Top panel: dominant formation and destruction pathways of the carbon hydrogenation chain in high temperature media (T ≥ 250 K) or in media irradiated by strong UV radiation field. Bottom panel: common forms of the density profile of CH⁺ in molecular shocks irradiated by an external radiation field of strength G₀. Although the scales are arbitrary, all the density profiles are shown in log–log space. Labels highlight parts of the trajectory where the evolution of CH⁺ abundance is driven by a specific thermochemical process (see text).