On March 25, The Astrophysical Journal published new results about convective overshooting in the envelopes of A-type stars reported by GUO Fei and LI Yan, who work in Yunnan Observatories of the Chinese Academy of Sciences.
Convection occurs where the buoyancy is in the same direction as the velocity. This convection is turbulent in the stellar. Convection happens due to the exchange of matter and energy within a star. In the stellar envelopes of 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 previous work, a k–ω model was proposed by Prof. LI Yan to deal with the mixing in convective and overshooting regions. The k–ω model is based on fluid dynamics and 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 found that there is material exchange between the hydrogen and helium convection zones through overshooting.
Besides, they obtain the overshooting distance of hydrogen and helium convective overshooting regions in a 2.3 solar mass star.
They also calibrate the free parameter for the classical overshooting model by using the k–ω model. As a result, they found that a suitable value of fov 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 the turbulent diffusion effect is particularly strong in the convective envelopes of A-type stars when they use the k–ω model to analyze.
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. They found that the typical size of the convective rolling cells is restricted by the actual thickness of the convective zones.
In summary, they found that the k–ω model can be used to describe complex convective regions and convective overshooting regions in the envelopes of A-type stars.
Contact:
GUO Fei, Yunnan Observatories, CAS
guofei@ynao.ac.cn
