Fractional population of level J = 6 from a model for a low mass-loss rate AGB star for ¹²CO (full red line) and ¹³CO (dashed blue line), with an isotopic ratio ¹²CO/¹³CO = 20. The vertical purple dashed line marks the point where critical density is reached for ¹²CO, calculated using the collisional coefficients between H₂ and CO for a 300 K gas. ¹³CO reaches the critical density at a 5% greater distance.