Line intensity of ¹³CO(1–0) and ¹³CO(2–1) (radiation temperature) and their ratio (2–1)/(1–0) in stable, starless thermally subcritical cores. The temperature structure for this model is shown in Fig. 8. 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 high densities at the edge of the core, the CO excitation temperature is always close to the gas temperature. Parameters are listed in Table 1.