Line ratios of ${\mathrm{C}\mathrm{H}}_3^{+}$ as a function of temperature. The blue line is the estimation of the ratios from LTE models. The gray area is the measurement of the ratio in the data in DF3. The uncertainties come from the estimation of the continuum. The red lines trace the temperature range where the models cross the observations. (Top) Ratio of the 7.12 μm line over the 7.2 μm line. (Bottom) Ratio of the 7.15 μm line over the 7.2 μm line. The excitation temperature of ${\mathrm{C}\mathrm{H}}_3^{+}$ in DF3 is around 1000–1500 K.