Skeletal Kinetics Reduction for Astrophysical Reaction Networks

A novel methodology is developed to extract accurate skeletal reaction models for nuclear combustion. Local sensitivities of isotope mass fractions with respect to reaction rates are modeled based on the forced optimally time-dependent (f-OTD) scheme. These sensitivities are then analyzed temporally to generate skeletal models. The methodology is demonstrated by conducting skeletal reduction of constant density and temperature burning of carbon and oxygen relevant to SNe Ia. The 495-isotopes Torch model is chosen as the detailed reaction network. A map of maximum production of ⁵⁶Ni in SNe Ia is produced for different temperatures, densities, and proton to neutron ratios. The f-OTD simulations and the sensitivity analyses are then performed with initial conditions from this map. A series of skeletal models are derived and their performances are assessed by comparison against currently existing skeletal models. Previous models have been constructed intuitively by assuming the dominance of α-chain reactions. The comparison of the newly generated skeletal models against previous models is based on the predicted energy release and ⁴⁴Ti and ⁵⁶Ni abundances by each model. The consequences of yₑ ≠ 0.5 in the initial composition are also explored where yₑ is the electron fraction. The simulated results show that ⁵⁶Ni production decreases by decreasing yₑ as expected, and that the ⁴³Sc is a key isotope in proton and neutron channels toward ⁵⁶Ni production. It is shown that an f-OTD skeletal model with 150 isotopes can accurately predict the ⁵⁶Ni abundance in SNe Ia for yₑ 0.5 initial conditions.