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 con-junction 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 deÐciencies. The predicted timescales for surface depletion due to microscopic di†usion are too long for signiÐcant Li and Be deÐciencies to develop in cool (T¹ 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 deÐciencies in cool dwarfs. A key meridional circulation prediction regarding the efficiency and severity of Li and Be dilu-tion 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 superÐcial angular momentum loss, predicts both the observed light element depletion morphology as well as the existence of 110 Her analogs. These ““YaleÏÏ mixing models provide, therefore, the most plausible explanation, of those presented, for the observed Li and Be abundances.

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