Illustration of the model strategy. Two phases are considered for which the level population of C⁺ is calculated as a function of density n, temperature T, and the ionization fraction x in the atomic medium. Collisional rates with each partner also depend on temperature. The [C II] intensity is calculated using the level population and the column density of H⁰ or H₂ and the fraction of C into C⁺ in each phase (see Lebouteiller et al. 2013 for details).