Upper panel: time evolution of convective zones in model M2 Z = 10^-8 during the PIE. Dark grey area: convective envelope, purple area between 0.24 and 0.51 M_⊙: HeCZ; light grey area: HCZ. Thick solid line represents the position where hydrogen mass fraction is 0.001; dot-dashed line represents the position of maximum energy release due to He-burning, respectively. The splitting of the HeCZ around t − t₀ = 0.451  yr is evident with the formation of the detached HCZ. Bottom panel: evolution of H-burning (solid line) and He-burning (dashed line) luminosities. t = t₀ corresponds to the time of maximum He-burning luminosity.