Modeled intensities from the radiative transfer in a spherically symmetric bubble. Top panel: modeled [C II] intensity for simple shell geometries (Fig. 14) for various optical depths of surrounding cold material. Bottom panel: modeled ¹²CO and ¹³CO (3 →2) intensities for a slightly more realistic configuration with a continuous density and temperature gradient. The modeled intensities are determined by solving the radiative transfer equations with SimLine (Ossenkopf et al. 2001).