Compactness parameter ξ_2.5 vs. the critical model time-averaged heating efficiency ${\overline{\eta }}_{\mathrm{heat}}^{\mathrm{crit}}$. The left, center, and right panels correspond to initial velocities V_ini = 0, 300 km s⁻¹, and 600 km s⁻¹, respectively. The data points in red, green, and black denote metallicities Z_⊙, 1/10 Z_⊙, and 1/50 Z_⊙. The squares mark the occurrence of CHE. The hollow inverted triangles were calculated by O’Connor & Ott (2011), and the combinations of EOS and models LS220-sWH07, LS180-sWH07, and LS220-uWHW02 are represented in red, green, and black, respectively.