[CII] emission map of MC2-HD at t_evol = 4 Myr for three different LOSs. The green and cyan circles highlight the stars (see Fig. 4 for a legend of the star symbols) responsible for the formation of ‘bubble 1’ and ‘bubble 2’, respectively. Because of the complex structure of the cloud, the shape of such HII regions is strongly dependent on the chosen LOS. As an example, “bubble 2” is not visible at all when looking along the z-axis (left panel). This explains the variations of the [CII] deficit as a function of the bubble radius for different LOS (Fig. 10).