Mass fraction q_AF of aromatic-feature-carrying grains in THEMIS as a function of the nano-grain dust-to-gas ratio M_a-C /M_H and the nano-grain minimum size a_{min, a-C}. The grey dot corresponds to the dust properties of the diffuse ISM-like dust (M_a-C /M_H = 0.17 × 10⁻² and a_{min, a-C} = 0.4 nm). The grey rectangle corresponds to the dust properties in the irradiated outer part of the Horsehead.