Convective Overshooting in the Envelopes of A-type Stars Using the k-ω Model

Mixing between convective zones is quite uncertain in the envelopes of A-type stars. To study the mixing in A-type envelopes, we use a new convection model, the k-omega model, in the MESA stellar evolution code. Using the k-omega model, we find that the overshooting regions of the He ii and H/He i convection zones are integrated with each other. There is material exchange between the He ii and H/He i convection zones through overshooting, in agreement with recent numerical simulations.We obtain the overshooting distance of about 3.5H(p) below the base of the H/He i convection zone in a 2.3 M star. There are two overshooting regions beyond the He ii convection zone in the same stellar model. We obtain that the overshooting distance of the upper one is about 3.9H(p) and about 2.0H(p) for the lower one. We find that the turbulent diffusion effect is particularly strong in the convective envelopes of A-type stars. In addition, we find that the typical size of the convective rolling cells is restricted by the actual thickness of the convective zones, because the thickness of the convection zones are usually smaller than or approximately equal to the local pressure scale height. Through comparisons with the results of the k-omega model, we find that a suitable value of f(ov) is about 0.45 for the H/He i convective overshooting region. It is about 0.27 for the upper He ii convective overshooting region and about 0.25 for the lower one.