Distribution of I_[CII] as a function of I_FIR for all pixels within 10 pc from the main star of MC2-HD, bubble 2, in the x − ɀ plane. The colour bar indicates the distance from the main star. To guide the readers eye, we show with the black line a constant ratio of I_[CII]/I_FIR = 10⁻². For pixels at close-by distances, the values of I_[CII] and I_FIR decrease over time due to the evacuation of the bubble, whereas at larger distances the values remain in a similar range. This leads to a separation into a “main branch”, formed by distant pixels, and a branch of close-by pixels, located below the main one. The squares show observational data from Orion A & B taken from Pabst et al. (2021, binned for the sake of readability) for comparison.