Relative level populations of the J = 3 and J = 2 levels of C¹⁷O, and the J = 4 level of H¹³CO⁺, as determined by the Monte Carlo method in ratran for the static spherical model. The population of each J level is given relative to the total population for that species. The gas temperature profile (T_kin) of the model cloud is shown by the solid blue line. The energy level of the J = 3 rotational state of C¹⁷O is indicated by the dotted horizontal line.