Temperature as a function of the optical depth at 1 μm for S-star model atmospheres. Left panel: for different effective temperatures, as labelled, but a typical gravity (log (g) = 0) and C/O ratio (C/O = 0.899) of S-type stars, the thermal structure shows only minor changes between non-enhanced ([s/Fe] = 0 dex, continuous lines) and enhanced ([s/Fe] = +2 dex, dashed lines) models. Right panel: same as left panel, but for a given effective temperature (T_eff = 3600 K) and varying gravities, as labelled.