Fig. 4: Observed Cl/S and K/Ar ratios in Cas A compared with representative nucleosynthesis models.

The abundance ratio is defined as the elemental abundance ratio normalized to the solar value: (K/Ar)/(K/Ar)_⊙ and (Cl/S)/(Cl/S)_⊙. Red regions indicate the 1σ statistical confidence intervals of observed abundance ratios in the SE region. Model predictions are shown for non-rotating single-star progenitors with initial masses of 13–25M_⊙ from ref. ²⁰ (grey crosses), rotating models from ref. ²¹ with initial masses of 13–25M_⊙ and rotational velocities of 300 km s⁻¹ (green squares), shell merger candidates from a subset of the models in ref. ²² with progenitor masses of ~19–27M_⊙ (red circles) and binary star models from ref. ¹² with initial masses of 13–25M_⊙ and companion stars with 80% of the primary mass (blue diamonds). All models shown assume solar metallicity. See Extended Data Fig. 2 for a more comprehensive comparison across a wider range of model parameters.