To explore the overshooting mixing beyond the convective core in core helium-burning stars,we use the k-ω model which is incorporated into the Modules of Experiments in Stellar Astrophysics（MESA） to investigate the overshooting mixing in evolution of subdwarf B（sdB）stars.The results of the evolutionary modelling of sdB stars are presented,and the development of the convective core can be divided into three stage.The mass of the convective core increases monotonically when the radiative temperature gradient monotonically,?rad,decreases outwardly,and the overshooting mixing presents exponential decay similar with Herwig（2000）.The splitting of the convective core occurred repeatedly when the minimum value of ?rad near the convective boundary is smaller than the adiabatic temperature gradient,?ad.The mass at the outer boundary of the convective shell Msc can exceed 0.2Msun after the central helium abundance drops to about Yc =0.45.It is close to the convective core masses derived by the asteroseismology for the younger models（0.22 ～ 0.28Msun）.In the final stage,the "core breathing pulses" happened two or three times.Helium injected into the convective core by the overshooting mixing,and increased the lifetime of sdB stars.The mass of the mixed region M mixed can raise to 0.303 Msun at last.The oxygen content in the central core of our g-mode sdB models are about 80 per cent by mass.For our sdB models with various overshooting schemes,the core breathing pulse is the key to the formation of the white dwarf with higher oxygen content.This may be an evidence to support the existence of the core breathing pulse.
修改评论