Line intensity of C¹⁸O(1–0) and C¹⁸O(2–1) (radiation temperature) and their ratio (2–1)/(1–0) in unstable, prestellar thermally supercritical cores. The temperature structure for this model is shown in Fig. 7. The density structure is shown in Fig. 9. The ratio of the CO lines is a function of the excitation temperature of the CO. In this model because of the low densities at the edge of the core, the CO excitation temperature is much lower than the gas temperature. Parameters are listed in Table 1.