Fraction of C⁺ (with respect to C⁺ and C²⁺) for MC2-HD at different times projected along the y-direction. As time evolves, C⁺ gets progressively converted into C²⁺ inside the HI regions due to further photo-ionisation by the radiation of the newly formed stars. This leads to a drop in the [CII] intensity, in particular in the region enclosed with the black rectangle (compare with Fig. 5).