Convective overshooting in sdB stars using the $k$-$\omega$ model

Abstract

Mixing in the convective core is quite uncertain in core helium burning stars. In order to explore the overshooting mixing beyond the convective core, we incorporated the $k$-$\omega$ proposed by Li (2012, 2017) into MESA, and investigated the overshooting mixing in evolution of sdB models. We found that the development of the convective core can be divided into three stages. When the radiative temperature gradient $\nabla_{\rm rad}$ monotonically decreases from the stellar center, the overshooting mixing presents a behaviour of exponential decay similar with Herwig (2000), and the overshooting distance is to make $\nabla_{\rm rad} \simeq \nabla_{\rm ad}$ at the boundary of the convective core, in agreement with the prediction of the self-driving mechanism Castellani, Giannone \& Renzini (1971a). When the radiative temperature gradient $\nabla_{\rm rad}$ shows a minimum value in the convective core, the convective core may be divided into two convection zones if the minimum value of $\nabla_{\rm rad}$ is smaller than the adiabatic temperature gradient $\nabla_{\rm ad}$. For the single-zone case, the overshooting mixing still shows an exponential decay behaviour, but the overshooting distance is much smaller than in the initial stage. For the double-zone case, the overshooting mixing is similar to that of the single-case beyond the convective core, while it almost stops on both sides of the above convective shell. Our overshooting mixing scheme is similar to the maximal overshoot scheme of Constantino et al. (2015). In the final stage, continuous injection of fresh helium into the convective core by the overshooting mixing happens, which is similar to the "core breathing pulses" (Sweigart \& Demarque 1973; Castellani et al. 1985).