Overshooting in the core helium burning stage of a $30M_{\odot}$ star using the $k$-$\omega$ model

Abstract

Overshooting and semiconvection are among the most uncertainties in the evolution of massive stars. Complete mixing over a certain distance beyond the convective boundary (Stothers \& Chin 1985) and an exponentially decaying diffusion outside the convection zone (Herwig 2000) are commonly adopted for the overshoot mixing. Recently, Li (2012, 2017) developed the $k$-$\omega$ model, which can be used in both convection zones and overshooting regions. We incorporated it in calculations of $30M_{\odot}$ stellar models. We find that in the main sequence stage, models with the $k$-$\omega$ model are almost identical to models with complete mixing in the overshooting region beyond the convective core, and the overshooting in the $k$-$\omega$ model is equivalent to an overshooting distance of about $0.15H_P$. In the post main sequence stage, we find that the overshooting below the bottom of the intermediate convection zone beyond the hydrogen-burning shell can significantly restrict the size of the hydrogen-depleted core, and can penetrate effectively into the hydrogen-burning shell. These two effects are crucial for the evolution of the core helium burning stage. During the core helium burning stage, we find that the overshooting model based on the $k$-$\omega$ model results in a similar complete mixing region but a much wider partial mixing region than the overshooting model based on Herwig (2000). In particular, the overshooting distance in the core helium burning stage may be significantly smaller than that in the main sequence phase for massive stars.