Structure and maximum neutron density during the third instability. Top: Kippenhahn diagram of the same model as in Fig. 1 around the proton ingestion event. The red circle corresponds to the time and location where the neutron density is maximal. The chemical composition at the time and location of the two colored circles are shown in Fig. 4. The time is indicated at specific moments (it does not run linearly) on the top axis, counted from t₀, which is the time corresponding to the black circle. The three important steps in this event are the maximal neutron density (red circle, t − t₀ = 1.182 yr), the splitting of the convective helium shell (t − t₀ = 1.184 yr) and the merging of the helium shell with the envelope (t − t₀ = 7.956 yr). Bottom: evolution of maximum neutron density found in the stellar model at a given time (or equivalently for a given evolutionary model number).