Relative Importance of Convective Uncertainties in Massive Stars

TL;DR

This study explores how uncertainties in convective boundary mixing affect the structure, evolution, and nucleosynthesis of massive stars, highlighting significant impacts on core sizes and surface evolution.

Contribution

It provides a detailed analysis of convective boundary mixing effects on massive star models using MESA, including impacts on core size, nucleosynthesis, and evolutionary tracks.

Findings

Broadened main-sequence with increased CBM aligns better with observations.

Convergence of convective boundary location during hydrogen burning is affected by CBM.

Uncertainties lead to up to 70% differences in core and total stellar masses.

Abstract

In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called `overshoot', namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses $15$, $20$ and $25\,\rm{M}_\odot$. We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main-sequence with more CBM, which is more in agreement with observations. Furthermore during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves red-wards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the $^{12}$C to $^{16}$O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduce and we find differences of up to $70\%$ for the core masses and the total mass of the star.