Convective boundary mixing in simulations of massive stars




Davis, Austin

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The turbulent convective mixing in the late stage evolution of the core of massive stars is not well understood. One-dimensional (1D) codes that simulate the lifetime of stars rely on models with free parameters in order to model convection. The free parameter determining the strength of the mixing across the boundary of a convection zone is undetermined for the majority of convective boundaries. In this thesis, the effects of convective boundary mixing (CBM) in stellar evolution models is investigated with a focus on the structural changes to massive star cores. An estimate for the amount of mixing present at a convective boundary is made from a three-dimensional O-shell simulation. Using this value, a set of simulations are computed in the 1D stellar evolution code, MESA, that test the effects of CBM of massive star carbon-oxygen (CO) cores. A three-dimensional (3D) simulation of the O-shell in a 25M_{sun} stellar model with Z=0.02 is summarized with a focus on determining the diffusion coefficient that would be necessary in a 1D stellar evolution model to reproduce the spherically averaged composition profiles. The diffusion coefficient was then fit with the exponential decaying CBM model (Freytag et al. 1996) and the free parameter, f_(cbm), was determined. The sensitivity of the late time evolution of the core in a 25M_{sun} 1D stellar model at Z=0.02 with respect to variation in the value of f_(cbm) was tested. The goal of this work was not to determine what values of f_(cbm) the stellar model should have, but to investigate the differences in the structure as a result of changing the f_(cbm) values. Past the onset of convection in the first C shell, the values of f_(cbm) change the structure of the star significantly, promoting dredge-ups that mix material from the core to the top of the C shell. The presupernova structure was investigated with a focus on the compactness parameter, xi_{2.5}={2.5 M_{sun}}/{R(2.5 M_{sun})}. The models show significant non-monotonic variation in xi_{2.5} with respect to f_(cbm), where xi_{2.5} spans a range of (0.12, 0.35). The abundances near collapse of the models were also investigated. It was found that Ne ash that was entrained into the C shells through dredge-ups and shell mergers was transported high enough in the star to be ejected by the supernova explosion. Informed by 3D simulations, this study shows that for values of f_(cbm) for the exponentially decaying CBM model, ranging from (0.002, 0.032), significantly affect the structure of the CO core. Interaction between the carbon (C), neon (Ne) and oxygen (O) convective shells change the core boundaries and therefore the structure. Although, during Si core burning the affect of the interaction is monotonic, as the simulations collapse shortly after this. The structure of the CO core determines the value of xi_{2.5}, determining whether the model will explode.