Determine bottleneck CNO nuclide abundances in the simplified network

Determine the abundances of the bottleneck nuclides 14N (for the cold CNO cycle) and 14O and 15O (for the hot CNO cycle) during CNO-cycle hydrogen burning in the collapse simulations of rotating supermassive star cores that evolve only H, He, and an aggregate "CNO species," in order to replace the current assumption that these bottleneck abundances equal the total CNO mass fraction.

Background

To model hydrogen burning, the simulations include only three nuclear species (H, He, and an aggregate CNO species) and implement both cold and hot CNO cycles using parameterized rates. Because individual CNO isotopes are not evolved, the simulations lack explicit abundances for specific nuclides that act as bottlenecks in the cycles.

The bottleneck species 14N (cold CNO) and 14O/15O (hot CNO) control the reaction flow and associated energy and neutrino production. Due to the limited network, the authors assume these bottleneck mass fractions are equal to the total CNO mass fraction, introducing an acknowledged uncertainty. Quantifying these abundances within the simulation framework would remove this approximation and refine the burning rates and neutrino source terms.