Expected I([C ii]) for n(H)= 4500 cm^-3 (left) and n(H)= 2300 cm^-3 (right) at different total hydrogen column densities N(H) and temperatures calculated from Eq. (5). The filled grey areas show the expected I([C ii]) for a T = 30 K. The blue areas show the expected I([C ii]) for a cold cloud with a kinetic gas temperature of 14 K, as indicated by the CO(2–1)/CO(1–0) ratio. The dashed lines (---) represent the I([C ii]) for a beam filling factor of Φ_b = 0.5. The upper and lower edge of the grey and blue filled areas shows the I([C ii]) for a beam filling factor of Φ_b = 1 and Φ_b = 0.1, respectively. The red areas mark the upper limits of the [C ii] observations.