Fig. A.1. Line and continuum luminosity ratios based on PDR models as a function of hydrogen number density, , of the model cloud. The assumed photoelectric efficiency is , independent of the energy of the incident photons. Otherwise, the models are equivalent to those for Fig. 5. The solid curves indicate the FIR luminosity ratio (, multiplied by ) of the [CII ] line to the continuum. The dashed curves indicate , where is the total luminosity of the CO rotational lines. Thin and thick curves are for and 10, respectively. The mean hydrogen column density of the cloud is assumed to be .