From top left to bottom right: vertical cuts at ~300 AU of the STN, α = 10^-2, 10^-3, and 10^-4 models. Legend: solid lines in black, red, green, and blue represent abundances of CO, C, C⁺, and CO ice, respectively. Dashed lines in orange and brown are T_gas and T_dust, respectively. We note that T_gas falls below T_dust at low values of α and that this thermal de-coupling starts at the C–CO transition.