Excess emission at 2.1 μm, ε_K shown as a function of R₅₅₀. The top panel plots the ratio between the observed flux and the intrinsic photospheric one (${ϵ}_K=\frac{F_{obs,K}}{F_{ph,K}}-1$). The bottom panel shows the ratio between the observed flux corrected for the extinction derived at optical wavelengths and the photospheric flux (${ϵ}_K=\frac{F_{obs,K}{e}^{\tau }}{F_{ph,K}}-1$). The grey area covers a ±20% region around ε_K = 0. Vertical error bars on ε_K are smaller than the size of the points.