Top: SEDs for the dust composition Mix 1:50 described in Sect. 2.1 and Table 1. The SEDs were obtained considering an optically thin medium with a column density N_H = 10²⁰ H cm⁻² illuminated by the standard ISRF scaled by G₀ = 10⁻³, 10⁻², 10⁻¹, 1, 10, 10², and 10³ plotted in orange, magenta, green, red, black, cyan, and brown, respectively. Bottom: corresponding spectral indices calculated over 1 GHz frequency steps. The spectral indices for a₀ = 1 cm and G₀ = 10⁻³, 10⁻², and 10⁻¹ are overlapping.