In a study published in The Astrophysical Journal, GUO Fei and LI Yan from Yunnan Observatories of the Chinese Academy of Sciences reported their new findings on convective overshooting in the envelopes of A-type stars.

Convection occurs where the buoyancy is in the same direction as the velocity. This convection is turbulent in the stellar. It happens due to the exchange of matter and energy within a star. In the stellar envelopes A-type stars, convective layers are caused by the ionization of hydrogen and helium. These convective layers are very thin and close to each other. It is difficult to simulate their material mixing with a convective model.

In the previous work, a k–ω model was proposed by LI Yan to deal with the mixing in convective and overshooting regions. This k–ω model is based on fluid dynamics and is more suitable to treat turbulent convection of the stars. It can describe not only the convection zone but also the convective overshoot zone.

By using the k–ω model in the stellar envelopes of A-type stars, the researchers in this study found that there is material exchange between the hydrogen and helium convection zones through overshooting. They obtained the overshooting distance of hydrogen and helium convective overshooting regions in a 2.3 solar mass star.

Besides, the researchers calibrated the free parameter for the classical overshooting model by using the k–ω model. They found that a suitable value of f_ov is about 0.45 for the hydrogen convective overshooting region. It is about 0.27 for the upper helium convective overshooting region and about 0.25 for the lower one.

One interesting result is that when the k–ω model was used, the researchers found that the turbulent diffusion effect is particularly strong in the convective envelopes of A-type stars.

Thermal convection in stars is characterized by fully developed turbulence and convective rolling cells of different scales. The convective rolling cells are described by using the typical size and typical tangential velocity. The researchers found that the typical size of the convective rolling cells is restricted by the actual thickness of the convective zones.

These findings showed that the k–ω model can be used to describe complex convective regions and convective overshooting regions in the envelopes of A-type stars.