Rotation diagram of ¹²CO. The black points with error bars represent the Herschel data, whereas the two black points connected by a bar are the lower and upper limit to the¹³ CO column density in the J = 2 level obtained from the¹³ CO(2–1) data of Cesaroni et al. (1999b) after multiplying the column density for a ¹²CO/¹³ CO abundance ratio of 72. The red points connected by the curve are the best fit to the Herschel data obtained with the model described in Appendix A.