Time evolution of the compactness parameter ξ_2.5 (top panel) and evolution in ρ_c−T_c space (bottom panel) from oxygen depletion to the onset of core collapse. Solid curves correspond to 15 M_⊙ models computed with η = 1.0, dashed and dot-dashed curves correspond to 25 M_⊙ and 30 M_⊙ models, respectively, computed with η = 0.33. The vertical dot-dashed line in the top panel indicates roughly the time of core silicon depletion (X(²⁸Si) ≤ 0.01).