Iron Convection Zones in B, A, and F Stars

Abstract

Stellar models, including all e †ects of atomic di †usion and radiative accelerations, are evolved from the pre-main sequence to the giant branch for stars of 1.3 to 4.0 with metallicity ranging from M _ , to 0.03. It is shown that radiative accelerations lead to the accumulation of iron-peak elements Z 0 \ 0.01 around 200,000 K ; this increases the opacity and causes the appearance of Fe convection zones when macroscopic motions are not rapid enough to wipe out the e †ects of particle transport. The behavior of Fe convection zones and conditions for their appearance are studied in detail. Iron-peak convection zones appear naturally in all solar metallicity models more massive than 1.5 In the 1.5 model, M _. M _ it is present only for a fraction of the main-sequence lifetime, but in models without turbulence of 1.7 and more, the Fe convection zone rapidly develops after arrival on the main-sequence and remains M _ until its end. For a metallicity of Z \ 0.01, an Fe convection zone appears even in a 1.3 model. M _ Moreover, the interaction between the di †usion velocities of di †erent species leads to an accumulation of heavy elements around the convective core, causing semiconvection. A detached semiconvection zone develops in the 1.5 model. Finally, the surface abundances are calculated using a number of turbu-M _ lence models and compared to observations of q UMa in order to show how abundance anomalies may be used to test various turbulence models ; the gravity at which abundance anomalies should be expected to disappear is determined. It is shown that in Am stars, the Ca underabundance should disappear during evolution at the same gravity as iron-peak overabundances.