Ratio of the CO and [O I] luminosities as a function of preshock density for UV irradiated C–shock models and observations of Gy 3-7 cores and IM YSOs from Matuszak et al. (2015; light blue box). All models correspond to UV fields parameterized by G₀ of 10 (in blue) and 0.1 (in orange). Solid lines connect models with shock velocities v_s of 20 km s⁻¹, and dashed lines – the models with v_s of 10 km s⁻¹.